A model based on disk-stability criteria to determine the number of spiral arms of a general disk galaxy with an exponential disk, a bulge and a dark halo described by a Hernquist model is presented. The multifold rotational symmetry of the spiral structure can be evaluated analytically once the structural properties of a galaxy, such as the circular speed curve, and the disk surface brightness, are known. By changing the disk mass, these models are aimed at varying the critical length scale parameter of the disk and lead to a different spiral morphology in agreement with prior models. Previous studies based on the swing amplification and disk stability have been applied to constrain the mass-to-light ratio in disk galaxies. This formalism provides an analytic expression to estimate the number of arms expected by swing amplification making its application straight-forward to large surveys. It can be applied to predict the number of arms in the Milky Way as a function of radius and to constrain the mass-to-light ratio in disk galaxies for which photometric and kinematic measurements are available, like in the DiskMass survey. Hence, the halo contribution to the total mass in the inner parts of disk galaxies can be inferred in light of the ongoing and forthcoming surveys.
The Prime Focus Spectrograph (PFS) is an optical/near-infrared multifiber spectrograph with 2394 science fibers distributed across a 1.3-deg diameter field of view at the Subaru 8.2-m telescope. The wide wavelength coverage from 0.38 {\mu}m to 1.26 {\mu}m, with a resolving power of 3000, simultaneously strengthens its ability to target three main survey programs: cosmology, galactic archaeology and galaxy/AGN evolution. A medium resolution mode with a resolving power of 5000 for 0.71 {\mu}m to 0.89 {\mu}m will also be available by simply exchanging dispersers. We highlight some of the technological aspects of the design. To transform the telescope focal ratio, a broad-band coated microlens is glued to each fiber tip. A higher transmission fiber is selected for the longest part of the cable system, optimizing overall throughput; a fiber with low focal ratio degradation is selected for the fiber-positioner and fiber-slit components, minimizing the effects of fiber movements and fiber bending. Fiber positioning will be performed by a positioner consisting of two stages of piezo-electric rotary motors. The positions of these motors are measured by taking an image of artificially back-illuminated fibers with the metrology camera located in the Cassegrain container; the fibers are placed in the proper location by iteratively measuring and then adjusting the positions of the motors. Target light reaches one of the four identical fast-Schmidt spectrograph modules, each with three arms. The PFS project has passed several project-wide design reviews and is now in the construction phase.
We present the results of our first year of quasar search in the on-going ESO public Kilo Degree Survey (KiDS) and VISTA Kilo-Degree Infrared Galaxy (VIKING) surveys. These surveys go up to 2 magnitudes fainter than other wide-field imaging surveys that uncovered predominantly very luminous quasars at z~6. This allows us to probe a more common, fainter population of z~6 quasars. From this first set of combined survey catalogues covering ~250 deg^2 we selected point sources down to Z_AB=22 that had a very red i-Z (i-Z>2.2) colour. After follow-up imaging and spectroscopy, we discovered four new quasars in the redshift range 5.8<z<6.0. The absolute magnitudes at a rest-frame wavelength of 1450 A are between -26.6 < M_1450 < -24.4, confirming that we can find quasars fainter than M^*, which at z=6 has been estimated to be between M^*=-25.1 and M^*=-27.6. The discovery of 4 quasars in 250 deg^2 of survey data is consistent with predictions based on the z~6 quasar luminosity function. We discuss various ways to push the candidate selection to fainter magnitudes and we expect to find about 30 new quasars down to an absolute magnitude of M_1450=-24. Studying this homogeneously selected faint quasar population will be important to gain insight into the onset of the co-evolution of the black holes and their stellar hosts.
Do spatial distributions of dust grains in galaxies have typical forms, as do spatial distributions of stars? We investigate whether or not the distributions resemble uniform foreground screens, as commonly assumed by the high-redshift galaxy community. We use rest-frame infrared, ultraviolet, and H$\alpha$ line luminosities of dust-poor and dusty galaxies at z ~ 0 and z ~ 1 to compare measured H$\alpha$ escape fractions with those predicted by the Calzetti attenuation formula. The predictions, based on UV escape fractions, overestimate the measured H$\alpha$ escape fractions for all samples. The interpretation of this result for dust-poor z ~ 0 galaxies is that regions with ionizing stars have more dust than regions with nonionizing UV-emitting stars. Dust distributions for these galaxies are nonuniform. The interpretation of the overestimates for dusty galaxies at both redshifts is less clear. If the attenuation formula is inapplicable to these galaxies, perhaps the disagreements are unphysical; perhaps dust distributions in these galaxies are uniform. If the attenuation formula does apply, then dusty galaxies have nonuniform dust distributions; the distributions are more uniform than they are in dust-poor galaxies. A broad range of H$\alpha$ escape fractions at a given UV escape fraction for z ~ 1 dusty galaxies, if real, indicates diverse dust morphologies and the implausibility of the screen assumption.
This chapter provides an overview of the possibilities for transient and variable-source astrophysics with the Square Kilometre Array. While subsequent chapters focus on the astrophysics of individual events, we focus on the broader picture, and how to maximise the science coming from the telescope. The SKA as currently designed will be a fantastic and ground-breaking facility for radio transient studies, but the scientifc yield will be dramatically increased by the addition of (i) near-real-time commensal searches of data streams for events, and (ii) on occasion, rapid robotic response to Target-of-Opprtunity style triggers.
Feedback from active galactic nuclei (AGN) has often been invoked both in simulations and in interpreting observations for regulating star formation and quenching cooling flows in massive galaxies. AGN activity can, however, also over-pressurise the dense star-forming regions of galaxies and thus enhance star formation, leading to a positive feedback effect. To understand this pressurisation better, we investigate the effect of an ambient external pressure on gas fragmentation and triggering of starburst activity by means of hydrodynamical simulations. We find that moderate levels of over-pressurisation of the galaxy boost the global star formation rate of the galaxy by an order of magnitude, turn stable discs unstable, and lead to significant fragmentation of the gas content of the galaxy, similar to what is observed in high redshift galaxies.
The abundance of compact and massive early-type galaxies (ETGs) can provide significant constraints on the galaxy merging history. The optical Kilo Degree Survey (KiDS), carried out with the VST, gives a unique opportunity to perform a complete census of the most compact galaxies in the Universe. This paper presents a first census of compact galaxy candidates from the first 156 square degrees of KIDS. Effective radii (Re) in the g-, r-, and i- bands are derived fitting galaxy images with PSF-convolved S\'ersic models, high-quality photo-z, are derived from machine learning techniques, and stellar masses, are calculated by fitting aperture photometry with predictions from stellar population models. After the morphological star/galaxy separation, massiveness ($M_{\star} > 8 \times 10^{10}\, \rm M_{\odot}$) and compactness ($R_{e} < 1.5 \, \rm kpc$) criteria are applied, and a visual inspection of the candidates plus near-IR photometry from VIKING-DR1 are used to refine our sample. The final catalog contains 92 compact systems in the redshift range $z \sim 0.2-0.7$. This sample, to be spectroscopically confirmed, represents the first attempt to select massive super-compact ETGs (MSCGs) in KiDS, a sample that we expect to increase, by a factor of ten, over the total survey area ($\sim 1500$ sq. deg.). We investigate the impact of redshift systematics in the selection, finding that, indeed, this seems a major source of contamination in our sample. Finally, we show that the number density of MSCGs , as a function of redshift, is mildly consistent with predictions from the Millennium Simulation for z>0.2, while, remarkably, no such system is found at z<0.2. (abridged)
We have started a systematic search of strong lens candidates in the ESO public survey KiDS based on the visual inspection of massive galaxies in the redshift range $0.1<z<0.5$. As a pilot program we have inspected 100 sq. deg., which overlap with SDSS and where there are known lenses to use as a control sample. Taking advantage of the superb image quality of VST/OmegaCAM, the colour information and accurate model subtracted images, we have found 18 new lens candidates, for which spectroscopic confirmation will be needed to confirm their lensing nature and study the mass profile of the lensing galaxies.
Observational bias against finding Milky Way (MW) dwarf galaxies at low Galactic latitudes (b < 20 deg) and at low surface brightnesses (fainter than 29 mag arcsec^-2, in the V-band) currently limits our understanding of the faintest limits of the galaxy luminosity function. This paper is a proof-of-concept that groups of two or more RR Lyrae stars reveal MW dwarf galaxies at d > 50 kpc in these unmined regions of parameter space, with only modest contamination from interloper groups when large halo structures are excluded. For example, a friends-of-friends (FOF) search with a linking length of 500 pc could reveal dwarf galaxies more luminous than M_V = -3.2 mag and with surface brightnesses as faint as 31 mag arcsec^-2 (or even fainter, depending on RR Lyrae specific frequency). Although existing public RR Lyrae catalogs are highly incomplete at d > 50 kpc and/or include <1% of the MW halo's volume, a FOF search reveals two known dwarfs (Bootes I and Sextans) and two dwarf candidate groups possibly worthy of follow-up. PanSTARRS 1 (PS1) may catalog RR Lyrae to 100 kpc which would include ~15% of predicted MW dwarf galaxies. Groups of PS1 RR Lyrae should therefore reveal very low surface brightness and low Galactic latitude dwarfs within its footprint, if they exist. With sensitivity to RR Lyrae to d >600 kpc, LSST is the only planned survey that will be both wide-field and deep enough to use RR Lyrae to definitively measure the Milky Way's dwarf galaxy census to extremely low surface brightnesses, and through the Galactic plane.
The Kilo-Degree Survey (KiDS) is an optical wide-field survey designed to map the matter distribution in the Universe using weak gravitational lensing. In this paper, we use these data to measure the density profiles and masses of a sample of $\sim \mathrm{1400}$ spectroscopically identified galaxy groups and clusters from the Galaxy And Mass Assembly (GAMA) survey. We detect a highly significant signal (signal-to-noise-ratio $\sim$ 120), allowing us to study the properties of dark matter haloes over one and a half order of magnitude in mass, from $M \sim 10^{13}-10^{14.5} h^{-1}\mathrm{M_{\odot}}$. We interpret the results for various subsamples of groups using a halo model framework which accounts for the mis-centring of the Brightest Cluster Galaxy (used as the tracer of the group centre) with respect to the centre of the group's dark matter halo. We find that the density profiles of the haloes are well described by an NFW profile with concentrations that agree with predictions from numerical simulations. In addition, we constrain scaling relations between the mass and a number of observable group properties. We find that the mass scales with the total r-band luminosity as a power-law with slope $1.16 \pm 0.13$ (1-sigma) and with the group velocity dispersion as a power-law with slope $1.89 \pm 0.27$ (1-sigma). Finally, we demonstrate the potential of weak lensing studies of groups to discriminate between models of baryonic feedback at group scales by comparing our results with the predictions from the Cosmo-OverWhelmingly Large Simulations (Cosmo-OWLS) project, ruling out models without AGN feedback.
The ESO Public Kilo-Degree Survey (KiDS) is an optical wide-field imaging survey carried out with the VLT Survey Telescope and the OmegaCAM camera. KiDS will scan 1500 square degrees in four optical filters (u, g, r, i). Designed to be a weak lensing survey, it is ideal for galaxy evolution studies, thanks to the high spatial resolution of VST, the good seeing and the photometric depth. The surface photometry have provided with structural parameters (e.g. size and S\'ersic index), aperture and total magnitudes have been used to derive photometric redshifts from Machine learning methods and stellar masses/luminositites from stellar population synthesis. Our project aimed at investigating the evolution of the colour and structural properties of galaxies with mass and environment up to redshift $z \sim 0.5$ and more, to put constraints on galaxy evolution processes, as galaxy mergers.
We use the first 100 sq. deg. of overlap between the Kilo-Degree Survey (KiDS) and the Galaxy And Mass Assembly (GAMA) survey to determine the galaxy halo mass of ~10,000 spectroscopically-confirmed satellite galaxies in massive ($M > 10^{13}h^{-1}{\rm M}_\odot$) galaxy groups. Separating the sample as a function of projected distance to the group centre, we jointly model the satellites and their host groups with Navarro-Frenk-White (NFW) density profiles, fully accounting for the data covariance. The probed satellite galaxies in these groups have total masses $\log M_{\rm sub} /(h^{-1}{\rm M}_\odot) \approx 11.7 - 12.2$ consistent across group-centric distance within the errorbars. Given their typical stellar masses, $\log M_{\rm \star,sat}/(h^{-2}{\rm M}_\odot) \sim 10.5$, such total masses imply stellar mass fractions of $M_{\rm \star,sat} /M_{\rm sub} \approx 0.04 h^{-1}$ . The average subhalo hosting these satellite galaxies has a mass $M_{\rm sub} \sim 0.015M_{\rm host}$ independent of host halo mass, in broad agreement with the expectations of structure formation in a $\Lambda$CDM universe.
The Kilo-Degree Survey (KiDS) is a multi-band imaging survey designed for
cosmological studies from weak lensing and photometric redshifts. It uses the
ESO VLT Survey Telescope with its wide-field camera OmegaCAM. KiDS images are
taken in four filters similar to the SDSS ugri bands. The best-seeing time is
reserved for deep r-band observations that reach a median 5-sigma limiting AB
magnitude of 24.9 with a median seeing that is better than 0.7arcsec.
Initial KiDS observations have concentrated on the GAMA regions near the
celestial equator, where extensive, highly complete redshift catalogues are
available. A total of 101 survey tiles, one square degree each, form the basis
of the first set of lensing analyses, which focus on measurements of halo
properties of GAMA galaxies. 9 galaxies per square arcminute enter the lensing
analysis, for an effective inverse shear variance of 69 per square arcminute.
Accounting for the shape measurement weight, the median redshift of the sources
is 0.53.
KiDS data processing follows two parallel tracks, one optimized for galaxy
shape measurement (for weak lensing), and one for accurate matched-aperture
photometry in four bands (for photometric redshifts). This technical paper
describes how the lensing and photometric redshift catalogues have been
produced (including an extensive description of the Gaussian Aperture and
Photometry pipeline), summarizes the data quality, and presents extensive tests
for systematic errors that might affect the lensing analyses. We also provide
first demonstrations of the suitability of the data for cosmological
measurements, and explain how the shear catalogues were blinded to prevent
confirmation bias in the scientific analyses.
The KiDS shear and photometric redshift catalogues, presented in this paper,
are released to the community through this http URL .
There is now strong evidence that some stars have been born with He mass fractions as high as $Y \approx 0.40$ (e.g., in $\omega$ Centauri). However, the advanced evolution, chemical yields, and final fates of He-rich stars are largely unexplored. We investigate the consequences of He-enhancement on the evolution and nucleosynthesis of intermediate-mass asymptotic giant branch (AGB) models of 3, 4, 5, and 6 M$_\odot$ with a metallicity of $Z = 0.0006$ ([Fe/H] $\approx -1.4$). We compare models with He-enhanced compositions ($Y=0.30, 0.35, 0.40$) to those with primordial He ($Y=0.24$). We find that the minimum initial mass for C burning and super-AGB stars with CO(Ne) or ONe cores decreases from above our highest mass of 6 M$_\odot$ to $\sim$ 4-5 M$_\odot$ with $Y=0.40$. We also model the production of trans-Fe elements via the slow neutron-capture process (s-process). He-enhancement substantially reduces the third dredge-up efficiency and the stellar yields of s-process elements (e.g., 90% less Ba for 6 M$_\odot$, $Y=0.40$). An exception occurs for 3 M$_\odot$, where the near-doubling in the number of thermal pulses with $Y=0.40$ leads to $\sim$ 50% higher yields of Ba-peak elements and Pb if the $^{13}$C neutron source is included. However, the thinner intershell and increased temperatures at the base of the convective envelope with $Y=0.40$ probably inhibit the $^{13}$C neutron source at this mass. Future chemical evolution models with our yields might explain the evolution of s-process elements among He-rich stars in $\omega$ Centauri.
The Kilo-Degree Survey (KiDS) is an optical wide-field imaging survey carried
out with the VLT Survey Telescope and the OmegaCAM camera. KiDS will image 1500
square degrees in four filters (ugri), and together with its near-infrared
counterpart VIKING will produce deep photometry in nine bands. Designed for
weak lensing shape and photometric redshift measurements, the core science
driver of the survey is mapping the large-scale matter distribution in the
Universe back to a redshift of ~0.5. Secondary science cases are manifold,
covering topics such as galaxy evolution, Milky Way structure, and the
detection of high-redshift clusters and quasars.
KiDS is an ESO Public Survey and dedicated to serving the astronomical
community with high-quality data products derived from the survey data, as well
as with calibration data. Public data releases will be made on a yearly basis,
the first two of which are presented here. For a total of 148 survey tiles
(~160 sq.deg.) astrometrically and photometrically calibrated, coadded ugri
images have been released, accompanied by weight maps, masks, source lists, and
a multi-band source catalog.
A dedicated pipeline and data management system based on the Astro-WISE
software system, combined with newly developed masking and source
classification software, is used for the data production of the data products
described here. The achieved data quality and early science projects based on
the data products in the first two data releases are reviewed in order to
validate the survey data. Early scientific results include the detection of
nine high-z QSOs, fifteen candidate strong gravitational lenses, high-quality
photometric redshifts and galaxy structural parameters for hundreds of
thousands of galaxies. (Abridged)
We present the methods and first results of the search for galaxy clusters in the Kilo Degree Survey (KiDS). The adopted algorithm and the criterium for selecting the member galaxies are illustrated. Here we report the preliminary results obtained over a small area (7 sq. degrees), and the comparison of our cluster candidates with those found in the RedMapper and SZ Planck catalogues; the analysis to a larger area (148 sq. degrees) is currently in progress. By the KiDS cluster search, we expect to increase the completeness of the clusters catalogue to z = 0.6-0.7 compared to RedMapper.
We estimated photometric redshifts (zphot) for more than 1.1 million galaxies of the ESO Public Kilo-Degree Survey (KiDS) Data Release 2. KiDS is an optical wide-field imaging survey carried out with the VLT Survey Telescope (VST) and the OmegaCAM camera, which aims at tackling open questions in cosmology and galaxy evolution, such as the origin of dark energy and the channel of galaxy mass growth. We present a catalogue of photometric redshifts obtained using the Multi Layer Perceptron with Quasi Newton Algorithm (MLPQNA) model, provided within the framework of the DAta Mining and Exploration Web Application REsource (DAMEWARE). These photometric redshifts are based on a spectroscopic knowledge base which was obtained by merging spectroscopic datasets from GAMA (Galaxy And Mass Assembly) data release 2 and SDSS-III data release 9. The overall 1 sigma uncertainty on Delta z = (zspec - zphot) / (1+ zspec) is ~ 0.03, with a very small average bias of ~ 0.001, a NMAD of ~ 0.02 and a fraction of catastrophic outliers (| Delta z | > 0.15) of ~0.4%.
A combined effort utilizing spectroscopy and photometry has revealed the existence of a new globular cluster class. These "anomalous" clusters, which we refer to as "iron-complex" clusters, are differentiated from normal clusters by exhibiting large (>0.10 dex) intrinsic metallicity dispersions, complex sub-giant branches, and correlated [Fe/H] and s-process enhancements. In order to further investigate this phenomenon, we have measured radial velocities and chemical abundances for red giant branch stars in the massive, but scarcely studied, globular cluster NGC 6273. The velocities and abundances were determined using high resolution (R~27,000) spectra obtained with the Michigan/Magellan Fiber System (M2FS) and MSpec spectrograph on the Magellan-Clay 6.5m telescope at Las Campanas Observatory. We find that NGC 6273 has an average heliocentric radial velocity of +144.49 km s^-1 (sigma=9.64 km s^-1) and an extended metallicity distribution ([Fe/H]=-1.80 to -1.30) composed of at least two distinct stellar populations. Although the two dominant populations have similar [Na/Fe], [Al/Fe], and [alpha/Fe] abundance patterns, the more metal-rich stars exhibit significant [La/Fe] enhancements. The [La/Eu] data indicate that the increase in [La/Fe] is due to almost pure s-process enrichment. A third more metal-rich population with low [X/Fe] ratios may also be present. Therefore, NGC 6273 joins clusters such as omega centauri, M 2, M 22, and NGC 5286 as a new class of iron-complex clusters exhibiting complicated star formation histories.
We present an X-ray analysis of a 4 Gyr old open cluster, M67, using archival XMM-Newton data. The aim of this study was to find new X-ray members of M67, and to use the updated member list for studying X-ray variability, and derive the X-ray luminosity functions (XLFs) of different stellar types and compare them with other star clusters of similar age. We report the detection of X-ray emission from 25 members of M67, with membership based primarily on their proper motion, of which one X-ray source is a new member. Supplementing this study with previous ROSAT and Chandra studies of M67, and using the most recent proper motion study by Vereshchagin et al., we have compiled a revised list of X-ray emitting members of M67 consisting of 43 stars. Sixteen of these are known RS CVn type binaries having orbital periods $<$ 10 days, and near-circular orbits, 5 are contact binaries with orbital periods $<$ 6 hours, 5 are yellow and blue stragglers, 2 are Algol-type binaries, and one source is a cataclysmic variable. Fourteen members do not have any orbital information and cannot be classified. Fourteen of the X-ray sources detected do not have any optical counterpart down to a magnitude of $V\simeq22$, and their membership is uncertain. Finally, we report the X-ray luminosity functions of RS CVn type and other types of stars in M67 and compare them with other open clusters of intermediate-to-old age.
This paper is the result of the International Cometary Workshop, held in Toulouse, France in April 2014, where the participants came together to assess our knowledge of comets prior to the ESA Rosetta Mission. In this paper, we look at the composition of the gas and dust from the comae of comets. With the gas, we cover the various taxonomic studies that have broken comets into groups and compare what is seen at all wavelengths. We also discuss what has been learned from mass spectrometers during flybys. A few caveats for our interpretation are discussed. With dust, much of our information comes from flybys. They include {\it in situ} analyses as well as samples returned to Earth for laboratory measurements. Remote sensing IR observations and polarimetry are also discussed. For both gas and dust, we discuss what instruments the Rosetta spacecraft and Philae lander will bring to bear to improve our understanding of comet 67P/Churyumov-Gerasimenko as "ground-truth" for our previous comprehensive studies. Finally, we summarize some of the initial Rosetta Mission findings.
We develop two general methods to infer the gravitational potential of a system using steady-state tracers, i.e., tracers with a time-independent phase-space distribution. Combined with the phase-space continuity equation, the time independence implies a universal Orbital Probability Density Function (oPDF) $\mathrm{d} P(\lambda|{\rm orbit})\propto \mathrm{d} t$, where $\lambda$ is the coordinate of the particle along the orbit. The oPDF is equivalent to Jeans theorem, and is the key physical ingredient behind most dynamical modelling of steady-state tracers. In the case of a spherical potential, we develop a likelihood estimator that fits analytical potentials to the system, and a non-parametric method ("Phase-Mark") that reconstructs the potential profile, both assuming only the oPDF. The methods involve no extra assumptions about the tracer distribution function and can be applied to tracers with any arbitrary distribution of orbits, with possible extension to non-spherical potentials. The methods are tested on Monte-Carlo samples of steady-state tracers in dark matter haloes to show that they are unbiased as well as efficient. A fully documented \textsc{C/Python} code implementing our method is freely available at a GitHub repository linked from this http URL
Using realistic cosmological simulations of Milky Way sized haloes, we study their dynamical state and the accuracy of inferring their mass profiles with steady-state models of dynamical tracers. We use a new method that describes the phase-space distribution of a steady-state tracer population in a spherical potential without any assumption regarding the distribution of their orbits. Applying the method to five haloes from the Aquarius $\Lambda$CDM N-body simulation, we find that dark matter particles are an accurate tracer that enables the halo mass and concentration parameters to be recovered with an accuracy of $5\%$. Assuming a potential profile of the NFW form does not significantly affect the fits in most cases, except for halo A whose density profile differs significantly from the NFW form, leading to a $30\%$ bias in the dynamically fitted parameters. The existence of substructures in the dark matter tracers only affects the fits by $\sim 1\%$. Applying the method to mock stellar haloes generated by a particle-tagging technique, we find the stars are farther from equilibrium than dark matter particles, yielding a systematic bias of $\sim 20\%$ in the inferred mass and concentration parameter. The level of systematic biases obtained from a conventional distribution function fit to stars is comparable to ours, while similar fits to DM tracers are significantly biased in contrast to our fits. In line with previous studies, the mass bias is much reduced near the tracer half-mass radius.
Several concepts now exist for small, space-based missions to directly characterize exoplanets in reflected light. Here, we develop an instrument noise model suitable for studying the spectral characterization potential of a coronagraph-equipped, space-based telescope. We adopt a baseline set of telescope and instrument parameters, including a 2 m diameter primary aperture, an operational wavelength range of 0.4-1.0 um, and an instrument spectral resolution of 70, and apply our baseline model to a variety of spectral models of different planet types, including Earth twins, Jupiter twins, and warm and cool Jupiters and Neptunes. With our exoplanet spectral models, we explore wavelength-dependent planet-star flux ratios for main sequence stars of various effective temperatures, and discuss how coronagraph inner and outer working angle constraints will influence the potential to study different types of planets. For planets most favorable to spectroscopic characterization---cool Jupiters and Neptunes as well as nearby Earth twins and super-Earths---we study the integration times required to achieve moderate signal-to-noise ratio spectra. We also explore the sensitivity of the integration times required to detect the base of key absorption bands (for methane, water vapor, and molecular oxygen) to coronagraph raw contrast performance, exozodiacal light levels, and the distance to the planetary system. Most modeled observations have noise dominated by dark current, indicating that improving CCD performance could substantially drive down requisite integration times. Finally, we briefly discuss the extension of our models to a more distant future Large UV-Optical-InfraRed (LUVOIR) mission.
We investigate astrophysical contributions to the statistical uncertainty of precision radial velocity measurements of stellar spectra. We analytically determine the uncertainty in centroiding isolated spectral lines broadened by Gaussian, Lorentzian, Voigt, and rotational profiles, finding that for all cases and assuming weak lines, the uncertainty is the line centroid is $\sigma_V\approx C\,\Theta^{3/2}/(W I_0^{1/2})$, where $\Theta$ is the full-width at half-maximum of the line, $W$ is the equivalent width, and $I_0$ is the continuum signal-to-noise ratio, with $C$ a constant of order unity that depends on the specific line profile. We use this result to motivate approximate analytic expressions to the total radial velocity uncertainty for a stellar spectrum with a given photon noise, resolution, wavelength, effective temperature, surface gravity, metallicity, macroturbulence, and stellar rotation. We use these relations to determine the dominant contributions to the statistical uncertainties in precision radial velocity measurements as a function of effective temperature and mass for main-sequence stars. For stars more than $\sim1.1\,M_\odot$ we find that stellar rotation dominates the velocity uncertainties for moderate and high resolution spectra ($R\gtrsim30,000$). For less massive stars, a variety of sources contribute depending on the spectral resolution and wavelength, with photon noise due to decreasing bolometric luminosity generally becoming increasingly important for low-mass stars at fixed exposure time and distance. In most cases, resolutions greater than 60,000 provide little benefit in terms of statistical precision. We determine the optimal wavelength range for stars of various spectral types, finding that the optimal region depends on the stellar effective temperature, but for mid M-dwarfs and earlier the most efficient wavelength region is from 6000A to 9000A.
Searches for magnetic fields in white dwarfs have clarified both the frequency of occurrence and the global structure of the fields found down to field strengths of the order of 500 kG. Below this level, the situation is still very unclear. We are studying the weakest fields found in white dwarfs to determine the frequency of such fields and their structure. We describe a very sensitive new method of measuring such fields in DA (H-rich) white dwarfs, and search for a field in the brightest such star, 40 Eri B. Our new method makes use of the strongly enhanced polarisation signal in the sharp core of Halpha. We find that with one-hour integrations with the high-resolution spectropolarimeter ESPaDOnS on the 3.6-m CFHT, we can reach a standard error fo the longitudinal field of about 85 G, the smallest error ever achieved for any white dwarf. Nevertheless, we do not detect a magnetic field in this star. Observations with ISIS at the WHT, and the Main Stellar Spectrograph at the SAO, support the absence of a field at somewhat lower precision. The new method is very efficient; it is shown that for suitable DA stars the integration time, with ESPaDOnS on a 3.6-m telescope, to reach a 500 G standard error on a white dwarf of V = 12.5, is about half an hour, about the same as the time required on an ESO 8-m telescope with FORS using conventional low-resolution spectropolarimetry.
Using high-dispersion, high-quality spectra of HD 30085 obtained with the echelle spectrograph SOPHIE at Observatoire de Haute Provence, we show that this star contains strong lines of the s-process elements Sr II, Y II and Zr II. Line syntheses of the lines yield large overabundances of Sr, Y, Zr which are characteristic of HgMn stars. The Sr-Y-Zr triad of abundances is inverted in HD 30085 compared to that in our solar system. The violation of the odd-even rule suggests that physical processes such as radiative diffusion, chemical fractionation and others must be at work in the atmosphere of HD 30085, and that the atmosphere is stable enough to sustain them.
Context. Oxygen sequence Wolf-Rayet (WO) stars represent a very rare stage in the evolution of massive stars. Their spectra show strong emission lines of helium-burning products, in particular highly ionized carbon and oxygen. The properties of WO stars can be used to provide unique constraints on the (post-)helium burning evolution of massive stars, as well as their remaining lifetime and the expected properties of their supernovae. Aims. We aim to homogeneously analyse the currently known presumed-single WO stars to obtain the key stellar and outflow properties and to constrain their evolutionary state. Methods. We use the line-blanketed non-local thermal equilibrium atmosphere code cmfgen to model the X-Shooter spectra of the WO stars and deduce the atmospheric parameters. We calculate dedicated evolutionary models to determine the evolutionary state of the stars. Results. The WO stars have extremely high temperatures that range from 150 kK to 210 kK, and very low surface helium mass fractions that range from 44% down to 14%. Their properties can be reproduced by evolutionary models with helium zero-age main sequence masses of $M_{\mathrm{He, ini}} = 15-25 M_{\odot}$ that exhibit a fairly strong (on the order of a few times $10^{-5} M_{\odot} \mathrm{yr}^{-1}$), homogeneous ($f_\mathrm{c} > 0.3$) stellar wind. Conclusions. WO stars represent the final evolutionary stage of stars with estimated initial masses of $M_{\mathrm{ini}} = 40-60 M_{\odot}$. They are post core-helium burning and predicted to explode as type Ic supernovae within a few thousand years.
We consider a simple scenario for the accretion of matter onto a neutron star in order to understand processes in the inner pulsar magnetosphere during the transition stage between different accretion modes. A simple quasi-spherical accretion process onto rotating, magnetized compact object is analyzed in order to search for the radiative signatures which could appear during transition between ejecting and accreting modes. It is argued that different accretion modes can be present in a single neutron star along different magnetic field lines for specific range of parameters characterising the pulsar (rotational period, surface magnetic field strength) and the density of surrounding medium. The radiation processes characteristic for the ejecting pulsar, i.e. curvature and synchrotron radiation produced by primary electrons in the pulsar outer gap, are expected to be modified by the presence of additional thermal radiation from the neutron star surface. We predict that during the transition from the pure ejector to the pure accretor mode (or vice versa) an intermediate accretion state can be distinguished which is characterized by the $\gamma$-ray spectra of pulsars truncated below ~1 GeV due to the absorption of synchro-curvature spectrum produced in the pulsar gaps.
Aims: Our aim is to explore the broad-band radio continuum spectrum of LSI+61303 during its outbursts by employing the available set of secondary focus receivers of the Effelsberg 100 m telescope. Methods: The clear periodicity of the system LSI+61303 allowed observations to be scheduled covering the large radio outburst in March-April 2012. We observed LSI+61303 on 14 consecutive days at 2.6, 4.85, 8.35, 10.45, 14.3, 23, and 32 GHz with a cadence of about 12 hours followed by two additional observations several days later. Based on these observations we obtained a total of 24 quasi-simultaneous broad-band radio spectra. Results: During onset, the main flare shows an almost flat broad-band spectrum, most prominently seen on March 27, 2012, where - for the first time - a flat spectrum (alpha=0.00+/-0.07, S nu^alpha) is observed up to 32 GHz (9 mm wavelength). The flare decay phase shows superimposed 'sub-flares' with the spectral index oscillating between -0.4 and -0.1 in a quasi-regular fashion. Finally, the spectral index steepens during the decay phase, showing optically thin emission with values alpha $\sim$ -0.5 to -0.7. Conclusions: The radio characteristics of LSI+61303 compare well with those of the microquasars XTE J1752-223 and Cygnus X-3. In these systems the flaring phase is actually also composed of a sequence of outbursts with clearly different spectral characteristics: a first outburst with a flat/inverted spectrum followed by a bursting phase of optically thin emission.
Clusters of galaxies usually contain rich populations of globular clusters (GCs). We investigate how different star formation histories (SFHs) shape the final mass distribution of star clusters. We assume that every star cluster population forms during a formation epoch of length dt at a constant star-formation rate (SFR). The mass distribution of such a population is described by the embedded cluster mass function (ECMF), which is a pure power law extending to an upper limit M_max. Since the SFR determines M_max, the ECMF implicitly depends on the SFR. Starting with different SFHs, each SFH is divided into formation epochs of length dt at different SFRs. The requested mass function arises from the superposition of the star clusters of all formation epochs. An improved optimal sampling technique is introduced that allows generating number and mass distributions, both of which accurately agree with the ECMF. Moreover, for each SFH the distribution function of all involved SFRs, F(SFR), is computed. For monotonically decreasing SFHs, F(SFR) always follows a power law. With F(SFR), we develope the theory of the integrated galactic embedded cluster mass function (IGECMF). It describes the distribution function of birth stellar masses of star clusters that accumulated over a formation episode much longer than dt. The IGECMF indeed reproduces the mass distribution of star clusters created according to the superposition principle. Interestingly, all considered SFHs lead to a turn-down with increasing star cluster mass in their respective IGECMFs. In the past, a turn-down at the high-mass end has been observed for GC systems in different galaxy clusters and in the cluster initial mass function. This behavior can be explained naturally if the observed star cluster ensembles are superpositions of several individual star cluster populations that formed at different times at different SFRs.
A recent report on the over saturation in SiPMs is puzzling. The measurements, using a variety of SiPMs, show an excess in signal far beyond the physical limit of the number of SiPM microcells without indication of an ultimate saturation. In this work I propose a solution to this problem. Different measurements and theoretical models of avalanche propagation indicate that multiple simultaneous primary avalanches produce an ever narrower and faster signal. This is because of a speed-up of effective avalanche propagation processes. It means that SiPMs, operated at their saturation regime, should become faster the more light they detect. Therefore, signal extraction methods that use the amplitude of the signal should see an over saturation effect. Measurements with a commercial SiPM illuminated with bright picosecond pulses in the saturation regime demonstrate that indeed the rising edge of the SiPM signal gets faster as the light pulses get brighter. A signal extractor based on the amplitude shows a nonlinear behavior in comparison to an integrating charge extractor. This supports the proposed solution for the over saturation effect. Furthermore I show that this effect can already be seen with a bandwidth of 300MHz, which means that it should be taken into account for fast sampling experiments.
We study the properties of a significant thermal emission component that was identified in 47 GRBs observed by different instruments. Within the framework of the "fireball" model, we deduce the values of the Lorentz factor Gamma, and the acceleration radius, r_0, for these bursts. We find that all the values of Gamma in our sample are in the range 10^2 <= Gamma <= 10^3, with <Gamma> = 310. We find a very weak dependence of Gamma on the acceleration radius r_0, Gamma ~ r_0^alpha with alpha = -0.10 +- 0.09 at sigma = 2.1 confidence level. The values of r_0 span a wide range, 10^7 <= r_0 <= 10^{9.5} cm, with mean value <r_0>~10^{8.5} cm. This is higher than the gravitational radius of a 10 M_sun black hole by a factor ~100. We argue that this result provides indirect evidence for jet propagation inside a massive star, and suggests the existence of recollimation shocks that take place close to this radius.
Active shielding is an effective technique to reduce background signals in
hard X-ray detectors and to enable observing darker sources with high
sensitivity in space. Usually the main detector is covered with some shield
detectors made of scintillator crystals such as BGO (Bi$_4$Ge$_3$O$_{12}$), and
the background signals are filtered out using anti-coincidence among them.
Japanese X-ray observing satellites "Suzaku" and "ASTRO-H" employed this
technique in their hard X-ray instruments observing at > 10 keV.
In the next generation X-ray satellites, such as the NGHXT proposal, a single
hybrid detector is expected to cover both soft (1-10 keV) and hard (> 10 keV)
X-rays for effectiveness. However, present active shielding is not optimized
for the soft X-ray band, 1-10 keV. For example, Bi and Ge, which are contained
in BGO, have their fluorescence emission lines around 10 keV. These lines
appear in the background spectra obtained by ASTRO-H Hard X-ray Imager, which
are non-negligible in its observation energy band of 5-80 keV.
We are now optimizing the design of active shields for both soft and hard
X-rays at the same time. As a first step, we utilized a BGO crystal as a
default material, and measured the L lines of Bi and K lines of Ge from it
using the X-ray SOIPIX, "XRPIX".
We extend our earlier multidimensional, magnetohydrodynamic simulations of coronal rain occurring in magnetic arcades with higher resolution, grid-adaptive computations covering a much longer ($>6$ hour) timespan. We quantify how in-situ forming blob-like condensations grow along and across field lines and show that rain showers can occur in limit cycles, here demonstrated for the first time in 2.5D setups. We discuss dynamical, multi-dimensional aspects of the rebound shocks generated by the siphon inflows and quantify the thermodynamics of a prominence-corona-transition-region like structure surrounding the blobs. We point out the correlation between condensation rates and the cross-sectional size of loop systems where catastrophic cooling takes place. We also study the variations of the typical number density, kinetic energy and temperature while blobs descend, impact and sink into the transition region. In addition, we explain the mechanisms leading to concurrent upflows while the blobs descend. As a result, there are plenty of shear flows generated with relative velocity difference around 80 km s$^{-1}$ in our simulations. These shear flows are siphon flows set up by multiple blob dynamics and they in turn affect the deformation of the falling blobs. In particular, we show how shear flows can break apart blobs into smaller fragments, within minutes.
Our analysis of archival VLBI data of PSR 0834+06 revealed that its scintillation properties can be precisely modelled using the inclined sheet model (Pen & Levin 2014), resulting in two distinct lens planes. These data strongly favour the grazing sheet model over turbulence as the primary source of pulsar scattering. This model can reproduce the parameters of the observed diffractive scintillation with an accuracy at the percent level. Comparison with new VLBI proper motion results in a direct measure of the ionized ISM screen transverse velocity. The results are consistent with ISM velocities local to the PSR 0834+06 sight-line (through the Galaxy). The simple 1D structure of the lenses opens up the possibility of using interstellar lenses as precision probes for pulsar lens mapping, precision transverse motions in the ISM, and new opportunities for removing scattering to improve pulsar timing. We describe the parameters and observables of this double screen system. While relative screen distances can in principle be accurately determined, a global conformal distance degeneracy exists that allows a rescaling of the absolute distance scale. For PSR B0834+06, we present VLBI astrometry results that provide (for the fist time) a direct measurement of the distance of the pulsar. For targets where independent distance measurements are not available, which are the cases for most of the recycled millisecond pulsars that are the targets of precision timing observations, the degeneracy presented in the lens modelling could be broken if the pulsar resides in a binary system.
IO:I is a new instrument that has recently been commissioned for the Liverpool Telescope, extending current imaging capabilities beyond the optical and into the near infrared. Cost has been minimised by use of a previously decommissioned instrument's cryostat as the base for a prototype and retrofitting it with Teledyne's 1.7$\mu m$ cutoff Hawaii-2RG HgCdTe detector, SIDECAR ASIC controller and JADE2 interface card. In this paper, the mechanical, electronic and cryogenic aspects of the cryostat retrofitting process will be reviewed together with a description of the software/hardware setup. This is followed by a discussion of the results derived from characterisation tests, including measurements of read noise, conversion gain, full well depth and linearity. The paper closes with a brief overview of the autonomous data reduction process and the presentation of results from photometric testing conducted on on-sky, pipeline processed data.
We analyse a controlled N-body + smoothed particle hydrodynamics simulation of a growing disc galaxy within a non-growing, live dark halo. The disc is continuously fed with gas and star particles on near-circular orbits and develops a bar comparable in size to the one of the Milky Way (MW). We extract line of sight velocity v_los distributions from the model and compare it to data recently obtained from the APOGEE survey which show distinct high velocity features around v_los ~ 200 km/s. With an APOGEE like selection function, but without any scaling nor adjustment, we find v_los distributions very similar to those in APOGEE. The stars that make up the high v_los features at positive longitudes l are preferentially young bar stars (age <~ 2-3 Gyr) which move away from us along the rear side of the bar. At negative l, we find the corresponding low v_los feature from stars moving towards us. At l>10 degrees the highest v_los stars are a mixture of bar and background disc stars which complicates the interpretation of observations. The main peak in v_los is dominated by fore- and background stars. At a given time, ~40-50 per cent of high v_los stars occupy x_1 like orbits, but a significant fraction are on more complex orbits. The observed feature is likely due to a population of dynamically cool, young stars formed from gas just outside the bar and subsequently captured by the growing bar. The high v_los features disappear at high latitudes |b|>~2 degrees which explains the non-detection of such features in other surveys.
The VST-VEGAS project is aimed at observing and studying a rich sample of nearby early-type galaxies in order to systematically characterize their properties over a wide baseline of sizes and out to the faint outskirts where data are rather scarce so far. The external regions of galaxies more easily retain signatures about the formation and evolution mechanisms which shaped them, as their relaxation time are longer, and they are more weakly influenced by processes such as mergers, secular evolution, central black hole activity, and supernova feedback on the ISM, which tend to level age and metallicity gradients. The collection of a wide photometric dataset of a large number of galaxies in various environmental conditions, may help to shed light on these questions. To this end VEGAS exploits the potential of the VLT Survey Telescope (VST) which provides high quality images of one square degree field of view in order to satisfy both the requirement of high resolution data and the need of studying nearby, and thus large, objects. We present a detailed study of the surface photometry of the elliptical galaxy NGC4472 and of smaller ETGs in its field, performed by using new g and i bands images to constrain the formation history of this nearby giant galaxy, and to investigate the presence of very faint substructures in its surroundings.
In this paper we study the phenomenology of stars and galaxies in massive bigravity. We give parameter conditions for the existence of viable star solutions when the radius of the star is much smaller than the Compton wavelength of the graviton. If these parameter conditions are not met, we constrain the ratio between the coupling constants of the two metrics, in order to give viable conditions for e.g. neutron stars. For galaxies, we put constraints on both the Compton wavelength of the graviton and the conformal factor and coupling constants of the two metrics. The relationship between black holes and stars, and whether the former can be formed from the latter, is discussed. We argue that the different asymptotic structure of stars and black holes makes it unlikely that black holes form from the gravitational collapse of stars in massive bigravity.
More than a dozen binary systems are now established as sources of variable, high energy (HE, 0.1-100 GeV) gamma rays. Five are also established sources of very high energy (VHE, >100 GeV) gamma rays. The mechanisms behind gamma-ray emission in binaries are very diverse. My current understanding is that they divide up into four types of systems: gamma-ray binaries, powered by pulsar rotation; microquasars, powered by accretion onto a black hole or neutron star; novae, powered by thermonuclear runaway on a white dwarf; colliding wind binaries, powered by stellar winds from massive stars. Some of these types had long been suspected to emit gamma rays (microquasars), others have taken the community by surprise (novae). My purpose here is to provide a brief review of the current status of gamma-ray emission from binaries, in the context of related objects where similar mechanisms are at work (pulsar wind nebulae, active galactic nuclei, supernova remnants).
Effects of velocity dispersion of dark matter particles on the CMB TT power spectrum and on the matter linear power spectrum are investigated using a modified CAMB code. Cold dark matter originated from thermal equilibrium processes does not produce appreciable effects but this is not the case if particles have a non-thermal origin. A cut-off in the matter power spectrum at small scales, similar to that produced by warm dark matter or that produced in the late forming dark matter scenario, appears as a consequence of velocity dispersion effects, which acts as a pressure perturbation.
We consider massive black hole binary systems and information that can be derived about their population and formation history solely from current and possible future pulsar timing array (PTA) results. We use models of the stochastic gravitational-wave background from circular massive black hole binaries with chirp mass in the range $10^6 - 10^{11} M_\odot$ evolving solely due to radiation reaction. Our parameterised models for the black hole merger history make only weak assumptions about the properties of the black holes merging over cosmic time. We show that current PTA results place a model-independent upper limit on the merger density of massive black hole binaries, but provide no information about their redshift or mass distribution. We show that even in the case of a detection resulting from a factor of 10 increase in amplitude sensitivity, PTAs will only put weak constraints on the source merger density as a function of mass, and will not provide any additional information on the redshift distribution. Without additional assumptions or information from other observations, a detection cannot meaningfully bound the massive black hole merger rate above zero for any particular mass.
We show that maximally helical hypermagnetic fields produced during pseudoscalar inflation can generate the observed baryon asymmetry of the universe via the B+L anomaly in the Standard Model. We find that most of the parameter space of pseudoscalar inflation that explains the cosmological data leads to baryon overproduction, hence the models of natural inflation are severely constrained. We also point out a connection between the baryon number and topology of the relic magnetic fields. Both the magnitude and sign of magnetic helicity can be detected in future diffuse gamma ray data. This will be a smoking gun evidence for a link between inflation and the baryon asymmetry of the Universe.
Regular bouncing solutions in the framework of a scalar-tensor gravity model were found in a recent work. We reconsider the problem in the Einstein frame (EF) in the present work. Singularities arising at the limit of physical viability of the model in the Jordan frame (JF) are either of the Big Bang or of the Big Crunch type in the EF. As a result we obtain integrable scalar field cosmological models in general relativity (GR) with inverted double-well potentials unbounded from below which possess solutions regular in the future, tending to a de Sitter space, and starting with a Big Bang. The existence of the two fixed points for the field dynamics at late times found earlier in the JF becomes transparent in the EF.
Recently it was argued that gravity with the squire of the Ricci tensor can be stabilized by adding constraints to the theory. This was so far demonstrated for fluctuations on the Minkowski/de Sitter background. We show that the same scheme works equally well for removing Ostrogradski's ghost from fluctuations on a cosmological background in generic $f(R,R_{\mu\nu}^2,C_{\mu\nu\rho\sigma}^2)$-type theories of gravity. We also derive the general formula for the spectrum of primordial tensor perturbations from the stabilized theory.
We study the diversities in the properties of the neutron stars arising due to the different choices for the cross-coupling between various mesons which governs the density dependence of the nuclear symmetry energy in the extended relativistic mean-field(RMF) model. For this purpose, we obtain two different families of the extended RMF model corresponding to different non-linear cross-coupling term in the isovector part of the effective Lagrangian density. The lowest order contributions for the $\delta$ mesons are also included. The different models within the same family are so obtained that they yield wide variation in the value of neutron-skin thickness in the $^{208}$Pb nucleus. These models are employed to compute the neutron star properties such as, core-crust transition density, radius and red shift at canonical mass ($1.4M_{\odot}$), tidal polarizability parameter, and threshold mass required for the enhanced cooling through direct Urca process. Most of the neutron star properties considered are significantly different(10\%-40\%) for the different families of models at a smaller neutron-skin thickness ($\sim 0.15$ fm) in the $^{208}$Pb nucleus.
Matter-neutrino resonances (MNR) can occur in environments where the flux of electron antineutrinos is greater than the flux of electron neutrinos. These resonances may result in dramatic neutrino flavor transformation. Compact object merger disks are an example of an environment where electron antineutrinos outnumber neutrinos. We study MNR resonances in several such disk configurations and find two qualitatively different types of matter-neutrino resonances: a standard MNR and a symmetric MNR. We examine the transformation that occurs in each type of resonance and explore the consequences for nucleosynthesis.
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We performed high resolution radio observations of a new sample of ten BAL quasars using both the VLBA and EVN at 5 GHz. All the selected sources have balnicity indices (BI) more than 0 and radio flux densities less than 80 mJy at 1.4 GHz. They are very compact with linear sizes of the order of a few tens of parsecs and radio luminosities at 1.4 GHz above the FRI-FRII luminosity threshold. Most of the observed objects have been resolved at 5 GHz showing one-sided, probably core-jet structures, typical for quasars. We discuss in detail their age and orientation based on the radio observations. We then used the largest available sample of BAL quasars to study the relationships between the radio and optical properties in these objects. We found that (1) the strongest absorption (high values of the balnicity index BI) is connected with the lower values of the radio-loudness parameter, logR_I<1.5, and thus probably with large viewing angles; (2) the large span of the BI values in each bin of the radio-loudness parameter indicates that the orientation is only one of the factors influencing the measured absorption; (3) most of the radio-loud BAL quasars are compact, low luminosity objects with a wide range of jet power (although the highest values of BI seem to be associated with the lower values of jet power). In addition, we suggest that the short lifetime postulated for some compact AGNs could also explain the scarcity of the large-scale radio sources among BAL quasars.
Using the most recent measurements of the ultraviolet (UV) luminosity functions (LFs) and dust estimates of early galaxies, we derive updated dust-corrected star-formation rate functions (SFRFs) at z~4-8, which we model to predict the evolution to higher redshifts, z>8. We employ abundance matching techniques to calibrate a relation between galaxy star formation rate (SFR) and host halo mass M{_h} by mapping the shape of the observed SFRFs at z~4-8 to that of the halo mass function. The resulting scaling law remains roughly constant over this redshift range. We apply the average SFR-M{_h} relation to reproduce the observed SFR functions at 4 <= z <= 8 and also derive the expected UV LFs at higher redshifts. At z~9 and z~10 these model LFs are in excellent agreement with current observed estimates. Our predicted number densities and UV LFs at z>10 indicate that JWST will be able to detect galaxies out to z~15 with an extensive treasury sized program. We also derive the redshift evolution of the star formation rate density and associated reionization history by galaxies for which we find that the inclusion of galaxies with SFRs well below the current detection limit leads to a fully reionized universe by z~6.5 and an optical depth of \tau~0.054, consistent with the recently derived Planck value at the 1\sigma level.
There are by now ten published detections of fast radio bursts (FRBs), single bright GHz-band millisecond pulses of unknown origin. Proposed explanations cover a broad range from exotic processes at cosmological distances to atmospheric and terrestrial sources. Loeb et al. have previously suggested that FRB sources could be nearby flare stars, and pointed out the presence of a W-UMa-type contact binary within the beam of one out of three FRB fields that they examined. Using time-domain optical photometry and spectroscopy, we now find possible flare stars in additional FRB fields, with one to three such cases among eight FRB fields studied. We evaluate the chance probabilities of these possible associations to be in the range 0.1% to 9%, depending on the input assumptions. Further, we re-analyze the probability that two FRBs recently discovered 3 years apart within the same radio beam are unrelated. Contrary to other claims, we conclude with 99% confidence that the two events are from the same repeating source. The different dispersion measures between the two bursts then rule out a cosmological origin for the dispersion measure, but are consistent with the flare-star scenario with a varying plasma blanket between bursts. Finally, we review some theoretical objections that have been raised against a local flare-star FRB origin, and show that they are incorrect.
There has been much recent work dedicated to exploring secondary correlations in the mass-metallicity relation, with significant dependence on both the star formation rate and HI content being demonstrated. Previously, a paucity of molecular gas data (combined with sample selection bias) hampered the investigation of any such relation with molecular gas content. In this work, we assemble a sample of 221 galaxies from a variety of surveys in the redshift range 0 < z < 2, to explore the connection between molecular gas content and metallicity. We find that the integrated Kennicut-Schmidt relation forms a single (i.e., non-bimodal) sequence across ~5 decades in SFR and M(H2), when adopting any metallicity-dependent CO/H2 conversion factor. Moreover, the normalisation of the K-S relation (i.e., the star formation efficiency), does not show any significant dependence on metallicity, in contrast with previous claims. We explore the effect of gas mass on the mass-metallicity relation, finding that the offset from the relation is negatively correlated against both molecular and total gas mass. We then employ a principle component analysis technique to explore secondary dependences in the mass-metallicity relation, finding that the secondary dependence with gas mass is significantly stronger than with star formation rate, and as such the underlying `Fundamental Metallicity Relation' is between stellar mass, metallicity, and gas mass. In particular, the metallicity dependence on SFR is simply a byproduct of the dependence on the molecular gas content, via the Kennicutt-Schmidt relation.
We present a three-dimensional map of interstellar dust reddening, covering three-quarters of the sky out to a distance of several kiloparsecs, based on Pan-STARRS 1 and 2MASS photometry. The map reveals a wealth of detailed structure, from filaments to large cloud complexes. The map has a hybrid angular resolution, with most of the map at an angular resolution of 3.4' to 13.7', and a maximum distance resolution of ~25%. The three-dimensional distribution of dust is determined in a fully probabilistic framework, yielding the uncertainty in the reddening distribution along each line of sight, as well as stellar distances, reddenings and classifications for 800 million stars detected by Pan-STARRS 1. We demonstrate the consistency of our reddening estimates with those of two-dimensional emission-based maps of dust reddening. In particular, we find agreement with the Planck 353 GHz optical depth-based reddening map to within 0.05 mag in E(B-V) to a depth of 0.5 mag, and explore systematics at reddenings less than E(B-V) ~ 0.08 mag. We validate our per-star reddening estimates by comparison with reddening estimates for stars with both SDSS photometry and SEGUE spectral classifications, finding per-star agreement to within 0.1 mag out to a stellar E(B-V) of 1 mag. We compare our map to two existing three-dimensional dust maps, by Marshall et al. (2006) and Lallement et al. (2013), demonstrating our finer angular resolution, and better distance resolution compared to the former within ~3 kpc. The map can be queried or downloaded at this http URL We expect the three-dimensional reddening map presented here to find a wide range of uses, among them correcting for reddening and extinction for objects embedded in the plane of the Galaxy, studies of Galactic structure, calibration of future emission-based dust maps and determining distances to objects of known reddening.
We present a 'two-fluid' implementation of dust in smoothed particle hydrodynamics (SPH) in the test particle limit. The scheme is able to handle both short and long stopping times and reproduces the short friction time limit, which is not properly handled in other implementations. We apply novel tests to verify its accuracy and limitations, including multi-dimensional tests that have not been previously applied to the drag-coupled dust problem and which are particularly relevant to self-gravitating protoplanetary discs. Our tests demonstrate several key requirements for accurate simulations of gas-dust mixtures. Firstly, in standard SPH particle jitter can degrade the dust solution, even when the gas density is well reproduced. The use of integral gradients, a Wendland kernel and a large number of neighbours can control this, albeit at a greater computational cost. Secondly, when it is necessary to limit the artificial viscosity we recommend using the Cullen & Dehnen (2010) switch, since the alternative, using ${\alpha} \sim 0.1$, can generate a large velocity noise up to ${{\sigma}_v} \lesssim 0.3 c_s$ in the dust particles. Thirdly, we find that an accurate dust density estimate requires $>400$ neighbours, since, unlike the gas, the dust particles do not feel regularization forces. This density noise applies to all particle-based two-fluid implementations of dust, irrespective of the hydro solver and could lead to numerically induced fragmentation. Although our tests show accurate dusty gas simulations are possible, care must be taken to minimize the contribution from numerical noise.
Potentially hazardous asteroids (PHAs) represent a unique opportunity for physical characterization during their close approaches to Earth. The proximity of these asteroids makes them accessible for sample-return and manned missions, but could also represent a risk for life on Earth in the event of collision. Therefore, a detailed mineralogical analysis is a key component in planning future exploration missions and developing appropriate mitigation strategies. In this study we present near-infrared spectra (0.7-2.55 microns) of PHA (214869) 2007 PA8 obtained with the NASA Infrared Telescope Facility during its close approach to Earth on November 2012. The mineralogical analysis of this asteroid revealed a surface composition consistent with H ordinary chondrites. In particular, we found that the olivine and pyroxene chemistries of 2007 PA8 are Fa18(Fo82) and Fs16, respectively. The olivine-pyroxene abundance ratio was estimated to be 47%. This low olivine abundance and the measured band parameters, close to the H4 and H5 chondrites, suggest that the parent body of 2007 PA8 experienced thermal metamorphism before being catastrophically disrupted. Based on the compositional affinity, proximity to the J5:2 resonance, and estimated flux of resonant objects we determined that the Koronis family is the most likely source region for 2007 PA8.
We present a hybrid code combining the OpenMP-parallel tree code VINE with an algorithmic chain regularization scheme. The new code, called "rVINE", aims to significantly improve the accuracy of close encounters of massive bodies with supermassive black holes in galaxy-scale numerical simulations. We demonstrate the capabilities of the code by studying two test problems, the sinking of a single massive black hole to the centre of a gas-free galaxy due to dynamical friction and the hardening of a supermassive black hole binary due to close stellar encounters. We show that results obtained with rVINE compare well with NBODY7 for problems with particle numbers that can be simulated with NBODY7. In particular, in both NBODY7 and rVINE we find a clear N-dependence of the binary hardening rate, a low binary eccentricity and moderate eccentricity evolution, as well as the conversion of the galaxy's inner density profile from a cusp to a a core via the ejection of stars at high velocity. The much larger number of particles that can be handled by rVINE will open up exciting opportunities to model stellar dynamics close to SMBHs much more accurately in a realistic galactic context. This will help to remedy the inherent limitations of commonly used tree solvers to follow the correct dynamical evolution of black holes in galaxy scale simulations.
We present results of high-cadence monitoring of the optical light curve of the nearby, Type Ia SN 2014J in M82 using the 2.3m Aristarchos telescope. $B$ and $V$-band photometry on days 15-18 after $t_{max}(B)$, obtained with a cadence of 2 min per band, reveals evidence for variability at the 0.02-0.05 mag level on timescales of 15-60 min on all four nights. The decline slope was measured to be steeper in the $B$-band than in $V$-band, and to steadily decrease in both bands from 0.15 mag/day (night 1) to 0.04 mag/day (night 4) in V and from 0.19 mag/day (night 1) to 0.06 mag/day (night 4) in B, corresponding to the onset of the secondary maximum. We propose that microvariability could be due to one or a combination of the following scenarios: the clumpiness of the ejecta, their interaction with circumstellar material, the asymmetry of the explosion, or the mechanism causing the secondary maximum in the near-infrared light curve. We encourage the community to undertake high-cadence monitoring of future, nearby and bright supernovae to investigate the intraday behavior of their light curves.
The early acceleration of stellar winds in massive stars is poorly
constrained. The scattering of hard X-ray photons emitted by the pulsar in the
high-mass X-ray binary Vela X-1 can be used to probe the stellar wind velocity
and density profile close to the surface of its supergiant companion HD 77581.
We built a high signal-to-noise and high resolution hard X-ray lightcurve of
Vela X-1 measured by Swift/BAT over 300 orbital periods of the system and
compared it with the predictions of a grid of hydrodynamic simulations.
We obtain a very good agreement between observations and simulations for a
narrow set of parameters, implying that the wind velocity close to the stellar
surface is twice larger than usually assumed with the standard beta law.
Locally a velocity gradient of $\beta\sim0.5$ is favoured. Even if still
incomplete, hydrodynamic simulations are successfully reproducing several
observational properties of Vela X-1.
We propose a novel theory of dark matter (DM) superfluidity that matches the successes of the LambdaCDM model on cosmological scales while simultaneously reproducing the MOdified Newtonian Dynamics (MOND) phenomenology on galactic scales. The DM and MOND components have a common origin, representing different phases of a single underlying substance. DM consists of axion-like particles with mass of order eV and strong self-interactions. The condensate has a polytropic equation of state P~rho^3 giving rise to a superfluid core within galaxies. Instead of behaving as individual collisionless particles, the DM superfluid is more aptly described as collective excitations. Superfluid phonons, in particular, are assumed to be governed by a MOND-like effective action and mediate a MONDian acceleration between baryonic matter particles. Our framework naturally distinguishes between galaxies (where MOND is successful) and galaxy clusters (where MOND is not): due to the higher velocity dispersion in clusters, and correspondingly higher temperature, the DM in clusters is either in a mixture of superfluid and normal phase, or fully in the normal phase. The rich and well-studied physics of superfluidity leads to a number of observational signatures: array of low-density vortices in galaxies, merger dynamics that depend on the infall velocity vs phonon sound speed; distinct mass peaks in bullet-like cluster mergers, corresponding to superfluid and normal components; interference patters in super-critical mergers. Remarkably, the superfluid phonon effective theory is strikingly similar to that of the unitary Fermi gas, which has attracted much excitement in the cold atom community in recent years. The critical temperature for DM superfluidity is of order mK, comparable to known cold atom Bose-Einstein condensates.
We report the discovery by the HATSouth network of HATS-7b, a transiting Super-Neptune with a mass of 0.120+/-0.012 M_Jup, a radius of 0.563+0.046-0.034 R_Jup, and an orbital period of 3.1853 days. The host star is a moderately bright (V = 13.340+/-0.010 mag, K_S = 10.976+/-0.026 mag) K dwarf star with a mass of 0.849+/-0.027 M_Sun, a radius of 0.815+0.049-0.035 R_Sun, and a metallicity of [Fe/H]= +0.250+/-0.080. The star is photometrically quiet to within the precision of the HATSouth measurements, has low RV jitter, and shows no evidence for chromospheric activity in its spectrum. HATS-7b is the second smallest radius planet discovered by a wide-field ground-based transit survey, and one of only a handful of Neptune-size planets with mass and radius determined to 10% precision. Theoretical modeling of HATS-7b yields a hydrogen-helium fraction of 18+/-4% (rock-iron core and H2-He envelope), or 9+/-4% (ice core and H2-He envelope), i.e.it has a composition broadly similar to that of Uranus and Neptune, and very different from that of Saturn, which has 75% of its mass in H2-He. Based on a sample of transiting exoplanets with accurately (<20%) determined parameters, we establish approximate power-law relations for the envelopes of the mass-density distribution of exoplanets. HATS-7b, which, together with the recently discovered HATS-8b, is one of the first two transiting Neptunes discovered in the Southern sky, is a prime target for additional follow-up observations with southern hemisphere facilities to characterize the atmospheres of super-Neptunes.
We develop a numerical tool for the fast computation of the temperature and polarization power spectra generated by domain wall networks, by extending the publicly available CMBACT code --- that calculates the CMB signatures generated by active sources --- to also describe domain wall networks. In order to achieve this, we adapt the Unconnected Segment model for cosmic strings to also describe domain wall networks, and use it to model the energy-momentum of domain wall networks throughout their cosmological history. We use this new tool to compute and study the TT, EE, TE and BB power spectra generated by standard domain wall networks, and derive a conservative constraint on the energy scale of the domain wall-forming phase transition of $\upeta <0.92\,\,{\rm MeV}$ (which is a slight improvement over the original Zel'dovich bound of $1\,\,{\rm MeV}$).
Context. Observations of the effect of microlensing in gravitationally lensed
quasars could potentially be used to study the structure of the source on
distance scales down to the size of the supermassive black hole powering the
quasar activity.
Aims. We search for the microlensing effect in the gamma-ray band using the
signal from a gravitationally lensed blazar B0218+357.
Methonds. We develop a method of deconvolution of contributions of two images
of the source into the gamma-ray band flaring lightcurve. We use this method to
study the evolution of the magnification factor ratio between the two images
throughout the flaring episodes. We interpret the time variability of the ratio
as a signature of the microlensing effect and derive constraints on the
physical parameters of the gamma-ray source by comparing the observed
variability properties of the magnification factor ratio with those derived
from numerical simulations of the microlensing caustics networks.
Results. We find that the magnification factor ratio has experienced a change
characteristic for a microlensing caustic crossing event during a 100 d flaring
period in 2012. It has further changed between 2012 and a recent flaring
episode in 2014. We use the measurement of the maximal magnification and
duration of the caustic crossing event to derive an estimate of the projected
size of the gamma-ray emission region in B0218+357, $R_\gamma \sim 10^{14}$ cm.
This estimate is compatible with a complementary estimate found from the
minimal variability time scale. The microlensing / minimal variability time
scale measurements of the source size suggest that the gamma-ray emission is
produced at the base of the blazar jet, in the direct vicinity of the central
supermassive black hole.
The (magnetic) chemically peculiar (CP) stars of the upper main sequence are well-suited laboratories for investigating the influence of magnetic fields on the stellar surface because they produce abundance inhomogeneities (spots), which results in photometric variability that is explained in terms of the oblique rotator model. CP stars exhibiting this phenomenon are normally classified as alpha2 Canum Venaticorum (ACV) variables. It is important to increase the sample of known rotational periods among CP stars by discovering new ACV variables. The ASAS-3 data were cross-correlated with the Catalogue of Ap, HgMn, and Am stars in order to analyse the light curves of bona fide CP and related stars. The light curves were downloaded and cleaned of outliers and data points with a flag indicating bad quality. Promising candidates showing a larger scatter than observed for constant stars in the corresponding magnitude range were searched for periodic signals using a standard Fourier technique. In total, we found 323 variables, from which 246 are reported here for the first time, and 77 were probably wrongly classified before. The observed variability pattern of most stars is in accordance with an ACV classification. For some cases, it is difficult to distinguish between the light curves of double-waved ACVs and the variability induced by orbital motion (ellipsoidal variables/eclipsing variables), especially for objects exhibiting very small amplitudes and/or significant scatter in their light curves. Thus, some eclipsing or rotating ellipsoidal variables might be present. However, we are confident that the given periods are the correct ones. There seems to be a possible weak correlation between the rotational period and colour, in the sense that cooler magnetic CP stars rotate more slowly. However, this correlation seems to disappear when correcting for the interstellar reddening.
Waldmeier in 1947 introduced a weighting (on a scale from 1 to 5) of the sunspot count made at Zurich and its auxiliary station Locarno, whereby larger spots were counted more than once. This counting method inflates the relative sunspot number over that which corresponds to the scale set by Wolfer and Brunner. Svalgaard re-counted some 60,000 sunspots on drawings from the reference station Locarno and determined that the number of sunspots reported were 'over counted' by 44% on average, leading to an inflation (measured by a weight factor) in excess of 1.2 for high solar activity. In a double-blind parallel counting by the Locarno observer Cagnotti, we determined that Svalgaard's count closely matches that of Cagnotti's, allowing us to determine the daily weight factor since 2003 (and sporadically before). We find that a simple empirical equation fits the observed weight factors well, and use that fit to estimate the weight factor for each month back to the introduction of weighting in 1947 and thus to be able to correct for the over-count and to reduce sunspot counting without weighting to the Wolfer method in use from 1893 onwards.
We describe the political and technical complications encountered during the astronomical CosmoGrid project. CosmoGrid is a numerical study on the formation of large scale structure in the universe. The simulations are challenging due to the enormous dynamic range in spatial and temporal coordinates, as well as the enormous computer resources required. In CosmoGrid we dealt with the computational requirements by connecting up to four supercomputers via an optical network and make them operate as a single machine. This was challenging, if only for the fact that the supercomputers of our choice are separated by half the planet, as three of them are located scattered across Europe and fourth one is in Tokyo. The co-scheduling of multiple computers and the 'gridification' of the code enabled us to achieve an efficiency of up to $93\%$ for this distributed intercontinental supercomputer. In this work, we find that high-performance computing on a grid can be done much more effectively if the sites involved are willing to be flexible about their user policies, and that having facilities to provide such flexibility could be key to strengthening the position of the HPC community in an increasingly Cloud-dominated computing landscape. Given that smaller computer clusters owned by research groups or university departments usually have flexible user policies, we argue that it could be easier to instead realize distributed supercomputing by combining tens, hundreds or even thousands of these resources.
The rotation rates in the deep interior and at the surface of 22 main-sequence stars with masses between $1.0$ and $1.6\,{\rm M}_{\odot}$ are constrained by combining asteroseismological analysis with spectroscopic measurements. The asteroseismic data of each star are taken by the {\it Kepler} or CoRoT space mission. It is found that the difference between the surface rotation rate and the average rotation rate (excluding the convective core) of most of stars is small enough to suggest that an efficient process of angular momentum transport operates during and/or before the main-sequence stage of stars. If each of the surface convective zone and the underlying radiative zone, for individual stars, is assumed to rotate uniformly, the difference in the rotation rate between the two zones turns out to be no more than a factor of two in most of the stars independently of their ages.
We combine a deep 0.5~deg$^2$, 1.4~GHz deep radio survey in the Lockman Hole with infrared and optical data in the same field, including the SERVS and UKIDSS near-infrared surveys, to make the largest study to date of the host galaxies of radio sources with typical radio flux densities $\sim 50 \;\mu$Jy. 87% (1274/1467) of radio sources have identifications in SERVS to $AB\approx 23.1$ at 3.6 or 4.5$\mu$m, and 9% are blended with bright objects (mostly stars), leaving only 4% (59 objects) which are too faint to confidently identify in the near-infrared. We are able to estimate photometric redshifts for 68% of the radio sources. We use mid-infrared diagnostics to show that the source population consists of a mixture of star forming galaxies, rapidly accreting (cold mode) AGN and low accretion rate, hot mode AGN, with neither AGN nor starforming galaxies clearly dominating. We see the breakdown in the $K-z$ relation in faint radio source samples, and show that it is due to radio source populations becoming dominated by sources with radio luminosities $\sim 10^{23}\;{\rm WHz^{-1}}$. At these luminosities, both the star forming galaxies and the cold mode AGN have hosts with stellar luminosities about a factor of two lower than those of hot mode AGN, which continue to reside in only the most massive hosts. We show that out to at least $z\sim 2$, galaxies with stellar masses $>10^{11.5}\, M_{\odot}$ have a radio-loud fraction up to $\sim 30$%. This is consistent with there being a sufficient number of radio sources that radio-mode feedback could play a role in galaxy evolution.
NOAA Active Region (AR) 11429 was the source of twin super-fast Coronal Mass Ejections (CMEs). The CMEs took place within a hour from each other, with the onset of the first taking place in the beginning of March 7, 2012. This AR fulfills all the requirements for a "super active region"; namely, Hale's law incompatibility and a $\delta$-spot magnetic configuration. One of the biggest storms of Solar Cycle 24 to date ($D_{st}=-143$ nT) was associated with one of these events. Magnetic Flux Ropes (MFRs) are twisted magnetic structures in the corona, best seen in $\sim$10 MK hot plasma emission and are often considered the core of erupting structures. However, their "dormant" existence in the solar atmosphere (i.e. prior to eruptions), is an open question. Aided by multi-wavelength observations (SDO/HMI/AIA and STEREO EUVI B) and a Non-Linear Force-Free (NLFFF) model for the coronal magnetic field, our work uncovers two separate, weakly-twisted magnetic flux systems which suggest the existence of pre-eruption MFRs that eventually became the seeds of the two CMEs. The MFRs could have been formed during confined (i.e. not leading to major CMEs) flaring and sub-flaring events which took place the day before the two CMEs in the host AR 11429.
We compare first order (refractive) ionospheric effects seen by the Murchison Widefield Array (MWA) with the ionosphere as inferred from Global Positioning System (GPS) data. The first order ionosphere manifests itself as a bulk position shift of the observed sources across an MWA field of view. These effects can be computed from global ionosphere maps provided by GPS analysis centres, namely the Center for Orbit Determination in Europe (CODE), using data from globally distributed GPS receivers. However, for the more accurate local ionosphere estimates required for precision radio astronomy applications, data from local GPS networks needs to be incorporated into ionospheric modelling. For GPS observations, the ionospheric parameters are biased by GPS receiver instrument delays, among other effects, also known as receiver Differential Code Biases (DCBs). The receiver DCBs need to be estimated for any non-CODE GPS station used for ionosphere modelling, a requirement for establishing dense GPS networks in arbitrary locations in the vicinity of the MWA. In this work, single GPS station-based ionospheric modelling is performed at a time resolution of 10 minutes. Also the receiver DCBs are estimated for selected Geoscience Australia (GA) GPS receivers, located at Murchison Radio Observatory (MRO1), Yarragadee (YAR3), Mount Magnet (MTMA) and Wiluna (WILU). The ionospheric gradients estimated from GPS are compared with the ionospheric gradients inferred from radio source position shifts observed with the MWA. The ionospheric gradients at all the GPS stations show a correlation with the gradients observed with the MWA. The ionosphere estimates obtained using GPS measurements show promise in terms of providing calibration information for the MWA.
We study the total density distribution in the central regions ($<\, 1$ effective radius, $R_{\rm e}$) of early-type galaxies (ETGs), using data from the SPIDER survey. We model each galaxy with two components (dark matter halo + stars), exploring different assumptions for the dark matter (DM) halo profile, and leaving stellar mass-to-light ($M_{\rm \star}/L$) ratios as free fitting parameters to the data. For a Navarro et al. (1996) profile, the slope of the total mass profile is non-universal. For the most massive and largest ETGs, the profile is isothermal in the central regions ($\sim R_{\rm e}/2$), while for the low-mass and smallest systems, the profile is steeper than isothermal, with slopes similar to those for a constant-M/L profile. For a concentration-mass relation steeper than that expected from simulations, the correlation of density slope with mass tends to flatten. Our results clearly point to a "non-homology" in the total mass distribution of ETGs, which simulations of galaxy formation suggest may be related to a varying role of dissipation with galaxy mass.
The Kilo Degree survey (KiDS) is a large-scale optical imaging survey carried out with the VLT Survey Telescope (VST), which is the ideal tool for galaxy evolution studies. We expect to observe millions of galaxies for which we extract the structural parameters in four wavebands (u, g, r and i). This sample will represent the largest dataset with measured structural parameters up to a redshift $z=0.5$. In this paper we will introduce the sample, and describe the 2D fitting procedure using the 2DPHOT environment and the validation of the parameters with an external catalog.
Many applications in Solar physics need a 1D atmospheric model as initial condition or as reference for inversions of observational data. The VAL atmospheric models are based on observations and are widely used since decades. Complementary to that, the FAL models implement radiative hydrodynamics and showed the shortcomings of the VAL models since almost equally long time. In this work, we present a new 1D layered atmosphere that spans not only from the photosphere to the transition region, but from the solar interior up to far in the corona. We also discuss typical mistakes that are done when switching on simulations based on such an initial condition and show how the initial condition can be equilibrated so that a simulation can start smoothly. The 1D atmosphere we present here served well as initial condition for HD and MHD simulations and should also be considered as reference data for solving inverse problems.
Total solar irradiance and UV spectral solar irradiance have been monitored since 1978 through a succession of space missions. This is accompanied by the development of models aimed at replicating solar irradiance by relating the variability to solar magnetic activity. The NRLSSI and SATIRE-S models provide the most comprehensive reconstructions of total and spectral solar irradiance over the period of satellite observation currently available. There is persistent controversy between the various measurements and models in terms of the wavelength dependence of the variation over the solar cycle, with repercussions on our understanding of the influence of UV solar irradiance variability on the stratosphere. We review the measurement and modelling of UV solar irradiance variability over the period of satellite observation. The SATIRE-S reconstruction is consistent with spectral solar irradiance observations where they are reliable. It is also supported by an independent, empirical reconstruction of UV spectral solar irradiance based on UARS/SUSIM measurements from an earlier study. The weaker solar cycle variability produced by NRLSSI between 300 and 400 nm is not evident in any available record. We show that although the method employed to construct NRLSSI is principally sound, reconstructed solar cycle variability is detrimentally affected by the uncertainty in the SSI observations it draws upon in the derivation. Based on our findings, we recommend, when choosing between the two models, the use of SATIRE-S for climate studies.
We have performed mid-infrared imaging of Barnard's Star, one of the nearest stars to the Sun, using CanariCam on the 10.4 m Gran Telescopio Canarias. We aim to investigate an area within 1-10 arcsec separations, which for the 1.83 pc distance of the star translates to projected orbital separations of 1.8-18 AU (P > 12 yr), which have not been explored yet with astrometry or radial velocity programs. It is therefore an opportunity to enter the domain of distances where most giant planets are expected to form. We performed deep imaging in the N-band window (Si-2 filter, 8.7 {\mu}m) reaching a 3{\sigma} detection limit of 0.85+/-0.18 mJy and angular resolution of 0.24 arcsec, close to the diffraction limit of the telescope at this wavelength. A total of 80 min on-source integration time data were collected and combined for the deepest image. We achieved a dynamical range of 8.0+/-0.1 mag in the 8.7 {\mu}m band, at angular separations from ~2 to 10 arcsec and of ~6-8 mag at 1-2 arcsec. No additional sources were found. Our detectability limits provide further constraints to the presence of substellar companions of the Barnard's Star. According to solar metallicity evolutionary models, we can exclude companions of masses larger than 15 MJup (Teff > 400 K), ages of a few Gyr, and located in ~3.6-18 AU orbits with a 3{\sigma} confidence level. This minimum mass is approximately 5 MJup smaller than any previous imaging survey that explored the surroundings of Barnard's Star could restrict.
Persistent plasma upflows were observed with Hinode's EUV Imaging Spectrometer (EIS) at the edges of active region (AR) 10978 as it crossed the solar disk. We analyze the evolution of the photospheric magnetic and velocity fields of the AR, model its coronal magnetic field, and compute the location of magnetic null-points and quasi-sepratrix layers (QSLs) searching for the origin of EIS upflows. Magnetic reconnection at the computed null points cannot explain all of the observed EIS upflow regions. However, EIS upflows and QSLs are found to evolve in parallel, both temporarily and spatially. Sections of two sets of QSLs, called outer and inner, are found associated to EIS upflow streams having different characteristics. The reconnection process in the outer QSLs is forced by a large-scale photospheric flow pattern which is present in the AR for several days. We propose a scenario in which upflows are observed provided a large enough asymmetry in plasma pressure exists between the pre-reconnection loops and for as long as a photospheric forcing is at work. A similar mechanism operates in the inner QSLs, in this case, it is forced by the emergence and evolution of the bipoles between the two main AR polarities. Our findings provide strong support to the results from previous individual case studies investigating the role of magnetic reconnection at QSLs as the origin of the upflowing plasma. Furthermore, we propose that persistent reconnection along QSLs does not only drive the EIS upflows, but it is also responsible for a continuous metric radio noise-storm observed in AR 10978 along its disk transit by the Nan\c{c}ay Radio Heliograph.
Comprehensive vibration-rotation line lists for eight isotopologues of carbon monosulphide (CS) ($^{12}$C$^{32}$S, $^{12}$C$^{33}$S, $^{12}$C$^{34}$S, $^{12}$C$^{36}$S, $^{13}$C$^{32}$S, $^{13}$C$^{33}$S, $^{13}$C$^{34}$S, $^{13}$C$^{36}$S) in their ground electronic states are calculated. These line lists are suitable for temperatures up to 3000 K. A spectroscopically-determined potential energy curve (PEC) and dipole moment curve (DMC) are taken from literature. This PEC is adapted to suit our method prior to the computation of ro-vibrational energies. The calculated energies are then substituted by experimental energies, where available, to improve the accuracy of the line lists. The {\it ab initio} DMC is used without refinement to generate Einstein A coefficients. Full line lists of vibration-rotation transitions and partition functions are made available in an electronic form as supporting information to this paper and at \url{www.exomol.com}.
Machine learning techniques offer a plethora of opportunities in tackling big data within the astronomical community. We present the set of Generalized Linear Models as a fast alternative for determining photometric redshifts of galaxies, a set of tools not commonly applied within astronomy, despite being widely used in other professions. With this technique, we achieve catastrophic outlier rates of the order of ~1%, that can be achieved in a matter of seconds on large datasets of size ~1,000,000. To make these techniques easily accessible to the astronomical community, we developed a set of libraries and tools that are publicly available.
Within the framework of the external shock model of gamma-ray bursts (GRBs) afterglows, we perform a morphological analysis of the early optical lightcurves to directly constrain model parameters. We define four morphological types, i.e. the reverse shock dominated cases with/without the emergence of the forward shock peak (Type I/ Type II), and the forward shock dominated cases without/with $\nu_m$ crossing the band (Type III/IV). We systematically investigate all the Swift GRBs that have optical detection earlier than 500 s and find 3/63 Type I bursts (4.8%), 12/63 Type II bursts (19.0%), 30/63 Type III bursts (47.6%), 8/63 Type IV bursts (12.7%) and 10/63 Type III/IV bursts (15.9%). We perform Monte Carlo simulations to constrain model parameters in order to reproduce the observations. We find that the favored value of the magnetic equipartition parameter in the forward shock ($\epsilon_B^f$) ranges from $10^{-6}$ to $10^{-2}$, and the reverse-to-forward ratio of $\epsilon_B$ ($R_B$) is about 100. The preferred electron equipartition parameter $\epsilon_e^{r,f}$ value is 0.01, which is smaller than the commonly assumed value, e.g., 0.1. This could mitigate the so- called "efficiency problem" for the internal shock model, if $\epsilon_e$ during the prompt emission phase (in the internal shocks) is large (say, $\sim 0.1$). The preferred $R_B$ value is in agreement with the results in previous works that indicates a moderately magnetized baryonic jet for GRBs.
In this paper, we examine the spacial distribution of gamma-ray bursts (GRBs) using a sample of 373 objects. We subdivide the GRB data into two redshift intervals over the redshift range $0<z< 6.7$. We measure the two-point correlation function (2PCF), $\xi(r)$ of the GRBs. In determining the separation distance of the GRB pairs, we consider two representative cosmological models: a cold dark matter universe plus a cosmological constant $\Lambda$, with $(\Omega_{{\rm m}}, \Omega_{{\rm \Lambda}})=(0.28,0.72)$ and an Einstein-de Sitter (EdS) universe, with $(\Omega_{{\rm m}}, \Omega_{{\rm \Lambda}})=(1,0)$. We find a $z$-decreasing correlation of the GRB distribution, which is in agreement with the predictions of the current structure formation theory. We fit a power-law model $\xi(r)=(r/r_0)^{-\gamma}$ to the measured $\xi(r)$ and obtain an amplitude and slope of $r_0= 1235.2 \pm 342.6~h^{-1}$ Mpc and $\gamma = 0.80\pm 0.19 $ ($1\sigma$ confidence level) over the scales $r=200$ to $10^4~h^{-1}$ Mpc. Our result provide a supplement to the measurement of matter correlation on large scales, while the matter distribution below $200~h^{-1}$ Mpc is usually described by the correlation function of galaxies.
In a previous paper we have investigated the molecular environment towards the eastern border of the SNR G18.8+0.3. Continuing with the study of the surroundings of this SNR, in this work we focus on its southern border, which in the radio continuum emission shows a very peculiar morphology with a corrugated corner and a very flattened southern flank. We observed two regions towards the south of SNR G18.8+0.3 using the Atacama Submillimeter Telescope Experiment (ASTE) in the 12CO J=3-2. One of these regions was also surveyed in 13CO and C18O J=3-2. The angular and spectral resolution of these observations were 22", and 0.11 km/s. We compared the CO emission to 20 cm radio continuum maps obtain as part of the Multi-Array Galactic Plane Imaging Survey (MAGPIS) and 870 um dust emission extracted from the APEX Telescope Large Area Survey of the Galaxy. We discovered a molecular feature with a good morphological correspondence with the SNR's southernmost corner. In particular, there are indentations in the radio continuum map that are complemented by protrusions in the molecular CO image, strongly suggesting that the SNR shock is interacting with a molecular cloud. Towards this region we found that the 12CO peak is not correlated with the observed 13CO peaks, which are likely related to a nearby \hii~region. Regarding the most flattened border of SNR G18.8+0.3, where an interaction of the SNR with dense material was previously suggested, our 12CO J=3-2 map show no obvious indication that this is occurring.
Redshifted 21cm signal is a promising tool to investigate the state of intergalactic medium (IGM) in the Cosmic Dawn (CD) and Epoch of Reionization(EoR). In our previous work (Shimabukuro et al 2015), we studied the variance and skewness to give a clear interpretation of 21cm power spectrum and found that skewness is a good indicator of the epoch when X-ray heating becomes effective. Thus, the non-Gaussian feature of the spatial distribution of the 21cm signal is expected to be useful to investigate the astrophysical effects in the CD and EoR. In this paper, in order to investigate such a non-Gaussian feature in more detail, we focus on the bispectrum of the 21cm signal. It is expected that the 21cm brightness temperature bispectrum is produced by non-gaussianity due to the various astrophysical effects such as Wouthysen-Field (WF) effect, X-ray heating and reionization. We study the various properties of 21cm bispectrum such as scale dependence, shape dependence and redshift evolution. And also we study the contribution from each component of 21cm bispectrum. We find that the contribution from each component has characteristic scale-dependent feature, and it is expected that we could obtain more detailed information on the IGM in the CD and EoR by using the 21cm bispectrum in the future experiments, combined with the power spectrum and skewness.
We present the VST Early-type GAlaxy Survey (VEGAS), which is designed to
obtain deep multiband photometry in g, r, i, of about one hundred nearby
galaxies down to 27.3, 26.8, and 26 mag/arcsec^2 respectively, using the ESO
facility VST/OmegaCAM.} The goals of the survey are 1) to map the light
distribution up to ten effective radii, r_e, 2) to trace color gradients and
surface brightness fluctuation gradients out to a few r_e for stellar
population characterization, and 3) to obtain a full census of the satellite
systems (globular clusters and dwarf galaxies) out to 20% of the galaxy virial
radius. The external regions of galaxies retain signatures of the formation and
evolution mechanisms that shaped them, and the study of nearby objects enables
a detailed analysis of their morphology and interaction features. To clarify
the complex variety of formation mechanisms of early-type galaxies (ETGs), wide
and deep photometry is the primary observational step, which at the moment has
been pursued with only a few dedicated programs. The VEGAS survey has been
designated to provide these data for a volume-limited sample with exceptional
image quality. In this commissioning photometric paper we illustrate the
capabilities of the survey using g- and i-band VST/OmegaCAM images of the
nearby galaxy NGC 4472 and of smaller ETGs in the surrounding field. Our
surface brightness profiles reach rather faint levels and agree excellently
well with previous literature. Genuine new results concern the detection of an
intracluster light tail in NGC 4472 and of various substructures at increasing
scales. We have also produced extended (g-i) color profiles.
The VST/OmegaCAM data that we acquire in the context of the VEGAS survey
provide an unprecedented view of substructures in the optical emission from
extended galaxies, which can be as faint as a hundred times below the sky
level.
Context. The chemical composition of planets is an important constraint for
planet formation and subsequent differentiation. While theoretical studies try
to derive the compositions of planets from planet formation models in order to
link the composition and formation process of planets, other studies assume
that the elemental ratios in the formed planet and in the host star are the
same.
Aims. Using a chemical model combined with a planet formation model, we aim
to link the composition of stars with solar mass and luminosity with the
composition of the hosted planets. For this purpose, we study the three most
important elemental ratios that control the internal structure of a planet:
Fe/Si, Mg/Si, and C/O.
Methods. A set of 18 different observed stellar compositions was used to
cover a wide range of these elemental ratios. The Gibbs energy minimization
assumption was used to derive the composition of planets, taking stellar
abundances as proxies for nebular abundances, and to generate planets in a
self-consistent planet formation model. We computed the elemental ratios Fe/Si,
Mg/Si and C/O in three types of planets (rocky, icy, and giant planets) formed
in different protoplanetary discs, and compared them to stellar abundances.
Results. We show that the elemental ratios Mg/Si and Fe/Si in planets are
essentially identical to those in the star. Some deviations are shown for
planets that formed in specific regions of the disc, but the relationship
remains valid within the ranges encompassed in our study. The C/O ratio shows
only a very weak dependence on the stellar value.
Context. In the Milky Way, most globular clusters are highly conspicuous
objects that were found centuries ago. However, a few dozen of them are faint,
sparsely populated systems identified largely during the second half of the
past century. One of the faintest is ESO 37-1 (E 3) and as such it remains
poorly studied, with no spectroscopic observations published so far, although
it was discovered in 1976.
Aims. We investigate the globular cluster E 3 in an attempt to better
constrain its fundamental parameters. Spectroscopy of stars in the field of E 3
is shown here for the first time.
Methods. Deep, precise VI CCD photometry of E 3 down to V=26 mag is presented
and analyzed. Low resolution, medium signal-to-noise ratio spectra of 9
candidate members are studied to derive radial velocity and metallicity. Proper
motions from the UCAC4 catalogue are used to explore the kinematics of the
bright members of E 3.
Results. Isochrone fitting indicates that E 3 is probably very old, with an
age of about 13 Gyr; its distance from the Sun is nearly 10 kpc. It is also
somewhat metal rich with [Fe/H]=-0.7. As for its kinematics, our tentative
estimate for the proper motions is (-7.0+/-0.8, 3.5+/-0.3) mas/yr (or a
tangential velocity of 382+/-79 km/s) and for the radial velocity is 45+/-5
km/s, in the solar rest frame.
Conclusions. E 3 is one of the most intriguing globular clusters in the
Galaxy. Having an old age and being metal rich is clearly a peculiar
combination, only seen in a handful of objects like the far more conspicuous
NGC 104 (47 Tucanae). In addition, its low luminosity and sparse population
make it a unique template to study the final evolutionary phases in the life of
a star cluster. Unfortunately, E 3 is among the most elusive and challenging
known globular clusters because field contamination severely hampers
spectroscopic studies.
Line emission from dark matter is well motivated for some candidates e.g. sterile neutrinos. We present the first search for dark matter line emission in the 3-80keV range in a pointed observation of the Bullet Cluster with NuSTAR. We do not detect any significant line emission and instead we derive upper limits (95% CL) on the flux, and interpret these constraints in the context of sterile neutrinos and more generic dark matter candidates. NuSTAR does not have the sensitivity to constrain the recently claimed line detection at 3.5keV, but improves on the constraints for energies of 10-25keV.
Gas in the Milky Way is driven inwards by its bar, some of it settling into a disk extending to Galactocentric radius $\sim 1.4 \kpc$. The stellar distribution in this region has not been well understood because of stellar crowding and high extinction. Here we use a high resolution simulation of a barred galaxy, which crucially includes gas and star formation, to guide our interpretation of the Apache Point Observatory Galactic Evolution Experiment (APOGEE) stellar velocity data for the inner Milky Way. We show that the data favor the presence of a thin, rapidly-rotating, nuclear disk extending to $\sim 1 \kpc$. This is the first detection of a nuclear stellar disk in the Milky Way.
Most of giant molecular clouds (GMCs) in M 33 are connected with spiral-like gaseous arms (filaments) with the exception of the inner 2 kpc region where the link between the arms and GMCs disappears (see Tosaki et al. 2011). We check whether it may be caused by the dynamic friction retarding the clouds. Using semi-analytical model for this galaxy we calculate the dynamics of GMCs of different masses situated at different initial galactocentric distances in the disk plane. We demonstrate that the dynamical friction may really change the orbits of GMCs in the central 2 kpc-size region. However in this case the typical lifetimes of GMCs should be close to or greater than $10^8$~yr, which is larger than the usually accepted values.
Recent high-resolution and high-cadence observations have surprisingly suggested that prominence barbs exhibit apparent rotating motions suggestive of a tornado-like structure. Additional evidence has been provided by Doppler measurements. The observations reveal opposite velocities for both hot and cool plasma on the two sides of a prominence barb. This motion is persistent for several hours and has been interpreted in terms of rotational motion of prominence feet. Several authors suggest that such barb motions are rotating helical structures around a vertical axis similar to tornadoes on Earth. One of the difficulties of such a proposal is how to support cool prominence plasma in almost-vertical structures against gravity. In this work we model analytically a tornado-like structure and try to determine possible mechanisms to support the prominence plasma. We have found that the Lorentz force can indeed support the barb plasma provided the magnetic structure is sufficiently twisted and/or significant poloidal flows are present.
We combine the Siding Spring Survey of RR Lyrae stars with the Southern Proper Motion Catalog 4, in order to detect and kinematically characterize overdensities in the inner halo of the Milky Way. We identify one such overdensity above the Galactic plane, in quadrant 4 of the Galaxy. The overdensity extends at least 20 degrees in longitude, has an average heliocentric distance of 8 kpc with a depth of 4 kpc, and is confined within 4 kpc of the Galactic plane. Its metallicity distribution is distinct from that of the field population having a peak at -1.3 and a pronounced tail to -2.0. Proper motions indicate a net vertical motion away from the plane, and a low orbital angular momentum. Qualitatively, these orbit properties suggest a possible association with omega Centauri's parent satellite. However, comparison to a specific omega Cen N-body disruption model does not give a good match with observations. Line-of-sight velocities, and more extensive N-body modelling will help clarify the nature of this overdensity.
We use the shear catalog from the CFHT Stripe-82 Survey to measure the subhalo masses of satellite galaxies in redMaPPer clusters. Assuming a Chabrier Initital Mass Function (IMF) and a truncated NFW model for the subhalo mass distribution, we find that the sub-halo mass to galaxy stellar mass ratio increases as a function of projected halo-centric radius $r_p$, from $M_{\rm sub}/M_{\rm star}=3.48^{+ 4.48}_{- 2.48}$ at $r_p \in [0.1,0.3] $ $\mpch$ to $M_{\rm sub}/M_{\rm star}=41.15^{+ 12.55}_{- 12.51}$ at $r_p \in [0.6,0.9]$ $\mpch$. We also investigate the dependence of subhalo masses on stellar mass by splitting satellite galaxies into two stellar mass bins: $10<\log(M_{\rm star}/\ms)<10.75$ and $10.75<\log(M_{\rm star}/\ms)<12$. The mean subhalo mass of the more massive satellite galaxy bin is about 5 times larger than that of the less massive satellites: $\log(M_{\rm sub}/\ms)=12.12 ^{+ 0.19 }_{- 0.19}$ ($M_{\rm sub}/M_{\rm star}=12^{+5}_{-5}$) versus $\log(M_{\rm sub}/\ms)=11.37 ^{+ 0.67 }_{- 0.90}$ ($M_{\rm sub}/M_{\rm star}=17^{+16}_{-16}$).
Parameters and abundances for 1133 stars of spectral types F, G, and K of
luminosity class III have been derived. In terms of stellar parameters, the
primary point of interest is the disagreement between gravities derived with
masses determined from isochrones, and gravities determined from an ionization
balance. This is not a new result per se; but the size of this sample
emphasizes the severity of the problem. A variety of arguments lead to the
selection of the ionization balance gravity as the working value. The derived
abundances indicate that the giants in the solar region have Sun-like total
abundances and abundance ratios. Stellar evolution indicators have also been
investigated with the Li abundances and the [C/Fe] and C/O ratios indicating
that standard processing has been operating in these stars. The more salient
result for stellar evolution is that the [C/Fe] data across the red-giant clump
indicates the presence of mass dependent mixing in accord with standard stellar
evolution predictions.
Keywords: stars: fundamental parameters - stars: abundances - stars:
evolution - Galaxy: abundances
We present a study of the Angular Resolution of the AGILE gamma-ray imaging detector (GRID) that is operational in space since April 2007. The AGILE instrument is made of an array of 12 planes each equipped with a Tungsten converter and Silicon micros trip detectors and is sensitive in the energy range 50 MeV - 10 GeV. Among the space instruments devoted to gamma-ray astrophysics, AGILE uniquely exploits an analog readout system with dedicated electronics coupled with Silicon detectors. We show the results of Monte Carlo simulations carried out to reproduce the gamma-ray detection by the GRID, and we compare them to in-flight data. We use the Crab (pulsar + Nebula) system for discussion of real data performance, since its E^{-2} energy spectrum is representative of the majority of gamma-ray sources. For Crab-like spectrum sources, the GRID angular resolution (FWHM of ~4deg at 100 MeV; ~0.8deg at 1 GeV; ~0.9deg integrating the full energy band from 100 MeV to tens of GeV) is stable across a large field of view, being characterized by a flat response up to 30deg off-axis. A comparison of the angular resolution obtained by the two operational gamma-ray instruments, AGILE-GRID and Fermi-LAT, is interesting in view of future gamma-ray missions, that are currently under study. The two instruments exploit different detector configurations affecting the angular resolution: the former being optimized in the readout and track reconstruction especially in the low-energy band, the latter in terms of converter thickness and power consumption. We show that, despite these differences, the angular resolution of both instruments is very similar between 100 MeV and a few GeV.
The latest Planck results reconfirm the existence of a slight but chronic tension between the best-fit CMB and low-redshift observables: power seems to be consistently lacking in the late universe across a range of observables (e.g. weak lensing, cluster counts). We propose a two-parameter model for dark energy where the dark energy is sufficiently like dark matter at large scales to keep the CMB unchanged but where it does not cluster at small scales, preventing concordance collapse and erasing power. We thus exploit the generic scale-dependence of DE instead of the more usual time-dependence to address the tension in the data. The combination of CMB, distance and weak lensing data somewhat prefer our model to $\Lambda$CDM, at $\Delta\chi^2=2.4$. Moreover, this improved solution has $\sigma_8=0.79 \pm 0.02$, consistent with the value implied by cluster counts.
We study the buoyant rise of magnetic flux tubes embedded in an adiabatic stratification using two-and three-dimensional, MHD simulations. We analyze the dependence of the tube evolution on the field line twist and on the curvature of the tube axis in different diffusion regimes. To be able to achieve a comparatively high spatial resolution we use the FLASH code, which has a built-in Adaptive Mesh Refinement (AMR) capability. Our 3D experiments reach Reynolds numbers that permit a reasonable comparison of the results with those of previous 2D simulations. When the experiments are run without AMR, hence with a comparatively large diffusivity, the amount of longitudinal magnetic flux retained inside the tube increases with the curvature of the tube axis. However, when a low-diffusion regime is reached by using the AMR algorithms, the magnetic twist is able to prevent the splitting of the magnetic loop into vortex tubes and the loop curvature does not play any significant role. We detect the generation of vorticity in the main body of the tube of opposite sign on the opposite sides of the apex. This is a consequence of the inhomogeneity of the azimuthal component of the field on the flux surfaces. The lift force associated with this global vorticity makes the flanks of the tube move away from their initial vertical plane in an antisymmetric fashion. The trajectories have an oscillatory motion superimposed, due to the shedding of vortex rolls to the wake, which creates a Von Karman street.
Recent spectropolarimetric surveys of main-sequence intermediate-mass stars have exhibited a dichotomy in the distribution of the observed magnetic field between the kG dipoles of Ap/Bp stars and the sub-Gauss magnetism of Vega and Sirius. We would like to test whether this dichotomy is linked to the stability versus instability of large-scale magnetic configurations in differentially rotating radiative zones. We computed the axisymmetric magnetic field obtained from the evolution of a dipolar field threading a differentially rotating shell. A full parameter study including various density profiles and initial and boundary conditions was performed with a 2D numerical code. We then focused on the ratio between the toroidal and poloidal components of the magnetic field and discuss the stability of the configurations dominated by the toroidal component using local stability criteria and insights from recent 3D numerical simulations. The numerical results and a simple model show that the ratio between the toroidal and the poloidal magnetic fields is highest after an Alfv\'en crossing time of the initial poloidal field. For high density contrasts, this ratio converges towards an asymptotic value that can thus be extrapolated to realistic stellar cases. We then consider the stability of the magnetic configurations to non-axisymmetric perturbations and find that configurations dominated by the toroidal component are likely to be unstable if the shear strength is significantly higher than the poloidal Alfv\'en frequency. An expression for the critical poloidal field below which magnetic fields are likely to be unstable is found and is compared to the lower bound of Ap/Bp magnetic fields.
We report the identification of 61.45 d^-1 (711.2 mu Hz) oscillations, with amplitudes of 62.6-mu mag, in KIC 4768731 (HD 225914) using Kepler photometry. This relatively bright (V=9.17) chemically peculiar star with spectral type A5 Vp SrCr(Eu) has previously been found to exhibit rotational modulation with a period of 5.21 d. Fourier analysis reveals a simple dipole pulsator with an amplitude that has remained stable over a 4-yr time span, but with a frequency that is variable. Analysis of high-resolution spectra yields stellar parameters of T_eff = 8100 +/- 200 K, log g = 4.0 +/- 0.2, [Fe/H] = +0.31 +/- 0.24 and v sin i = 14.8 +/- 1.6 km/s. Line profile variations caused by rotation are also evident. Lines of Sr, Cr, Eu, Mg and Si are strongest when the star is brightest, while Y and Ba vary in anti-phase with the other elements. The abundances of rare earth elements are only modestly enhanced compared to other roAp stars of similar T_eff and log g. Radial velocities in the literature suggest a significant change over the past 30 yr, but the radial velocities presented here show no significant change over a period of 4 yr.
A grid of evolutionary sequences of stars in the mass range $1.2$-$7$ M$_{\odot}$, with solar-like initial composition is presented. We focus on this mass range in order to estimate the masses and calculate the CNO surface abundances of a sample of observed red giants. The stellar models are calculated from the zero-age main sequence till the early asymptotic giant branch (AGB) phase. Stars of M $\leqslant$ $2.2$M$_{\odot}$ are evolved through the core helium flash. In this work, an approach is adopted that improves the mass determination of an observed sample of 21 RGB and early AGB stars. This approach is based on comparing the observationally derived effective temperatures and absolute magnitudes with the calculated values based on our evolutionary tracks in the Hertzsprung-Russell diagram. A more reliable determination of the stellar masses is achieved by using evolutionary tracks extended to the range of observation. In addition, the predicted CNO surface abundances are compared to the observationally inferred values in order to show how far standard evolutionary calculation can be used to interpret available observations and to illustrate the role of convective mixing. We find that extra mixing beyond the convective boundary determined by the Schwarzschild criterion is needed to explain the observational oxygen isotopic ratios in low mass stars. The effect of recent determinations of proton capture reactions and their uncertainties on the $^{16}$O$/^{17}$O and $^{14} $N$/^{15}$N ratios is also shown. It is found that the $^{14}$N$($ p$,\gamma)^{15}$O reaction is important for predicting the $^{14}$N$/^{15}$N ratio in red giants.
We report the results of the narrow band photometry and imaging monitoring of comet C/2013 R1 (Lovejoy) with the robotic telescope TRAPPIST (La Silla observatory). We gathered around 400 images over 8 months pre- and post-perihelion between September 12, 2013 and July 6, 2014. We followed the evolution of the OH, NH, CN, C3 , and C2 production rates computed with the Haser model as well as the evolution of the dust production. All five gas species display an asymmetry about perihelion, the rate of brightening being steeper than the rate of fading. The study of the coma morphology reveals gas and dust jets which indicate one or several active zone(s) on the nucleus. The dust, C2 , and C3 morphologies present some similarities while the CN morphology is different. OH and NH are enhanced in the tail direction. The study of the evolution of the comet activity shows that the OH, NH, and C2 production rates evolution with the heliocentric distance is correlated to the dust evolution. The CN and, to a lesser extent, the C3 do not display such a correlation with the dust. These evidences and the comparison with parent species production rates indicate that C2 and C3 on one side and OH and NH on the other side could be -at least partially- released from organic-rich grains and icy grains. On the contrary, all evidences point to HCN being the main parent of CN in this comet.
Low-mass X-ray binaries hosting neutron stars (NS) exhibit thermonuclear (type-I) X-ray bursts, which are powered by unstable nuclear burning of helium and/or hydrogen into heavier elements deep in the NS "ocean". In some cases the burning ashes may rise from the burning depths up to the NS photosphere by convection, leading to the appearance of the metal absorption edges in the spectra, which then force the emergent X-ray burst spectra to shift toward lower energies. These effects may have a substantial impact on the color correction factor $f_c$ and the dilution factor $w$, the parameters of the diluted blackbody model $F_E \approx w B_E(f_c T_{eff})$ that is commonly used to describe the emergent spectra from NSs. The aim of this paper is to quantify how much the metal enrichment can change these factors. We have developed a new NS atmosphere modeling code, which has a few important improvements compared to our previous code required by inclusion of the metals. The opacities and the internal partition functions (used in the ionization fraction calculations) are now taken into account for all atomic species. In addition, the code is now parallelized to counter the increased computational load. We compute a detailed grid of atmosphere models with different exotic chemical compositions that mimic the presence of the burning ashes. From the emerging model spectra we compute the color correction factors $f_c$ and the dilution factors $w$ that can then be compared to the observations. We find that the metals may change $f_c$ by up to about 40%, which is enough to explain the scatter seen in the blackbody radius measurements. The presented models open up the possibility for determining NS mass and radii more accurately, and may also act as a tool to probe the nuclear burning mechanisms of X-ray bursts.
The Hubble Frontier Fields (HFF) are six clusters of galaxies, all showing indications of recent mergers, which have recently been been observed for lensed images. As such they are the natural laboratories to study the merging history of galaxy clusters. In this work, we explore the 2D power spectrum of the mass distribution $P_{\rm M}(k)$ as a measure of substructure. We compare $P_{\rm M}(k)$ of these clusters (obtained using strong gravitational lensing) to that of $\Lambda$CDM simulated clusters of similar mass. To compute lensing $P_{\rm M}(k)$, we produced free-form lensing mass reconstructions of HFF clusters, without any light traces mass (LTM) assumption. The inferred power at small scales tends to be larger if (i) the cluster is at lower redshift, and/or (ii) there are deeper observations and hence more lensed images. In contrast, lens reconstructions assuming LTM show higher power at small scales even with fewer lensed images; it appears the small scale power in the LTM reconstructions is dominated by light information, rather than the lensing data. The average lensing derived $P_{\rm M}(k)$ shows lower power at small scales as compared to that of simulated clusters at redshift zero, both dark-matter only and hydrodynamical. The possible reasons are: (i) the available strong lensing data are limited in their effective spatial resolution on the mass distribution, (ii) HFF have yet to build the small scale power they would have at $z\sim 0$, or (iii) simulations are somehow overestimating the small scale power.
We study the background cosmological dynamics with a three component source content: radiation fluid, a barotropic fluid to mimic the matter sector and a single scalar field which can act as dark energy giving rise to the late-time accelerated phase. Using the well-known dimensionless variables, we cast the dynamical equations into an autonomous system of ordinary differential equations (ASODE), which are studied by computing the fixed points and the conditions for their stability. The matter fluid and the scalar field are taken to be uncoupled at first and later, we consider a coupling between the two of the form $Q = \sqrt{2/3}\kappa\beta\rho_m\dot{\phi}$ where $\rho_m$ is the barotropic fluid density. The key point of our analysis is that for the closure of ASODE, we only demand that the jerk, $\Gamma = V V"/V'^2$ is a function of acceleration, $z = - M_p V'/ V$, that is, $\Gamma = 1+ f(z)$. In this way, we are able to accommodate a large class of potentials that goes beyond the simple exponential potentials. The analysis is completely generic and \emph{independent} of the form of potential for the scalar field. As an illustration and confirmation of the analysis, we consider $f(z)$ of the forms $\mu/z^2$, $\mu/z$, $(\mu-z)/z^2$ and $(\mu-z)$ to numerically compute the evolution of cosmological parameters with and without coupling. Implications of the approach and the results are discussed.
The abundance of fluorine is determined from the (2-0) R9 2.3358 micron feature of the molecule HF for several dozen normal G and K stars in the Galactic thin disk from spectra obtained with the Phoenix IR spectrometer on the 2.1-m telescope at Kitt Peak. The abundances are analyzed in the context of Galactic chemical evolution to explore the contributions of supernovae and asymptotic giant branch (AGB) stars to the abundance of fluorine in the thin disk. The average abundance of fluorine in the thin disk is found to be [F/Fe] = +0.23 +/- 0.03, and the [F/Fe] ratio is flat or declines slowly with metallicity in the range from -0.6 < [Fe/H] < +0.3, within the limits of our estimated uncertainty. The measured abundance of fluorine and lack of variation with metallicity in Galactic thin disk stars suggest neutrino spallation in Type II supernovae contributes significantly to the Galactic fluorine abundance, although contributions from AGB stars may also be important.
A one parameter model to describe the individual metallicity distributions and mass-metallicity relation for dwarf galaxies is presented. This multiple-burst model is based on an accretion scenario, accomodates the observational constraint between $\overline{z}$ and $\sigma_{z}^{2}$ recently established by Leaman (2012), and predicts a slope consistent with the mass-metallicity relation of Kirby et al (2013) who showed that the local group dwarf spheroidal and dwarf irregular galaxies lie on the same relation. One interpretation of the model is that it describes star formation occuring either in gas rich mergers or at the intersection of colliding gas streams.
We examine the relationship between star formation and AGN activity by constructing matched samples of local ($0<z<0.6$) radio-loud and radio-quiet AGN in the $\textit{Herschel}$-ATLAS fields. Radio-loud AGN are classified as high-excitation and low-excitation radio galaxies (HERGs, LERGs) using their emission lines and $\textit{WISE}$ 22-$\mu$m luminosity. AGN accretion and jet powers in these active galaxies are traced by [OIII] emission-line and radio luminosity, respectively. Star formation rates (SFRs) and specific star formation rates (SSFRs) were derived using $\textit{Herschel}$ 250-$\mu$m luminosity and stellar mass measurements from the SDSS$-$MPA-JHU catalogue. In the past, star formation studies of AGN have mostly focused on high-redshift sources to observe the thermal dust emission that peaks in the far-infrared, which limited the samples to powerful objects. However, with $\textit{Herschel}$ we can expand this to low redshifts. Our stacking analyses show that SFRs and SSFRs of both radio-loud and radio-quiet AGN increase with increasing AGN power but that radio-loud AGN tend to have lower SFR. Additionally, radio-quiet AGN are found to have approximately an order of magnitude higher SSFRs than radio-loud AGN for a given level of AGN power. The difference between the star formation properties of radio-loud and -quiet AGN is also seen in samples matched in stellar mass.
We present imaging and spectroscopic observations from the Interface Region Imaging Spectrograph (IRIS) of the evolution of the flare ribbon in the SOL2014-04-18T13:03 M-class flare event, at high spatial resolution and time cadence. These observations reveal small-scale substructure within the ribbon, which manifests as coherent quasi-periodic oscillations in both position and Doppler velocities. We consider various alternative explanations for these oscillations, including modulation of chromospheric evaporation flows. Among these we find the best support for some form of wave localized to the coronal current sheet, such as a tearing mode or Kelvin-Helmholtz instability.
We report interferometric mapping of the bipolar pre-planetary nebula IRAS 08005-2356 with an angular-resolution of ~1"-5", using the Submillimeter Array (SMA), in the 12CO J=2-1, 3-2, 13CO J=2-1 and SiO J=5-4 (v=0) lines. Single-dish observations, using the SMT 10-m, were made in these lines as well as in the CO J=4-3 and SiO J-6-5 (v=0) lines. The lines profiles are very broad, showing the presence of a massive (>0.1 Msun), extreme high-velocity outflow (V~200 km/s) directed along the nebular symmetry axis derived from the HST imaging of this object. The outflow's scalar momentum far exceeds that available from radiation pressure of the central post-AGB star, and it may be launched from an accretion disk around a main-sequence companion. We provide indirect evidence for such a disk from its previously published, broad H-alpha emission profile, which we propose results from Ly-beta emission generated in the disk followed by Raman-scattering in the innermost regions of a fast, neutral wind.
Only a handful of quasars have been identified as kinetically dominated, their long term time averaged jet power, $\overline{Q}$, exceeds the bolometric thermal emission, $L_{bol}$, associated with the accretion flow. This letter presents the first extreme ultraviolet (EUV) spectrum of a kinetically dominated quasar, 3C 270.1. The EUV continuum flux density of 3C 270.1 is very steep, $F_{\nu} \sim \nu^{-\alpha_{EUV}}$, $\alpha_{EUV} =2.98\pm 0.15$. This value is consistent with the correlation of $\overline{Q}/L_{bol}$ and $\alpha_{EUV}$ found in previous studies of the EUV continuum of quasars, the EUV deficit of radio loud quasars. Curiously, although ultraviolet broad absorption line (BAL) troughs in quasar spectra are anti-correlated with $\overline{Q}$, 3C 270.1 has been considered a BAL quasar based on an SDSS spectrum. This claim is examined in terms of the EUV spectrum of OVI 1and the highest resolution CIV spectrum in the archival data and the SDSS spectrum. First, from [OIII]4959,5007 (IR) observations and the UV spectral lines, it is concluded that the correct redshift for 3C 270.1 is 1.5266. It is then found that the standard measure of broad absorption, BALnicity = 0, for MgII 2800, CIV 1549 and OVI 1032 in all epochs.
The cosmic microwave background (CMB) energy spectrum is a near-perfect blackbody. The standard model of cosmology predicts small spectral distortions to this form, but no such distortion of the sky-averaged CMB spectrum has yet been measured. We calculate the largest expected distortion, which arises from the inverse Compton scattering of CMB photons off hot, ionized electrons in the universe, known as the thermal Sunyaev-Zel'dovich (tSZ) effect. We show that the predicted signal is roughly one order of magnitude below the current bound from the COBE-FIRAS experiment, but will be detected at enormous significance ($\gtrsim 1000\sigma$) by the proposed Primordial Inflation Explorer (PIXIE). Although cosmic variance reduces the effective signal-to-noise to $230\sigma$, PIXIE will still yield a sub-percent constraint on the total thermal energy in electrons in the observable universe. Furthermore, we show that PIXIE will detect subtle relativistic effects in the sky-averaged tSZ signal at $30\sigma$, which directly probe moments of the optical depth-weighted intracluster medium electron temperature distribution. PIXIE will thus determine the global thermodynamic properties of ionized gas in the universe with unprecedented precision. These measurements will impose a fundamental "integral constraint" on models of galaxy formation and the injection of feedback energy over cosmic time.
We study the possibility of asymmetric dark matter with self-interactions forming compact stable objects. We solve the Tolman-Oppenheimer-Volkoff equation and find the mass-radius relation of such "dark stars", their density profile and their Chandrasekhar mass limit. We consider fermionic asymmetric dark matter with Yukawa-type self-interactions appropriate for solving the well known problems of the collisionless dark matter paradigm. We find that in several cases the relativistic effects are significant.
We study the evolution of neutrinos in a background matter moving with a linear acceleration. The Dirac equation for a massive neutrino electroweakly interacting with background fermions is obtained in a comoving frame where matter is at rest. We solve this Dirac equation for ultrarelativistic neutrinos. The neutrino quantum states in matter moving with a linear acceleration are obtained. We demonstrate that the neutrino electroweak interaction with an accelerated matter leads to the vacuum instability which results in the neutrino-antineutrino pairs creation. We rederive the temperature of the Unruh radiation and find the correction to the Unruh effect due to the specific neutrino interaction with background fermions. As a possible application of the obtained results we discuss the neutrino pairs creation in a core collapsing supernova. The astrophysical upper limit on the neutrino masses is obtained.
A modification of general relativity is presented in which Newton's constant and the cosmological constant become a conjugate pair of dynamical variables.
In the comment of Avelino, Sousa and Lobo [arXiv:1506.06028], it is claimed that networks of topological defects cannot be the origin of the pulsar glitch phenomenon. Here, we point out that topological defects may trigger pulsar glitches within traditional scenarios, such as vortex unpinning and crustal fracture, in which the source of angular momentum required for a glitch event is provided by the pulsar itself.
On February 23, 1987 we collected 24 neutrinos from the explosion of a blue super-giant star in the Large Magellanic Cloud confirming the basic paradigm of core-collapse supernova. During the many years we have been waiting for a repeat of that momentous day, the number and size of neutrino detectors around the world has grown considerably. If the neutrinos from the next supernova in our Galaxy arrive tomorrow we shall collect upwards of tens of thousands of events and next generation detectors will increase the amount of data we collect by more than an order of magnitude. But it is also now apparent that the message is much more complex than previously thought because many time, energy and neutrino flavor dependent features are imprinted upon the signal either at emission or by the passage through the outer layers of the star. These features arise due to the explosion dynamics, the physics of nuclei at high temperatures and densities, and the properties of neutrinos. In this proceedings I will present some aspects of the physics of supernova neutrino oscillations and what we should expect to observe when the neutrinos from the next Galactic supernova (eventually) arrive.
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We present a novel unsupervised learning approach to automatically segment and label images in astronomical surveys. Automation of this procedure will be essential as next-generation surveys enter the petabyte scale: data volumes will exceed the capability of even large crowd-sourced analyses. We demonstrate how a growing neural gas (GNG) can be used to encode the feature space of imaging data. When coupled with a technique called hierarchical clustering, imaging data can be automatically segmented and labelled by organising nodes in the GNG. The key distinction of unsupervised learning is that these labels need not be known prior to training, rather they are determined by the algorithm itself. Importantly, after training a network can be be presented with images it has never 'seen' before and provide consistent categorisation of features. As a proof-of-concept we demonstrate application on data from the Hubble Space Telescope Frontier Fields: images of clusters of galaxies containing a mixture of galaxy types that would easily be recognised and classified by a human inspector. By training the algorithm using one field (Abell 2744) and applying the result to another (MACS0416.1-2403), we show how the algorithm can cleanly separate image features that a human would associate with early and late type galaxies. We suggest that the algorithm has potential as a tool in the automatic analysis and data mining of next-generation imaging and spectral surveys, and could also find application beyond astronomy.
We analyse the clustering of cosmic large scale structure using a consistent modified gravity perturbation theory, accounting for anisotropic effects along and transverse to the line of sight. The growth factor has a particular scale dependence in f(R) gravity and we fit for the shape parameter f_{R0} simultaneously with the distance and the large scale (general relativity) limit of the growth function. Using more than 690,000 galaxies in the Baryon Oscillation Spectroscopy Survey Data Release 11, we find no evidence for extra scale dependence, with the 95\% confidence upper limit |f_{R0}| <8 \times 10^{-4}. Future clustering data, such as from the Dark Energy Spectroscopic Instrument, can use this consistent methodology to impose tighter constraints.
We measure the two-point correlation function of G-dwarf stars within 1-3 kpc of the Sun in multiple lines-of-sight using the Schlesinger et al. G-dwarf sample from the SDSS SEGUE survey. The shapes of the correlation functions along individual SEGUE lines-of-sight depend sensitively on both the stellar-density gradients and the survey geometry. We fit smooth disk galaxy models to our SEGUE clustering measurements, and obtain strong constraints on the thin- and thick-disk components of the Milky Way. Specifically, we constrain the values of the thin- and thick-disk scale heights with 3% and 2% precision, respectively, and the values of the thin- and thick-disk scale lengths with 20% and 8% precision, respectively. Moreover, we find that a two-disk model is unable to fully explain our clustering measurements, which exhibit an excess of clustering at small scales (< 50 pc). This suggests the presence of small-scale substructure in the disk system of the Milky Way.
We present the analysis of high-resolution spectra obtained with UVES and UVES-FLAMES at the Very Large Telescope of 17 giants in the globular cluster M22, a stellar system suspected to have an intrinsic spread in the iron abundance. We find that when surface gravities are derived spectroscopically (by imposing to obtain the same iron abundance from FeI and FeII lines) the [Fe/H] distribution spans ~0.5 dex, according to previous analyses. However, the gravities obtained in this way correspond to unrealistic low stellar masses (0.1-0.5 Msun) for most of the surveyed giants. Instead, when photometric gravities are adopted, the [FeII/H] distribution shows no evidence of spread at variance with the [FeI/H] distribution. This difference has been recently observed in other clusters and could be due to non-local thermodynamical equilibrium effects driven by over-ionization mechanisms, that mainly affect the neutral species (thus providing lower [FeI/H]) but leave [FeII/H] unaltered. We confirm that the s-process elements show significant star-to-star variations and their abundances appear to be correlated with the difference between [FeI/H] and [FeII/H]. This puzzling finding suggests that the peculiar chemical composition of some cluster stars may be related to effects able to spuriously decrease [FeI/H]. We conclude that M22 is a globular cluster with no evidence of intrinsic iron spread, ruling out that it has retained the supernovae ejecta in its gravitational potential well.
The recently developed method (Paper 1) enabling one to investigate the evolution of dynamical systems with an accuracy not dependent on time, is developed further. The classes of dynamical systems which can be studied by that method are much extended, now including; 1) non-Hamiltonian, conservative; 2) Hamiltonian with time-dependent perturbation; 3) non-conservative (with dissipation). These systems cover various types of N-body gravitating systems of astrophysical and cosmological interest, such as the orbital evolution of planets, minor planets, artificial satellites due to tidal, non-tidal perturbations and thermal thrust, evolving close binary stellar systems, dynamics of accretion disks.
We present ground-based and Swift photometric and spectroscopic observations of the candidate tidal disruption event (TDE) ASASSN-14li, found at the center of PGC 043234 ($d\simeq90$~Mpc) by the All-Sky Automated Survey for SuperNovae (ASAS-SN). The source had a peak bolometric luminosity of $L\simeq10^{44}$ ergs s$^{-1}$ and a total integrated energy of $E\simeq7\times10^{50}$ ergs radiated over the $\sim6$ months of observations presented. The UV/optical emission of the source is well-fit by a blackbody with roughly constant temperature of $T\sim35,000$ K, while the luminosity declines by roughly a factor of 16 over this time. The optical/UV luminosity decline is broadly consistent with an exponential decline, $L\propto e^{-t/t_0}$, with $t_0\simeq60$ days. ASASSN-14li also exhibits soft X-ray emission comparable in luminosity to the optical and UV emission but declining at a slower rate, and the X-ray emission now dominates. Spectra of the source show broad Balmer and helium lines in emission as well as strong blue continuum emission at all epochs. We use the discoveries of ASASSN-14li and ASASSN-14ae to estimate the TDE rate implied by ASAS-SN, finding an average rate of $r \simeq 5.4 \times 10^{-5}~{\rm yr}^{-1}$ per galaxy with a 90\% confidence interval of $(2.2 - 17.0) \times 10^{-5}~{\rm yr}^{-1}$ per galaxy. ASAS-SN found roughly 1 TDE for every 70 Type Ia supernovae in 2014, a rate that is much higher than that of other surveys.
We use semi-analytic models and cosmological merger trees to provide the initial conditions for multi-merger numerical hydrodynamic simulations, and exploit these simulations to explore the effect of galaxy interaction and merging on star formation (SF). We compute numerical realisations of twelve merger trees from z=1.5 to z=0. We include the effects of the large hot gaseous halo around all galaxies, following recent obervations and predictions of galaxy formation models. We find that including the hot gaseous halo has a number of important effects. Firstly, as expected, the star formation rate on long timescales is increased due to cooling of the hot halo and refuelling of the cold gas reservoir. Secondly, we find that interactions do not always increase the SF in the long term. This is partially due to the orbiting galaxies transferring gravitational energy to the hot gaseous haloes and raising their temperature. Finally we find that the relative size of the starburst, when including the hot halo, is much smaller than previous studies showed. Our simulations also show that the order and timing of interactions are important for the evolution of a galaxy. When multiple galaxies interact at the same time, the SF enhancement is less than when galaxies interact in series. All these effects show the importance of including hot gas and cosmologically motivated merger trees in galaxy evolution models.
While recent supernova cosmology research has benefited from improved measurements, current analysis approaches are not statistically optimal and will prove insufficient for future surveys. This paper discusses the limitations of current supernova cosmological analyses in treating outliers, selection effects, shape- and color-standardization relations, intrinsic dispersion, and heterogeneous observations. We present a new Bayesian framework, called UNITY (Unified Nonlinear Inference for Type-Ia cosmologY), that incorporates significant improvements in our ability to confront these effects. We apply the framework to real supernova observations and demonstrate smaller statistical and systematic uncertainties. We verify earlier results that SNe Ia require nonlinear shape and color standardizations, but we now include these nonlinear relations in a statistically well-justified way. This analysis was blinded, in that the method was first validated on simulated data, and no analysis changes were made after transitioning to real data. We discuss possible extensions of the method.
Here we extend the exploration of significantly super-Chandrasekhar magnetised white dwarfs by numerically computing axisymmetric stationary equilibria of differentially rotating magnetised polytropic compact stars in general relativity (GR), within the ideal magnetohydrodynamic regime. We use a general relativistic magnetohydrodynamic (GRMHD) framework that describes rotating and magnetised axisymmetric white dwarfs, choosing appropriate rotation laws and magnetic field profiles (toroidal and poloidal). The numerical procedure for finding solutions in this framework uses the 3+1 formalism of numerical relativity, implemented in the open source XNS code. We construct equilibrium sequences by varying different physical quantities in turn, and highlight the plausible existence of super-Chandrasekhar white dwarfs, with masses in the range of 2-3 solar mass, with central (deep interior) magnetic fields of the order of $10^{14}$ Gauss and differential rotation with surface time periods of about 1-10 seconds. We note that such white dwarfs are candidates for the progenitors of peculiar, overluminous type Ia supernovae, to which observational evidence ascribes mass in the range 2.1-2.8 solar mass. We also present some interesting results related to the structure of such white dwarfs, especially the existence of polar hollows in special cases.
Although WASP-14 b is one of the most massive and densest exoplanets on a tight and eccentric orbit, it has never been a target of photometric follow-up monitoring or dedicated observing campaigns. We report on new photometric transit observations of WASP-14 b obtained within the framework of "Transit Timing Variations @ Young Exoplanet Transit Initiative" (TTV@YETI). We collected 19 light-curves of 13 individual transit events using six telescopes located in five observatories distributed in Europe and Asia. From light curve modelling, we determined the planetary, stellar, and geometrical properties of the system and found them in agreement with the values from the discovery paper. A test of the robustness of the transit times revealed that in case of a non-reproducible transit shape the uncertainties may be underestimated even with a wavelet-based error estimation methods. For the timing analysis we included two publicly available transit times from 2007 and 2009. The long observation period of seven years (2007-2013) allowed us to refine the transit ephemeris. We derived an orbital period 1.2 s longer and 10 times more precise than the one given in the discovery paper. We found no significant periodic signal in the timing-residuals and, hence, no evidence for TTV in the system.
Using the Wind/WAVES radio observations from 2010-2013, we present an analysis of the 123 decametric-hectometric (DH) type II solar radio bursts during this period, the associated type III burst properties, and their correlation with solar energetic proton (SEP) properties determined from analysis of the Geostationary Operational Environmental Satellite (GOES) observations. We present a useful catalog of the type II burst, type III burst, Langmuir wave, and proton flux properties for these 123 events, which we employ to develop a statistical relationship between the radio properties and peak proton flux that can be used to forecast SEP events. We find that all SEP events with a peak > 10 MeV flux above 15 pfu are associated with a type II burst and virtually all SEP events, 92%, are also associated with a type III radio burst. Based on a principal component analysis, the radio burst properties that are most highly correlated with the occurrence of gradual SEP events and account for the most variance in the radio properties are the type III burst intensity and duration. Further, a logistic regression analysis with the radio-derived principal component (dominated by the type III and type II radio burst intensity and type III duration) obtains SEP predictions approaching the human forecaster rates, with a false alarm rate of 22%, a probability of detection of 62%, and with 85% of the classifications correct. Therefore, type III radio bursts that occur along with a DH type II burst are shown to be an important diagnostic that can be used to forecast SEP events.
We present observations of Epsilon Eridani from the Submillimeter Array (SMA) at 1.3 millimeters and from the Australia Telescope Compact Array (ATCA) at 7 millimeters that reach an angular resolution of ~4" (13 AU). These first millimeter interferometer observations of Epsilon Eridani, which hosts the closest debris disk to the Sun, reveal two distinct emission components: (1) the well-known outer dust belt, which, although patchy, is clearly resolved in the radial direction, and (2) an unresolved source coincident with the position of the star. We use direct model-fitting of the millimeter visibilities to constrain the basic properties of these two components. A simple Gaussian shape for the outer belt fit to the SMA data results in a radial location of $64.4^{+2.4}_{-3.0}$ AU and FWHM of $20.2^{+6.0}_{-8.2}$ AU (fractional width $\Delta R/R = 0.3$. Similar results are obtained taking a power law radial emission profile for the belt, though the power law index cannot be usefully constrained. Within the noise obtained (0.2 mJy/beam), these data are consistent with an axisymmetric belt model and show no significant azimuthal structure that might be introduced by unseen planets in the system. These data also limit any stellocentric offset of the belt to $<9$ AU, which disfavors the presence of giant planets on highly eccentric ($>0.1$) and wide (10's of AU) orbits. The flux density of the unresolved central component exceeds predictions for the stellar photosphere at these long wavelengths, by a marginally significant amount at 1.3 millimeters but by a factor of a few at 7 millimeters (with brightness temperature $13000 \pm 1600$ K for a source size of the optical stellar radius). We attribute this excess emission to ionized plasma from a stellar corona or chromosphere.
We employ very high resolution simulations of isolated Milky Way-like galaxies to study the effect of triaxial dark matter haloes on exponential stellar disks. Non-adiabatic halo shape changes can trigger two-armed grand-design spiral structures which extend all the way to the edge of the disk. Their pattern speed coincides with the inner Lindblad resonance indicating that they are kinematic density waves which can persist up to several Gyrs. In dynamically cold disks grand-design spirals are swing amplified and after a few Gyrs can lead to the formation of (multi-armed) transient recurrent spirals. Stellar disks misaligned to the principal planes of the host triaxial halo develop characteristic integral shaped warps, but otherwise exhibit very similar spiral structures as aligned disks. For the grand-design spirals in our simulations their strength dependence with radius is determined by the torque on the disk, suggesting that by studying grand-design spirals without bars it may be possible to set constraints on the tidal field and host dark matter halo shape.
Many early universe theories predict the creation of Primordial Black Holes (PBHs). The PBHs could have masses ranging from the Planck mass to 10^5 solar masses or higher depending on the formation scenario. Hawking showed that any Black Hole (BH) has a temperature which is inversely proportional to its mass. Hence a sufficiently small BH will thermodynamically radiate particles at an ever-increasing rate, continually decreasing its mass and raising its temperature. The final moments of this evaporation phase should be explosive. In this work, we investigate the final few seconds of the BH burst using the Standard Model of particle physics and calculate the energy dependent burst time profiles in the GeV/TeV range. We use the HAWC (High Altitude Water Cherenkov) observatory as a case study and calculate PBH burst light curves which would be observed by HAWC.
We use Minkowski Functionals to explore the presence of non-Gaussian signatures in simulated cosmic microwave background (CMB) maps. Precisely, we analyse the non-Gaussianities produced from the angular power spectra emerging from a class of inflationary models with a primordial step-like potential. This class of models are able to perform the best-fit of the low-$\ell$ `features', revealed first in the CMB angular power spectrum by the WMAP experiment and then confirmed by the Planck collaboration maps. Indeed, such models generate oscillatory features in the primordial power spectrum of scalar perturbations, that are then imprinted in the large scales of the CMB field. Interestingly, we discover Gaussian deviations in the CMB maps simulated from the power spectra produced by these models, as compared with Gaussian $\Lambda$CDM maps. Moreover, we also show that the kind and level of the non-Gaussianities produced in these simulated CMB maps are compatible with that found in the four foreground-cleaned Planck maps. Our results indicate that inflationary models with a step-like potential are not only able to improve the best-fit respect to the $\Lambda$CDM model accounting well for the `features' observed in the CMB angular power spectrum, but also suggesting a possible origin for certain non-Gaussian signatures observed in the Planck data.
We investigate the origin of the most iron-poor stars including SMSS J031300.36-670839.3 with [Fe/H] < -7.52. We compute the change of surface metallicity of stars with the accretion of interstellar matter (ISM) after their birth using the chemical evolution model within the framework of the hierarchical galaxy formation. The predicted metallicity distribution function agrees very well with that observed from extremely metal-poor stars. In particular, the lowest metallicity tail is well reproduced by the Population III stars whose surfaces are polluted with metals through ISM accretion. This suggests that the origin of iron group elements is explained by ISM accretion for the stars with [Fe/H]$\lesssim -5$. The present results give new insights into the nature of the most metal-poor stars and the search for Population III stars with pristine abundances.
We have used public data from the Next Generation Virgo Survey (NGVS) to investigate the dwarf galaxy population of the Virgo cluster beyond what has previously been discovered. We initially mask and smooth the data, and then use the object detection algorithm Sextractor to make our initial dwarf galaxy selection. All candidates are then visually inspected to remove artefacts and duplicates. We derive Sextractor parameters to best select low surface brightness galaxies using g band central surface brightness values of 22.5 to 26.0 mag sq arc sec and exponential scale lengths of 3.0 - 10.0 arc sec to identify 443 cluster dwarf galaxies - 303 of which are new detections. These new detections have a surface density that decreases with radius from the cluster centre. We also apply our selection algorithm to 'background', non-cluster, fields and find zero detections. In combination, this leads us to believe that we have isolated a cluster dwarf galaxy population. The range of objects we are able to detect is limited because smaller scale sized galaxies are confused with the background, while larger galaxies are split into numerous smaller objects by the detection algorithm. Using data from previous surveys combined with our data, we find a faint end slope to the luminosity function of -1.35+/-0.03, which does not significantly differ to what has previously been found for the Virgo cluster, but is a little steeper than the slope for field galaxies. There is no evidence for a faint end slope steep enough to correspond with galaxy formation models, unless those models invoke either strong feedback processes or use warm dark matter.
Ocean planets are volatile rich planets, not present in our Solar System,
which are thought to be dominated by deep, global oceans. This results in the
formation of high-pressure water ice, separating the planetary crust from the
liquid ocean and, thus, also from the atmosphere. Therefore, instead of a
carbonate-silicate cycle like on the Earth, the atmospheric carbon dioxide
concentration is governed by the capability of the ocean to dissolve carbon
dioxide (CO2).
In our study, we focus on the CO2 cycle between the atmosphere and the ocean
which determines the atmospheric CO2 content. The atmospheric amount of CO2 is
a fundamental quantity for assessing the potential habitability of the planet's
surface because of its strong greenhouse effect, which determines the planetary
surface temperature to a large degree. In contrast to the stabilising
carbonate-silicate cycle regulating the long-term CO2 inventory of the Earth
atmosphere, we find that the CO2 cycle feedback on ocean planets is negative
and has strong destabilising effects on the planetary climate. By using a
chemistry model for oceanic CO2 dissolution and an atmospheric model for
exoplanets, we show that the CO2 feedback cycle can severely limit the
extension of the habitable zone for ocean planets.
Using high-resolution images from 1.6 m New Solar Telescope (NST) at Big Bear Solar Observatory (BBSO), we report the direct evidence of chromospheric reconnection at the polarity inversion line (PIL) between two small opposite polarity sunspots. Small jet-like structures (with velocities of ~20-55 km/s) were observed at the reconnection site before the onset of the first M1.0 flare. The slow rise of untwisting jets was followed by the onset of cool plasma inflow (~10 km/s) at the reconnection site, causing the onset of a two-ribbon flare. The reconnection between two sheared J-shaped cool H$\alpha$ loops causes the formation of a small twisted flux rope (S shaped) in the chromosphere. In addition, Helioseismic and Magnetic Imager (HMI) magnetograms show the flux cancellation (both positive and negative) during the first M1.0 flare. The emergence of negative flux and cancellation of positive flux (with shear flows) continue until the successful eruption of the flux rope. The newly formed chromospheric flux rope becomes unstable and rises slowly with the speed of ~108 km/s during a second C8.5 flare that occurred after ~3 hours of the first M1.0 flare. The flux rope was destroyed by repeated magnetic reconnection induced by its interaction with the ambient field (fan-spine toplology) and looks like an untwisting surge (~170 km/s) in the coronal images recorded by Solar Dynamic Observatory/Atmospheric Imaging Assembly (SDO/AIA). These observations suggest the formation of a chromospheric flux rope (by magnetic reconnection associated with flux cancellation) during the first M1.0 flare and its subsequent eruption/disruption during the second C8.5 flare.
The ongoing 11-year cycle of solar activity is considerably less vigorous than the three cycles before. It was preceded by a very deep activity minimum with a low polar magnetic flux, the source of the toroidal field responsible for solar magnetic activity in the subsequent cycle. Simulation of the evolution of the solar surface field shows that the weak polar fields and thus the weakness of the present cycle 24 are mainly caused by a number of bigger bipolar regions emerging at low latitudes with a `wrong' (i.e., opposite to the majority for this cycle) orientation of their magnetic polarities in the North-South direction, which impaired the growth of the polar field. These regions had a particularly strong effect since they emerged within $\pm10^\circ$ latitude from the solar equator.
We present an analysis of the 3-79 keV NuSTAR spectrum of the low-luminosity active galactic nucleus NGC 7213. In agreement with past observations, we find a lower limit to the high-energy cut-off of Ec > 140 keV, no evidence for a Compton-reflected continuum, and the presence of an iron Kalpha complex, possibly produced in the broad-line region. From the application of the MYTorus model, we find that the line-emitting material is consistent with the absence of a significant Compton reflection if arising from a Compton-thin torus of gas with a column density of 5.0(+2.0,-1.6) cm^-2. We report variability of the equivalent width of the iron lines on the time-scale of years using archival observations from XMM-Newton, Chandra and Suzaku. This analysis suggests a possible contribution from dusty gas. A fit with a Comptonization model indicates the presence of a hot corona with a temperature kTe > 40 keV and an optical depth tau < 1, assuming a spherical geometry.
The core of the nearby galaxy NGC660 has recently undergone a spectacular radio outburst; using a combination of archival radio and Chandra X-ray data, together with new observations, the nature of this event is investigated. Radio observations made using e-MERLIN in mid-2013 show a new compact and extremely bright continuum source at the centre of the galaxy. High angular resolution observations carried out with the European VLBI Network show an obvious jet-like feature to the north east and evidence of a weak extension to the west, possibly a counter-jet. We also examine high angular resolution HI spectra of these new sources, and the radio spectral energy distribution using the new wide-band capabilities of e-MERLIN. We compare the properties of the new object with possible explanations, concluding that we are seeing a period of new AGN activity in the core of this polar ring galaxy.
We analyze 134 ks Chandra ACIS-I observations of the Galactic Centre (GC) performed in July 2011. The X-ray image with the field of view $17' \times 17'$ contains the hot plasma surrounding the Sgr~A*. The obtained surface brightness map allow us to fit Bondi hot accretion flow to the innermost hot plasma around the GC. We have fitted spectra from region up to $5"$ from Sgr~A* using a thermal bremsstrahlung model and four Gaussian profiles responsible for K$_{\alpha}$ emission lines of Fe, S, Ar, and Ca. The X-ray surface brightness profile up to $3"$ from Sgr~A* found in our data image, was successfully fitted with the dynamical model of Bondi spherical accretion. By modelling the surface brightness profile, we derived the temperature and number density profiles in the vicinity of the black hole. The best fitted model of spherical Bondi accretion shows that this type of flow works only up to $3"$ and implies outer plasma density and temperature to be: $n_{\rm e}^{\rm out}=18.3 \pm {0.1}$ cm$^{-3}$ and $T_{\rm e}^{\rm out}= 3.5 \pm {0.3}$ keV respectively. We show that the Bondi flow can reproduce observed surface brightness profile up to $3"$ from Sgr~A* in the Galactic Center. This result strongly suggests the position of stagnation radius in the complicated dynamics around GC. The Faraday rotation computed from our model towards the pulsar PSR J1745-2900 near the GC agrees with the observed one, recently reported.
Hyperaccretion flows with mass accretion rates far above the Eddington rate have an N-shaped equilibrium curve on the $\Sigma$-$\dot{M}$ plane (with $\Sigma$ and $\dot{M}$ being surface density and mass accretion rate, respectively). The accretion flow on the lower $\Sigma$ branch of the N-shape is optically thick, advection-dominated accretion flow (ADAF) while that on the upper one is neutrino-dominated accretion flow (NDAF). The middle branch has a negative slope on the $\Sigma$-$\dot{M}$ plane, meaning that the flow on this branch is secularly unstable. To investigate how the instability affects the flow structure and what observable signatures are produced, we study the time evolution of the unstable hyperaccretion flow in response to variable mass injection rates by solving the height-averaged equations for viscous accretion flows. When a transition occurs from the lower branch to the upper branch (or from the upper branch to the lower branch), the surface density rapidly increases (decreases) around that transition region, which induces locally enhanced mass flow (referred to as non-steady mass flow) into (out of) that region. We confirm that the non-steady flow can create a kind of disturbance and that it propagates over the whole disk. However, the non-steady mass flow is not strong enough to induce coherent transition over the whole disk, unless the mass injection rate varies with time. When the injection rate continuously changes, the neutrino luminosity varies intermittently, thus producing step-function-like light curves, as the radiation efficiency discontinuously changes every time the local transition occurs. The effects of changing the N-shape and possible observational consequences on the gamma-ray burst emission are briefly discussed in relation to gamma-ray bursts.
The deflection, potential, shear and magnification of a gravitational lens following an elliptical power law mass model are investigated. This mass model is derived from the circular power law profile through a rescaling of the axes, similar to the case of a singular isothermal ellipsoid. The resulting deflection can be calculated explicitly and given in terms of the Gaussian hypergeometric function. Analytic expressions for the remaining lensing properties are found as well. Because the power law profile lens contains a number of well-known lens models as special cases, the equivalence of the new expressions with known results is checked. Finally, it is shown how these results naturally lead to a fast and accurate numerical scheme for computing the deflection and other lens quantities, making this method a useful tool for realistically modelling observed lenses.
We present the first detections by the NASA K2 Mission of oscillations in solar-type stars, using short-cadence data collected during K2 Campaign\,1 (C1). We understand the asteroseismic detection thresholds for C1-like levels of photometric performance, and we can detect oscillations in subgiants having dominant oscillation frequencies around $1000\,\rm \mu Hz$. Changes to the operation of the fine-guidance sensors are expected to give significant improvements in the high-frequency performance from C3 onwards. A reduction in the excess high-frequency noise by a factor of two-and-a-half in amplitude would bring main-sequence stars with dominant oscillation frequencies as high as ${\simeq 2500}\,\rm \mu Hz$ into play as potential asteroseismic targets for K2.
Blue supergiant stars are known to display photometric and spectroscopic variability that is suggested to be linked to stellar pulsations. Pulsational activity in massive stars strongly depends on the star's evolutionary stage and is assumed to be connected with mass-loss episodes, the appearance of macroturbulent line broadening, and the formation of clumps in the wind. To investigate a possible interplay between pulsations and mass-loss, we carried out an observational campaign of the supergiant 55 Cyg over a period of five years to search for photospheric activity and cyclic mass-loss variability in the stellar wind. We modeled the H, He I, Si II and Si III lines using the nonlocal thermal equilibrium atmosphere code FASTWIND and derived the photospheric and wind parameters. In addition, we searched for variability in the intensity and radial velocity of photospheric lines and performed a moment analysis of the line profiles to derive frequencies and amplitudes of the variations. The Halpha line varies with time in both intensity and shape, displaying various types of profiles: P Cygni, pure emission, almost complete absence, and double or multiple peaked. The star undergoes episodes of variable mass-loss rates that change by a factor of 1.7-2 on different timescales. We also observe changes in the ionization rate of Si II and determine a multiperiodic oscillation in the He I absorption lines, with periods ranging from a few hours to 22.5 days. We interpret the photospheric line variations in terms of oscillations in p-, g-, and strange modes. We suggest that these pulsations can lead to phases of enhanced mass loss. Furthermore, they can mislead the determination of the stellar rotation. We classify the star as a post-red supergiant, belonging to the group of alpha Cyg variables.
We perform a series of 2D smoothed particle hydrodynamics (SPH) simulations of gas accretion onto binaries via a circumbinary disc, for a range of gas temperatures and binary mass ratios ($q$). We show that increasing the gas temperature increases the accretion rate onto the primary for all values of the binary mass ratio: for example, for $q=0.1$ and a fixed binary separation, an increase of normalised sound speed by a factor of $5$ (from our "cold" to "hot" simulations) changes the fraction of the accreted gas that flows on to the primary from $ 10\%$ to $\sim40\%$. We present a simple parametrisation for the average accretion rate of each binary component accurate to within a few percent and argue that this parametrisation (rather than those in the literature based on warmer simulations) is relevant to supermassive black hole accretion and all but the widest stellar binaries. We present trajectories for the growth of $q$ during circumbinary disc accretion and argue that the period distribution of stellar "twin" binaries is strong evidence for the importance of circumbinary accretion. We also show that our parametrisation of binary accretion increases the minimum mass ratio needed for spin alignment of supermassive black holes to $q \sim 0.4$, with potentially important implications for the magnitude of velocity kicks acquired during black hole mergers.
Aperture synthesis techniques are increasingly being employed to provide high angular resolution images in situations where the object of interest is in the near field of the interferometric array. Previous work has showed that an aperture synthesis array can be refocused on an object in the near field of an array, provided that the object is smaller than the effective Fresnel zone size corresponding to the array-object range. We show here that, under paraxial conditions, standard interferometric techniques can be used to image objects which are substantially larger than this limit. We also note that interferometric self-calibration and phase-closure image reconstruction techniques can be used to achieve near-field refocussing without requiring accurate object range information. We use our results to show that the field of view for high-resolution aperture synthesis imaging of geosynchronous satellites from the ground can be considerably larger than the largest satellites in Earth orbit.
We present Arecibo, GBT, VLA and WIYN/pODI observations of the ALFALFA source AGC 226067. Originally identified as an ultra-compact high velocity cloud and candidate Local Group galaxy, AGC 226067 is spatially and kinematically coincident with the Virgo cluster, and the identification by multiple groups of an optical counterpart with no resolved stars supports the interpretation that this systems lies at the Virgo distance (D=17 Mpc). The combined observations reveal that the system consists of multiple components: a central HI source associated with the optical counterpart (AGC 226067), a smaller HI-only component (AGC 229490), a second optical component (AGC 229491), and extended low surface brightness HI. Only ~1/4 of the single-dish HI emission is associated with AGC 226067; as a result, we find M_HI/L_g ~ 6 Msun/Lsun, which is lower than previous work. At D=17 Mpc, AGC 226067 has an HI mass of 1.5 x 10^7 Msun and L_g = 2.4 x 10^6 Lsun, AGC 229490 (the HI-only component) has M_HI = 3.6 x 10^6 Msun, and AGC 229491 (the second optical component) has L_g = 3.6 x 10^5 Lsun. The nature of this system of three sources is uncertain: AGC 226067 and AGC 229490 may be connected by an HI bridge, and AGC 229490 and AGC 229491 are separated by only 0.5'. The current data do not resolve the HI in AGC 229490 and its origin is unclear. We discuss possible scenarios for this system of objects: an interacting system of dwarf galaxies, accretion of material onto AGC 226067, or stripping of material from AGC 226067.
We present a new local method for optimally estimating the local effects of magnification from weak gravitational lensing, using a comparison of number counts in an arbitrary region of space to the expected unmagnified number counts. This method has equivalent statistical power to the optimally-weighted correlation function method previously employed to measure magnification, but has the potential to be used for purposes such as mass mapping, and is also significantly computationally faster. We present a proof-of-principle test of this method on data from the CFHTLenS, showing that its calculated magnification signals agree with predictions from model fits to shear data. Finally, we investigate how magnification data can be used to supplement shear data in determining the best-fit model mass profiles for galaxy dark matter haloes. We find that at redshifts greater than z~0.6, the inclusion of magnification can often significantly improve the constraints on the components of the mass profile which relate to galaxies' local environments relative to shear alone, and in high-redshift, low-mass bins, it can have a higher signal-to-noise than the shear signal.
Here we report on the first successful exoplanet transit observation with the Stratospheric Observatory for Infrared Astronomy (SOFIA). We observed a single transit of the hot Jupiter HD 189733 b, obtaining two simultaneous primary transit lightcurves in the B and z' bands as a demonstration of SOFIA's capability to perform absolute transit photometry. We present a detailed description of our data reduction, in particular the correlation of photometric systematics with various in-flight parameters unique to the airborne observing environment. The derived transit depths at B and z' wavelengths confirm a previously reported slope in the optical transmission spectrum of HD 189733 b. Our results give new insights to the current discussion about the source of this Rayleigh scattering in the upper atmosphere and the question of fixed limb darkening coefficients in fitting routines.
In the solar corona, magnetic flux rope is believed to be a fundamental structure accounts for magnetic free energy storage and solar eruptions. Up to the present, the extrapolation of magnetic field from boundary data is the primary way to obtain fully three-dimensional magnetic information of the corona. As a result, the ability of reliable recovering coronal magnetic flux rope is important for coronal field extrapolation. In this paper, our coronal field extrapolation code (CESE-MHD-NLFFF, Jiang & Feng 2012) is examined with an analytical magnetic flux rope model proposed by Titov & Demoulin (1999), which consists of a bipolar magnetic configuration holding an semi-circular line-tied flux rope in force-free equilibrium. By using only the vector field in the bottom boundary as input, we test our code with the model in a representative range of parameter space and find that the model field is reconstructed with high accuracy. Especially, the magnetic topological interfaces formed between the flux rope and the surrounding arcade, i.e., the "hyperbolic flux tube" and "bald patch separatrix surface", are also reliably reproduced. By this test, we demonstrate that our CESE-MHD-NLFFF code can be applied to recovering magnetic flux rope in the solar corona as long as the vector magnetogram satisfies the force-free constraints.
A non-minimal coupling between the dark matter and dark energy components may offer a way of solving the so-called coincidence problem. In this paper we propose a low-$z$ test for such hypothesis using measurements of the gas mass fraction $f_{\rm{gas}}$ in relaxed and massive galaxy clusters. The test applies to any model whose dilution of dark matter is modified with respect to the standard $a^{-3}$ scaling, as usual in interacting models, where $a$ is the cosmological scale factor. We apply the test to current $f_{\rm{gas}}$ data and perform Monte Carlo simulations to forecast the necessary improvements in number and accuracy of upcoming observations to detect a possible interaction in the cosmological dark sector. Our results show that improvements in the present relative error $\sigma_{\rm{gas}}/f_{\rm{gas}}$ are more effective to achieve this goal than an increase in the size of the $f_{\rm{gas}}$ sample.
We present an extended grid of mean three-dimensional (3D) spectra for low-mass, pure-hydrogen atmosphere DA white dwarfs (WDs). We use CO5BOLD radiation-hydrodynamics 3D simulations covering Teff = 6000-11,500 K and logg = 5-6.5 (cgs units) to derive analytical functions to convert spectroscopically determined 1D temperatures and surface gravities to 3D atmospheric parameters. Along with the previously published 3D models, the 1D to 3D corrections are now available for essentially all known convective DA WDs (i.e., logg = 5-9). For low-mass WDs, the correction in temperature is relatively small (a few per cent at the most), but the surface gravities measured from the 3D models are lower by as much as 0.35 dex. We revisit the spectroscopic analysis of the extremely low-mass (ELM) WDs, and demonstrate that the 3D models largely resolve the discrepancies seen in the radius and mass measurements for relatively cool ELM WDs in eclipsing double WD and WD + milli-second pulsar binary systems. We also use the 3D corrections to revise the boundaries of the ZZ Ceti instability strip, including the recently found ELM pulsators.
By means of radiative transfer simulation we study the evolution of the far-infrared colours of protoplanetary discs undergoing inside-out dispersal, often referred to as transition discs. We show that a brightening of the mid and far-infrared emission from these objects is a natural consequence of the removal of the inner disc. Our results can fully explain recent observations of transition discs in the Chamaleon and Lupus star forming regions from the Herschel Gould Belt Survey, which show a higher median for the 70?um (Herschel PACS 1) band of known transition objects compared with primordial discs. Our theoretical results hence support the suggestion that the 70?um band may be a powerful diagnostic for the identi?cation of transition discs from photometry data, provided that the inner hole is larger than tens of AU, depending on spectral type. Furthermore we show that a comparison of photometry in the K , 12?um and 7u0?m bands to model tracks can provide a rough, but quick estimate of the inner hole size of these objects, provided their inclination is below ?85 degrees and the inner hole size is again larger than tens of AU.
Minimal Dark Matter (MDM) stands as one of the simplest dark matter scenarios. In MDM models, annihilation and co-annihilation processes among the members of the MDM multiplet are usually very efficient, pushing the dark matter mass above $\mathcal{O}(10)$ TeV in order to reproduce the observed dark matter relic density. Motivated by this little drawback, in this paper we consider an extension of the MDM scenario by three right-handed neutrinos. Two specific choices for the MDM multiplet are studied: a fermionic $SU(2)_L$ quintuplet and a scalar $SU(2)_L$ septuplet. The lightest right-handed neutrino, with tiny Yukawa couplings, never reaches thermal equilibrium in the early universe and is produced by freeze-in. This creates a link between dark matter and neutrino physics: dark matter can be non-thermally produced by the decay of the lightest right-handed neutrino after freeze-out, allowing to lower significantly the dark matter mass. We discuss the phenomenology of the non-thermally produced MDM and, taking into account significant Sommerfeld corrections, we find that the dark matter mass must have some specific values in order not to be in conflict with the current bounds from gamma-ray observations.
Different types of essentially nongeodesic motions of highly relativistic spinning particles in Schwarzschild's and Kerr's background which follows from the Mathisson-Papapetrou (MP) equations are considered. It is shown that dependently on the correlation of signs of the spin and the particle's orbital velocity the spin-gravity coupling acts as a significant repulsive or attractive force. Numerical estimates for electrons, protons, and neutrinos in the gravitational field of black holes are presented. The correspondence between the general relativistic Dirac equation and MP equations is discussed. It is stressed that for the highly relativistic motions the adequate supplementary condition for the MP equations is the Mathisson-Pirani condition. In the following it is important to study the possible role of the highly relativistic spin-gravity coupling in astrophysics, cosmology, and high energy physics.
The Multi-Band Template Analysis is a low-latency analysis pipeline for the detection of gravitational waves to triggering electromagnetic follow up observations. Coincident observation of gravitational waves and an electromagnetic counterpart will allow us to develop a complete picture of energetic astronomical events. We give an outline of the MBTA pipeline, as well as the procedure for distributing gravitational wave candidate events to our astronomical partners. We give some details of the recent work that has been done to improve the MBTA pipeline and are now making preparations for the advanced detector era.
We use our newly constructed quantum Gibbs ensemble Monte Carlo algorithm to perform computer experiments for the two phase coexistence of a hydrogen-helium mixture. Our results are in quantitative agreement with the experimental results of C. M. Sneed, W. B. Streett, R. E. Sonntag, and G. J. Van Wylen. The difference between our results and the experimental ones is in all cases less than 15% relative to the experiment, reducing to less than 5% in the low helium concentration phase. At the gravitational inversion between the vapor and the liquid phase, at low temperatures and high pressures, the quantum effects become relevant. At extremely low temperature and pressure the first component to show superfluidity is the helium in the vapor phase.
We describe a simple computational model of cosmic logic suitable for analysis of, for example, discretized cosmological systems. The construction is based on a particular model of computation, developed by Alan Turing, with cosmic observers (CO), cosmic measures (CM) and cosmic symmetries (CS) described by Turing machines. CO machines always start with a blank tape and CM machines take CO's Turing number (also known as description number or G{\" o}del number) as input and output the corresponding probability. Similarly, CS machines take CO's Turing number as input, but output either one if the CO machines are in the same equivalence class or zero otherwise. We argue that CS machines are more fundamental than CM machines and, thus, should be used as building blocks in constructing CM machines. We prove the non-computability of a CS machine which discriminates between two classes of CO machines: mortal that halts in finite time and immortal that runs forever. In context of eternal inflation this result implies that it is impossible to construct CM machines to compute probabilities using cut-off prescriptions or that all of the cut-off measures are non-computable.
In this paper we perform stability analysis for exponential solutions in Einstein-Gauss-Bonnet and cubic Lovelock gravity. We report our findings, provide areas on parameters space and discuss familiarities and differences between cases. Analysis suggests that only several cases out of numerous found solutions could be called stable. In particular, cases with three-dimensional isotropic subspace which could give rise to successful compactification are diminished to one general case and one additional partial solution in the cubic Lovelock case.
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Intermediate mass protostarsprovide a bridge between theories of low- and high-mass star formation. Emerging molecular outflows can be used to determine the influence of fragmentation and multiplicity on protostellar evolution through the correlation of outflow forces of intermediate mass protostars with the luminosity. The aim of this paper is to derive outflow forces from outflows of six intermediate mass protostellar regions and validate the apparent correlation between total luminosity and outflow force seen in earlier work, as well as remove uncertainties caused by different methodology. By comparing CO 6--5 observations obtained with APEX with non-LTE radiative transfer model predictions, optical depths, temperatures, densities of the gas of the molecular outflows are derived. Outflow forces, dynamical timescales and kinetic luminosities are subsequently calculated. Outflow parameters, including the forces, were derived for all sources. Temperatures in excess of 50 K were found for all flows, in line with recent low-mass results. However, comparison with other studies could not corroborate conclusions from earlier work on intermediate mass protostars which hypothesized that fragmentation enhances outflow forces in clustered intermediate mass star formation. Any enhancement in comparison with the classical relation between outflow force and luminosity can be attributed the use of a higher excitation line and improvement in methods; They are in line with results from low-mass protostars using similar techniques. The role of fragmentation on outflows is an important ingredient to understand clustered star formation and the link between low and high-mass star formation. However, detailed information on spatial scales of a few 100 AU, covering all individual members is needed to make the necessary progress.
The presence of stellar companions around planet hosting stars influences the architecture of their planetary systems. To find and characterise these companions and determine their orbits is thus an important consideration to understand planet formation and evolution. For transiting systems even unbound field stars are of interest if they are within the photometric aperture of the light curve measurement. Then they contribute a constant flux offset to the transit light curve and bias the derivation of the stellar and planetary parameters if their existence is unknown. Close stellar sources are, however, easily overlooked by common planet surveys due to their limited spatial resolution. We therefore performed high angular resolution imaging of 49 transiting exoplanet hosts to identify unresolved binaries, characterize their spectral type, and determine their separation. The observations were carried out with the Calar Alto 2.2m telescope using the Lucky Imaging camera AstraLux Norte. All targets were imaged in i' and z' passbands. We found new companion candidates to WASP-14 and WASP-58, and we re-observed the stellar companion candidates to CoRoT-2, CoRoT-3, CoRoT-11, HAT-P-7, HAT-P-8, HAT-P-41, KIC 10905746, TrES-2, TrES-4, and WASP-2. We deduce from the stellar density around all sources that two companion candidates out of the targets with the first position measurement (CoRoT-11, HAT-P-41, KIC 10905746, WASP-14 and WASP-58) are probably unbound. In addition, we re-analyse the influence of the sources close to WASP-14 and WASP-58 on the planetary parameters given in the literature and find no significant changes.
Massive galaxy clusters are filled with a hot, turbulent and magnetized intra-cluster medium. Still forming under the action of gravitational instability, they grow in mass by accretion of supersonic flows. These flows partially dissipate into heat through a complex network of large-scale shocks [1], while residual transonic flows create giant turbulent eddies and cascades [2,3]. Turbulence heats the intra-cluster medium [4] and also amplifies magnetic energy by way of dynamo action [5-8]. However, the pattern regulating the transformation of gravitational energy into kinetic, thermal, turbulent and magnetic energies remains unknown. Here we report that the energy components of the intra-cluster medium are ordered according to a permanent hierarchy, in which the ratio of thermal to turbulent to magnetic energy densities remains virtually unaltered throughout the cluster's history, despite evolution of each individual component and the drive towards equipartition of the turbulent dynamo. This result revolves around the approximately constant efficiency of turbulence generation from the gravitational energy that is freed during mass accretion, revealed by our computational model of cosmological structure formation [3,9]. The permanent character of this hierarchy reflects yet another type of self-similarity in cosmology [10-13], while its structure, consistent with current data [14-18], encodes information about the efficiency of turbulent heating and dynamo action.
The cumulative comoving number-density of galaxies as a function of stellar mass or central velocity dispersion is commonly used to link galaxy populations across different epochs. By assuming that galaxies preserve their number-density in time, one can infer the evolution of their properties, such as masses, sizes, and morphologies. However, this assumption does not hold in the presence of galaxy mergers or when rank ordering is broken owing to variable stellar growth rates. We present an analysis of the evolving comoving number density of galaxy populations found in the Illustris cosmological hydrodynamical simulation focused on the redshift range $0\leq z \leq 3$. Our primary results are as follows: 1) The inferred average stellar mass evolution obtained via a constant comoving number density assumption is systematically biased compared to the merger tree results at the factor of $\sim$2(4) level when tracking galaxies from redshift $z=0$ out to redshift $z=2(3)$; 2) The median number density evolution for galaxy populations tracked forward in time is shallower than for galaxy populations tracked backward in time; 3) A similar evolution in the median number density of tracked galaxy populations is found regardless of whether number density is assigned via stellar mass, stellar velocity dispersion, or dark matter halo mass; 4) Explicit tracking reveals a large diversity in galaxies' assembly histories that cannot be captured by constant number-density analyses; 5) The significant scatter in galaxy linking methods is only marginally reduced by considering a number of additional physical and observable galaxy properties as realized in our simulation. We provide fits for the forward and backward median evolution in stellar mass and number density and discuss implications of our analysis for interpreting multi-epoch galaxy property observations.
We present a comprehensive study of the abundance of carbon dioxide in exoplanetary atmospheres. We construct analytical models of systems in chemical equilibrium that include carbon monoxide, carbon dioxide, water, methane and acetylene and relate the equilibrium constants of the chemical reactions to temperature and pressure via the tabulated Gibbs free energies. We prove that such chemical systems may be described by a quintic equation for the mixing ratio of methane. By examining the abundances of these molecules across a broad range of temperatures (spanning equilibrium temperatures from 600 to 2500 K), pressures (via temperature-pressure profiles that explore albedo and opacity variations) and carbon-to-oxygen ratios (from 0.1 to 100), we conclude that carbon dioxide is subdominant compared to carbon monoxide and water. Atmospheric mixing does not alter this conclusion if carbon dioxide is subdominant everywhere in the atmosphere. Carbon dioxide and carbon monoxide may attain comparable abundances if the metallicity is greatly enhanced, but this property is negated by temperatures above 1000 K. For hydrogen-dominated atmospheres, our generic result has the implication that retrieval studies need to set the subdominance of carbon dioxide as a prior of the calculation and not let its abundance completely roam free as a fitting parameter, because it directly affects the inferred value of the carbon-to-oxygen ratio and may produce unphysical conclusions. We discuss the relevance of these implications for the hot Jupiter WASP-12b and suggest that some of the previous results are chemically impossible. The relative abundance of carbon dioxide to acetylene is potentially a sensitive diagnostic of the carbon-to-oxygen ratio.
We present results on gas flows in the halo of a Milky Way-like galaxy at z=0.413 based on high-resolution spectroscopy of a background galaxy. This is the first study of circumgalactic gas at high spectral resolution towards an extended background source (i.e., a galaxy rather than a quasar). Using longslit spectroscopy of the foreground galaxy, we observe spatially extended H alpha emission with circular rotation velocity v=270 km/s. Using echelle spectroscopy of the background galaxy, we detect Mg II and Fe II absorption lines at impact parameter rho=27 kpc that are blueshifted from systemic in the sense of the foreground galaxy's rotation. The strongest absorber EW(2796) = 0.90 A has an estimated column density (N_H>10^19 cm-2) and line-of-sight velocity dispersion (sigma=17 km/s) that are consistent with the observed properties of extended H I disks in the local universe. Our analysis of the rotation curve also suggests that this r=30 kpc gaseous disk is warped with respect to the stellar disk. In addition, we detect two weak Mg II absorbers in the halo with small velocity dispersions (sigma<10 km/s). While the exact geometry is unclear, one component is consistent with an extraplanar gas cloud near the disk-halo interface that is co-rotating with the disk, and the other is consistent with a tidal feature similar to the Magellanic Stream. We can place lower limits on the cloud sizes (l>0.4 kpc) for these absorbers given the extended nature of the background source. We discuss the implications of these results for models of the geometry and kinematics of gas in the circumgalactic medium.
Subhalo abundance matching (SHAM) is a widely-used method to connect galaxies with dark matter structures in numerical simulations. SHAM predictions agree remarkably well with observations, yet they still lack strong theoretical support. Here we examine the performance, search for the best implementation, and analyse the key assumptions of SHAM using cosmological simulations from the EAGLE project. We find that $V_{\rm relax}$, the highest value of the circular velocity attained by a subhalo while it satisfies a relaxation criterion, is the subhalo property that correlates most strongly with galaxy stellar mass ($M_{\rm star}$). Using this parameter in SHAM, we retrieve the real-space clustering of EAGLE to within our statistical uncertainties on scales greater than $2$ Mpc for galaxies with $8.77<\log_{10}(M_{\rm star}[M_\odot])<10.77$. On the other hand, clustering is overestimated by $30\%$ on scales below $2$ Mpc because SHAM slightly overpredicts the fraction of satellites in massive haloes. The agreement is even better in redshift space, where the clustering in EAGLE is recovered to within our statistical uncertainties for all masses and separations. Additionally, we analyse the dependence of galaxy clustering on properties other than halo mass, i.e. the assembly bias. We demonstrate that assembly bias alters the clustering in EAGLE by $25\%$ and that $V_{\rm relax}$ captures its effect to within $15\%$. We trace the small but systematic difference in the predicted clustering of SHAM and EAGLE galaxies to the failure of a fundamental assumption of SHAM: for the same $V_{\rm relax}$, central and satellite subhaloes do not host statistically the same galaxies independently of the host halo mass.
(Abridged) We present results from 13776 radial-velocity (RV) measurements of
1278 candidate members of the old (4 Gyr) open cluster M67 (NGC 2682). The
measurements are the results of a long-term survey that includes data from
seven telescopes with observations for some stars spanning over 40 years. For
narrow-lined stars, RVs are measured with precisions ranging from about 0.1 to
0.8 km/s. The combined stellar sample reaches from the brightest giants in the
cluster down to about 4 magnitudes below the main-sequence turnoff (V = 16.5),
covering a mass range of about 1.34 MSun to 0.76 MSun. Spatially, the sample
extends to a radius of 30 arcmin (7.4 pc in projection at a distant of 850 pc
or 6-7 core radii). We find M67 to have a mean RV of +33.64 km/s (with an
internal precision of +/- 0.03 km/s). For stars with >=3 measurements, we
derive RV membership probabilities and identify RV variables, finding 562
cluster members, 142 of which show significant RV variability. We use these
cluster members to construct a color-magnitude diagram and identify a rich
sample of stars that lie far from the standard single star isochrone, including
the well-known blue stragglers, sub-subgiants and yellow giants. These exotic
stars have a binary frequency of (at least) 80%, more than three times that
detected for stars in the remainder of the sample. We confirm that the cluster
is mass segregated, finding the binaries to be more centrally concentrated than
the single stars in our sample at the 99.8% confidence level. The blue
stragglers are centrally concentrated as compared to the solar-type
main-sequence single stars in the cluster at the 99.7% confidence level.
Accounting for both measurement precision and undetected binaries, we derive a
RV dispersion in M67 of 0.59 +0.07 -0.06 km/s, which yields a virial mass for
the cluster of 2100 +610 -550 MSun.
WIYN Open Cluster Study. LXVII.
Efficient thermalization of overlapping supernovae within star-forming galaxies may produce a supernova-heated fluid that drives galactic winds. For fiducial assumptions about the timescale for Kelvin-Helmholz (KH) instabilities from high-resolution simulations (which neglect magnetic fields) we show that cool clouds with temperature from T_c ~ 10^2-10^4 K seen in emission and absorption in galactic winds cannot be accelerated to observed velocities by the ram pressure of a hot wind. Taking into account both the radial structure of the hot flow and gravity, we show that this conclusion holds over a wide range of galaxy, cloud, and hot wind properties. This finding calls into question the prevailing picture whereby the cool atomic gas seen in galactic winds is entrained and accelerated by the hot flow. Given these difficulties with ram pressure acceleration, we discuss alternative models for the origin of high velocity cool gas outflows. Another possibility is that magnetic fields in cool clouds are sufficiently important that they prolong the cloud's life. For T_c = 10^3 K and 10^4 K clouds, we show that if conductive evaporation can be neglected, the KH timescale must be ~ 10 and 3 times longer, respectively, than the values from hydrodynamical simulations in order for cool cloud velocities to reach those seen in observations.
The Siding Spring Southern Seyfert Spectroscopic Snapshot Survey (S7) is a targeted survey probing the narrow-line regions (NLRs) of a representative sample of ~140 nearby (z<0.02) Seyfert galaxies by means of optical integral field spectroscopy. The survey is based on a homogeneous data set observed using the Wide Field Spectrograph WiFeS. The data provide a 25x38 arcsec$^2$ field-of-view around the galaxy centre at typically ~1.5 arcsec spatial resolution and cover a wavelength range between ~3400 - 7100 $\AA$ at spectral resolutions of ~100 km s$^{-1}$ and ~50 km s$^{-1}$ in the blue and red parts, respectively. The survey is primarily designed to study gas excitation and star formation around AGN, with a special focus on the shape of the AGN ionising continuum, the interaction between radio jets and the NLR gas, and the nature of nuclear LINER emission. We provide an overview of the current status of S7-based results and present new results for NGC 6300.
The statistics of peaks in weak gravitational lensing maps is a promising technique to constrain cosmological parameters in present and future surveys. Here we investigate its power when using general extreme value statistics which is very sensitive to the exponential tail of the halo mass function. To this end, we use an analytic method to quantify the number of weak lensing peaks caused by galaxy clusters, large-scale structures and observational noise. Doing so, we further improve the method in the regime of high signal-to-noise ratios dominated by non-linear structures by accounting for the embedding of those counts into the surrounding shear caused by large scale structures. We derive the extreme value and order statistics for both over-densities (positive peaks) and under-densities (negative peaks) and provide an optimized criterion to split a wide field survey into sub-fields in order to sample the distribution of extreme values such that the expected objects causing the largest signals are mostly due to galaxy clusters. We find good agreement of our model predictions with a ray-tracing $N$-body simulation. For a Euclid-like survey, we find tight constraints on $\sigma_8$ and $\Omega_\text{m}$ with relative uncertainties of $\sim 10^{-3}$. In contrast, the equation of state parameter $w_0$ can be constrained only with a $10\%$ level, and $w_\text{a}$ is out of reach even if we include redshift information.
We present a Magellan/MIKE high-resolution (R ~ 35,000) spectrum of the ancient star SD 1313-0019 which has an iron abundance of [Fe/H] = -5.0, paired with a carbon enhancement of [C/Fe] ~ 3.0. The star was initially identified by Allende Prieto et al. in the BOSS survey. Its medium-resolution spectrum suggested a higher metallicity of [Fe/H] = -4.3 due to the CaII K line blending with a CH feature which is a common issue related to the search for the most iron-poor stars. This star joins several other, similar stars with [Fe/H] < -5.0 that all display a combination of low iron and high carbon abundances. Other elemental abundances of SD 1313-0019 follow that of more metal-rich halo stars. From fitting the abundance pattern with yields of Population III supernova, we conclude that SD 1313-0019 had only one massive progenitor star with 20 - 30 M_sun that must have undergone a mixing and fallback episode. Overall, there are now five stars known with [Fe/H] < -5.0 (1D LTE abundances). This population of second-generation stars strongly suggests massive first stars that almost exclusively produced large amounts of carbon through stellar winds and/or their mixing and fallback supernova explosions. As a consequence, their natal clouds -- presumably some early minihalo structures -- contained ample amounts of carbon and oxygen that likely facilitated the formation of these first low-mass stars.
We present a model where sterile neutrinos with rest masses in the range ~ keV to ~ MeV can be the dark matter and be consistent with all laboratory, cosmological, large scale structure, and X-ray constraints. These sterile neutrinos are assumed to freeze out of thermal and chemical equilibrium with matter and radiation in the very early universe, prior to an epoch of prodigious entropy generation ("dilution") from out-of-equilibrium decay of heavy particles. In this work, we consider heavy, entropy-producing particles in the ~ TeV to ~ EeV rest mass range, possibly associated with new physics at high energy scales. The process of dilution can give the sterile neutrinos the appropriate relic densities, but it also alters their energy spectra so that they could act like cold dark matter, despite relatively low rest masses as compared to conventional dark matter candidates. Moreover, since the model does not rely on active-sterile mixing for producing the relic density, the mixing angles can be small enough to evade current X-ray/lifetime constraints. Nevertheless, we discuss how future X-ray observations, future lepton number constraints, and future observations and sophisticated simulations of large scale structure could, in conjunction, provide evidence for this model and/or constrain and probe its parameters.
The Mars flyby of C/2013 A1 (Siding Spring) represented a unique opportunity for imaging a long-period comet and resolving its nucleus and rotation period. Because of the small encounter distance and the high relative velocity, the goal of successfully observing C/2013 A1 from the Mars orbiting spacecrafts posed strict accuracy requirements on the comet's ephemerides. These requirements were hard to meet, as comets are known for being highly unpredictable: astrometric observations can be significantly biased and nongravitational perturbations affect comet trajectories. Therefore, even prior to the encounter, we remeasured a couple of hundred astrometric images obtained with ground-based and Earth-orbiting telescopes. We also observed the comet with the Mars Reconnaissance Orbiter's High Resolution Imaging Science Experiment (HiRISE) camera on 2014 October 7. In particular, these HiRISE observations were decisive in securing the trajectory and revealed that out-of-plane nongravitational perturbations were larger than previously assumed. Though the resulting ephemeris predictions for the Mars encounter allowed observations of the comet from the Mars orbiting spacecrafts, post-encounter observations show a discrepancy with the pre-encounter trajectory. We reconcile this discrepancy by employing the Rotating Jet Model, which is a higher fidelity model for nongravitational perturbations and provides an estimate of C/2013 A1's spin pole.
We present Hubble Space Telescope observations of active asteroid 313P/Gibbs (formerly P/2014 S4) taken over the five month interval from 2014 October to 2015 March. This object has been recurrently active near perihelion (at 2.4 AU) in two different orbits, a property that is naturally explained by the sublimation of near surface ice but which is difficult to reconcile with other activity mechanisms. We find that the mass loss peaks near 1 kg s$^{-1}$ in October and then declines over the subsequent months by about a factor of five, at nearly constant heliocentric distance. This decrease is too large to be caused by the change in heliocentric distance during the period of observation. However, it is consistent with sublimation from an ice patch shadowed by local topography, for example in a pit like those observed on the nuclei of short-period comet 67P/Churyumov-Gerasimenko. While no unique interpretation is possible, a simple self shadowing model shows that sublimation from a pit with depth to diameter ratio near 1/2 matches the observed rate of decline of the activity, while deeper and shallower pits do not. We estimate the nucleus radius to be 700$\pm$100 m (geometric albedo 0.05 assumed). Measurements of the spatial distribution of the dust were obtained from different viewing geometries. They show that dust was ejected continuously not impulsively, that the effective particle size is large, $\sim$50 $\mu m$, and that the ejection speed is $\sim$2.5 m s$^{-1}$. The total dust mass ejected is $\sim$10$^7$ kg, corresponding to $\sim$10$^{-5}$ of the nucleus mass. The observations are consistent with partially shadowed sublimation from $\sim$10$^4$ m$^2$ of ice, corresponding to $\sim$0.2\% of the nucleus surface. For ice to survive in 313P for billion-year timescales requires that the duty cycle for sublimation be $\lesssim$10$^{-3}$.
A generic synchrotron external shock model is the widely preferred paradigm used to interpret the broad-band afterglow data of gamma-ray bursts (GRBs), including predicted observable signatures from a reverse shock which have been confirmed by observations. Investigations of the nature of the reverse shock emission can provide valuable insights into the intrinsic properties of the GRB ejecta. Here we briefly review the standard and the extended models of the reverse shock emission, discussing the connection between the theory and observations, including the implications of the latest observational advances.
The recent detection of a 3.5 keV X-ray line from the centres of galaxies and clusters by Bulbul et al. (2014a) and Boyarsky et al. (2014a) has been interpreted as emission from the decay of 7 keV sterile neutrinos which could make up the (warm) dark matter (WDM). As part of the COpernicus COmplexio (COCO) programme, we investigate the properties of dark matter haloes formed in a high-resolution cosmological $N$-body simulation from initial conditions similar to those expected in a universe in which the dark matter consists of 7 keV sterile neutrinos. This simulation and its cold dark matter (CDM) counterpart have $\sim13.4$bn particles, each of mass $\sim 10^5\, h^{-1} M_\odot$, providing detailed information about halo structure and evolution down to dwarf galaxy mass scales. Non-linear structure formation on small scales ($M_{200}\, \leq\, 2 \times 10^9~h^{-1}\,M_\odot$) begins slightly later in COCO-Warm than in COCO-Cold. The halo mass function at the present day in the WDM model begins to drop below its CDM counterpart at a mass $\sim 2 \times 10^{9}~h^{-1}\,M_\odot$ and declines very rapidly towards lower masses so that there are five times fewer haloes of mass $M_{200}= 10^{8}~h^{-1}\,M_\odot$ in COCO-Warm than in COCO-Cold. Halo concentrations on dwarf galaxy scales are correspondingly smaller in COCO-Warm, and we provide a simple functional form that describes its evolution with redshift. The shapes of haloes are similar in the two cases, but the smallest haloes in COCO-Warm rotate slightly more slowly than their CDM counterparts.
We present comprehensive X-ray point source catalogs of NGC~55, NGC~2403, and NGC~4214 as part of the Chandra Local Volume Survey. The combined archival observations have effective exposure times of 56.5 ks, 190 ks, and 79 ks for NGC~55, NGC~2403, and NGC~4214, respectively. When combined with our published catalogs for NGC 300 and NGC 404, our survey contains 629 X-ray sources total down to a limiting unabsorbed luminosity of $\sim5\times10^{35}$ erg s$^{-1}$ in the 0.35-8 keV band in each of the five galaxies. We present X-ray hardness ratios, spectral analysis, radial source distributions, and an analysis of the temporal variability for the X-ray sources detected at high significance. To constrain the nature of each X-ray source, we carried out cross-correlations with multi-wavelength data sets. We searched overlapping Hubble Space Telescope observations for optical counterparts to our X-ray detections to provide preliminary classifications for each X-ray source as a likely X-ray binary, background AGN, supernova remnant, or foreground star.
We investigate a scalar field dark energy model (i.e., $\phi$CDM model) with massive neutrinos, where the scalar field possesses an inverse power-law potential, i.e., $V(\phi)\propto {\phi}^{-\alpha}$ ($\alpha>0$). We find that the sum of neutrino masses $\Sigma m_{\nu}$ and the parameter $\alpha$ both have significant impacts on the CMB temperature power spectrum and on the matter power spectrum. A joint sample, including CMB data from Planck 2013 and WMAP9, galaxy clustering data from WiggleZ and BOSS DR11, and JLA compilation of Type Ia supernova observations, is adopted to confine the parameters. Within the context of the $\phi$CDM model under consideration, the joint sample determines the cosmological parameters to high precision. It turns out that $\alpha < 3.785$ at 95% CL for the $\phi$CDM model. And yet, the $\Lambda$CDM scenario corresponding to $\alpha = 0$ is not ruled out at 95% CL. Moreover, we get $\Sigma m_{\nu}< 0.046$ eV at 95% CL for the $\phi$CDM model, while the corresponding one for the $\Lambda$CDM model is $\Sigma m_{\nu} < 0.293$ eV. Obviously, the allowed scale of $\Sigma m_\nu$ in the $\phi$CDM model is greatly smaller than that in the $\Lambda$CDM model. It is consistent with the qualitative analysis, which reveals that the increases of $\alpha$ and $\Sigma m_\nu$ both can result in the suppression of the matter power spectrum. As a consequence, when $\alpha$ is larger, in order to avoid suppressing the matter power spectrum too much, the value of $\Sigma m_\nu$ should be smaller.
We investigate quasi-periodic coronal pulsations during the decay phase of an X 3.2 class flare on 14 May 2013, using soft X-ray data from the RHESSI satellite. Periodogram analyses of soft X-ray light curves show that 53 s and 72 s periods co-exist in the 3-6, 6-12 and 12-25 KeV energy bands. Considering the typical length of the flaring loop system and observed periodicities, we find that they are associated with multiple (first two harmonics) of fast magnetoacoustic sausage waves. The phase relationship of soft X-ray emissions in different energy bands using cross-correlation technique show that these modes are standing in nature as we do not find the phase lag. Considering the period ratio, we diagnose the local plasma conditions of the flaring region by invoking MHD seismology. The period ratio P1/2P2 is found to be 0.65, which indicates that such oscillations are most likely excited in longitudinal density stratified loops.
Zonal and meridional axisymmetric flows can deeply impact the rotational and chemical evolution of stars. Therefore, momentum exchanges between waves propagating in stars, differential rotation, and meridional circulation must be carefully evaluated. In this work, we study axisymmetric mean flows in rapidly and initially uniformly rotating massive stars driven by small amplitude non-axisymmetric $\kappa$-driven oscillations. We treat them as perturbations of second-order of the oscillation amplitudes and derive their governing equations as a set of coupled linear ordinary differential equations. This allows us to compute 2-D zonal and meridional mean flows driven by low frequency $g$- and $r$-modes in slowly pulsating B (SPB) stars and $p$-modes in $\beta$ Cephei stars. Oscillation-driven mean flows usually have large amplitudes only in the surface layers. In addition, the kinetic energy of the induced 2-D zonal rotational motions is much larger than that of the meridional motions. In some cases, meridional flows have a complex radial and latitudinal structure. For SPB stars, we find that there are low frequency retrograde $g$-modes and $r$-modes that drive mean flows with positive velocity components in the radial and azimuthal directions at the stellar surface. This suggests that low frequency retrograde modes can transport angular momentum to the surface layers, which may help to form a circumstellar gaseous disc. Moreover, pulsation-driven and rotation-driven meridional flows can have similar amplitudes. These results show the importance of taking wave-- mean flow interactions into account when studying the evolution of massive stars.
Based on SDSS g, r and SCUSS (South Galactic Cap of u-band Sky Survey) $u$ photometry, we develop a photometric calibration for estimating the stellar metallicity from $u-g$ and $g-r$ colors by using the SDSS spectra of 32,542 F- and G-type main sequence stars, which cover almost $3700$ deg$^{2}$ in the south Galactic cap. The rms scatter of the photometric metallicity residuals relative to spectrum-based metallicity is $0.14$ dex when $g-r<0.4$, and $0.16$ dex when $g-r>0.4$. Due to the deeper and more accurate magnitude of SCUSS $u$ band, the estimate can be used up to the faint magnitude of $g=21$. This application range of photometric metallicity calibration is wide enough so that it can be used to study metallicity distribution of distant stars. In this study, we select the Sagittarius (Sgr) stream and its neighboring field halo stars in south Galactic cap to study their metallicity distribution. We find that the Sgr stream at the cylindrical Galactocentric coordinate of $R\sim 19$ kpc, $\left| z\right| \sim 14$ kpc exhibits a relative rich metallicity distribution, and the neighboring field halo stars in our studied fields can be modeled by two-Gaussian model, with peaks respectively at [Fe/H]$=-1.9$ and [Fe/H]$=-1.5$.
The W3 GMC is a prime target for investigating the formation of high-mass stars and clusters. This second study of W3 within the HOBYS Key Program provides a comparative analysis of subfields within W3 to further constrain the processes leading to the observed structures and stellar population. Probability density functions (PDFs) and cumulative mass distributions (CMDs) were created from dust column density maps, quantified as extinction Av. The shape of the PDF, typically represented with a lognormal function at low Av "breaking" to a power-law tail at high Av, is influenced by various processes including turbulence and self-gravity. The breaks can also be identified, often more readily, in the CMDs. The PDF break from lognormal (Av(SF)= 6-10 mag) appears to shift to higher Av by stellar feedback, so that high-mass star-forming regions tend to have higher PDF breaks. A second break at Av > 50 mag traces structures formed or influenced by a dynamic process. Because such a process has been suggested to drive high-mass star formation in W3, this second break might then identify regions with potential for hosting high-mass stars/clusters. Stellar feedback appears to be a major mechanism driving the local evolution and state of regions within W3. A high initial star formation efficiency in a dense medium could result in a self-enhancing process, leading to more compression and favourable star-formation conditions (e.g., colliding flows), a richer stellar content, and massive stars. This scenario would be compatible with the "convergent constructive feedback" model introduced in our previous Herschel study.
We want to get insight into the nature, i.e. the formation mechanism and the evolution, of UGC 7639, a dwarf galaxy in the Canes Venatici I Cloud (CVnIC). We used archival GALEX (FUV and NUV) and SDSS images, as well as Hyperleda and NED databases, to constrain its global properties. GALEX FUV/NUV images show that UGC 7639 inner regions are composed mostly by young stellar populations. In addition, we used smoothed particle hydrodynamics (SPH) simulations with chemo-photometric implementation to account for its formation and evolution. UGC 7639 is an example of blue dwarf galaxy whose global properties are well matched by our multi-wavelength and multi-technique approach, that is also a suitable approach to highlight the evolution of these galaxies as a class. We found that the global properties of UGC 7639, namely its total absolute B-band magnitude, its whole spectral energy distribution (SED), and its morphology are well-matched by an encounter with a system four times more massive than our target. Moreover, the current star formation rate (SFR) of the simulated dwarf, ~0.03 M_sun yr-1, is in good agreement with our UV-based estimate. For UGC 7639, we estimated a galaxy age of 8.6 Gyr. Following our simulation, the ongoing star formation will extinguish within 1.6 Gyr, thus leaving a red dwarf galaxy.
The Second Planck Catalogue of Compact Sources is a catalogue of sources detected in single-frequency maps from the full duration of the Planck mission and supersedes previous versions of the Planck compact source catalogues. It consists of compact sources, both Galactic and extragalactic, detected over the entire sky. Compact sources detected in the lower frequency channels are assigned to the PCCS2, while at higher frequencies they are assigned to one of two sub-catalogues, the PCCS2 or PCCS2E, depending on their location on the sky. The first of these catalogues covers most of the sky and allows the user to produce subsamples at higher reliabilities than the target 80% integral reliability of the catalogue. The PCCS2E contains sources detected in sky regions where the diffuse emission makes it difficult to quantify the reliability of the detections. Both the PCCS2 and PCCS2E include polarization measurements, in the form of polarized flux densities, or upper limits, and orientation angles for all seven polarization-sensitive Planck channels. The improved data-processing of the full-mission maps and their reduced noise levels allow us to increase the number of objects in the catalogue, improving its completeness for the target 80 % reliability as compared with the previous versions, the PCCS and ERCSC catalogues.
We have applied the graphical user interfaced close binary system analysis program WinFitter to an intensive study of Kepler 91 using all the available photometry from the NASA Exoplanet Archive (NEA) at the Caltech website: this http URL Our fitting function for the tidal distortion derives from the relevant Radau equation and includes terms up to the fifth power of the fractional radius. This results in a systematic improvement in the mass ratio estimation over that of Lillo Box et al (2014a) and our derived value for the mass ratio is in close agreement with that inferred from recent high-resolution spectroscopic data. It is clear that the data analysis in terms of simply an eclipsing binary system is compromised by the presence of significant other causes of light variation, in particular non-radial pulsations. We apply a low-frequency filtering procedure to separate out some of this additional light variation. Whilst the derived eccentricity appears then reduced, an eccentric effect remains in the light curve. We consider how this may be maintained in spite of likely frictional effects operating over a long time. There are also indications of the possible effects of Trojan or other period-resonant mass concentrations. Suggestions of a possible secular period variation are briefly discussed.
Dark matter only simulations of galaxy formation predict many more subhalos around a Milky Way like galaxy than the number of observed satellites. Proposed solutions require the satellites to inhabit dark matter halos with masses between one to ten billion solar masses at the time they fell into the Milky Way. Here we use a modelling approach, independent of cosmological simulations, to obtain a preinfall mass of 360 (+380,-230) million solar masses for one of the Milky Way's satellites: Carina. This determination of a low halo mass for Carina can be accommodated within the standard model only if galaxy formation becomes stochastic in halos below ten billion solar masses. Otherwise Carina, the eighth most luminous Milky Way dwarf, would be expected to inhabit a significantly more massive halo. The implication of this is that a population of "dark dwarfs" should orbit the Milky Way: halos devoid of stars and yet more massive than many of their visible counterparts.
Astrometric surveys such as Gaia and LSST will measure parallaxes for hundreds of millions of stars. Yet they will not measure a single distance. Rather, a distance must be estimated from a parallax. In this didactic article, I show that doing this is not trivial once the fractional parallax error is larger than about 20%, which will be the case for about 80% of stars in the Gaia catalogue. Estimating distances is an inference problem in which the use of prior assumptions is unavoidable. I investigate the properties and performance of various priors and examine their implications. A supposed uninformative uniform prior in distance is shown to give very poor distance estimates (large bias and variance). Any prior with a sharp cut-off at some distance has similar problems. The choice of prior depends on the information one has available - and is willing to use - concerning, for example, the survey and the Galaxy. I demonstrate that a simple prior which decreases asymptotically to zero at infinite distance has good performance, accommodates non-positive parallaxes, and does not require a bias correction.
Atmospheric refraction affects to various degrees exoplanet transit, lunar eclipse, as well as stellar occultation observations. Exoplanet retrieval algorithms often use analytical expressions for the column abundance along a ray traversing the atmosphere as well as for the deflection of that ray, which are first order approximations valid for low densities in a spherically symmetric homogeneous isothermal atmosphere. We derive new analytical formulae for both of these quantities, which are valid for higher densities, and use them to refine and validate a new ray tracing algorithm which can be used for arbitrary atmospheric temperature-pressure profiles. We illustrate with simple isothermal atmospheric profiles the consequences of our model for different planets: temperate Earth-like and Jovian-like planets, as well as HD189733b, and GJ1214b. We find that, for both hot exoplanets, our treatment of refraction does not make much of a difference to pressures as high as 10 atmosphere, but that it is important to consider the variation of gravity with altitude for GJ1214b. However, we find that the temperate atmospheres have an apparent scale height significantly smaller than their actual density scale height at densities larger than 1 amagat, thus increasing the difficulty of detecting spectral features originating in these regions. These denser atmospheric regions form a refractive boundary layer where column abundances and ray deflection increases dramatically with decreasing impact parameter. This refractive boundary layer mimics a surface, and none of the techniques mentioned above can probe atmospheric regions denser than about 4 amagat on these temperate planets.
Two-ribbon brightenings are one of the most remarkable characteristics of an eruptive solar flare and are often used for predicting the occurrence of coronal mass ejections (CMEs). Nevertheless, it was called in question recently whether all two-ribbon flares are eruptive. In this paper, we investigate a two ribbon-like white-light (WL) flare that is associated with a failed magnetic flux rope (MFR) eruption on 2015 January 13, which has no accompanying CME in the WL coronagraph. Observations by \textit{Optical and Near-infrared Solar Eruption Tracer} and \textit{Solar Dynamics Observatory} reveal that, with the increase of the flare emission and the acceleration of the unsuccessfully erupting MFR, two isolated kernels appear at the WL 3600 {\AA} passband and quickly develop into two elongated ribbon-like structures. The evolution of the WL continuum enhancement is completely coincident in time with the variation of \textit{Fermi} hard X-ray 26--50 keV flux. Increase of continuum emission is also clearly visible at the whole FUV and NUV passbands observed by \textit{Interface Region Imaging Spectrograph}. Moreover, in one WL kernel, the \ion{Si}{4}, \ion{C}{2}, and \ion{Mg}{2} h/k lines display significant enhancement and non-thermal broadening. However, their Doppler velocity pattern is location-dependent. At the strongly bright pixels, these lines exhibit a blueshift; while at moderately bright ones, the lines are generally redshifted. These results show that the failed MFR eruption is also able to produce a two-ribbon flare and high-energy electrons that heat the lower atmosphere, causing the enhancement of the WL and FUV/NUV continuum emissions and chromospheric evaporation.
The character of star formation is intimately related to the supersonic
magnetohydrodynamic (MHD) turbulent dynamics of the giant molecular clouds in
which stars form. A significant amount of the turbulent energy dissipates in
low-velocity shock waves. These shocks cause molecular line cooling of the
compressed and heated gas, and so their radiative signatures probe the nature
of the turbulence. In MHD fluids the three distinct families of shocks---fast,
intermediate and slow---differ in how they compress and heat the molecular gas,
and so observational differences between them may also distinguish driving
modes of turbulent regions.
Here we use a two-fluid model to compare the characteristics of
one-dimensional fast and slow MHD shocks propagating at low speeds (a few km/s)
in molecular clouds. Fast MHD shocks are magnetically driven, forcing ion
species to stream through the neutral gas ahead of the shock front. This
magnetic precursor heats the gas sufficiently to create a large, warm
transition zone where all the fluid variables smoothly change in the shock
front. In contrast, slow MHD shocks are driven by gas pressure, and neutral
species collide with ion species in a thin hot slab that closely resembles an
ordinary gas dynamic shock.
We computed observational diagnostics for fast and slow shocks at velocities
$v_s = 2-4$ km/s and preshock Hydrogen nuclei densities $n_H = 10^2-10^4$
cm$^{-3}$. We followed the abundances of molecules relevant for a simple oxygen
chemistry and include cooling by CO, H$_2$ and H$_2$O. Estimates of intensities
of CO rotational lines show that high-$J$ lines, above $J = 6 \to 5$, are more
strongly excited in slow MHD shocks. We show that these slow shock signatures
may have already been observed in strong high-$J$ CO lines from infrared dark
clouds in the Milky Way.
We study dynamics of $\Lambda(t)$ cosmological models which are a natural
generalization of the standard cosmological model (the $\Lambda$CDM model). We
consider a class of models: the ones with a prescribed form of
$\Lambda(t)=\Lambda_{\text{bare}}+\frac{\alpha^2}{t^2}$. This type of a
$\Lambda(t)$ parametrization is motivated by different cosmological approaches.
To guarantee the covariance principle in general relativity we interpreted
$\Lambda(t)$ relation as $\Lambda(\phi(t))$, where $\phi(t)$ is a scalar field
with a self-interacting potential $V(\phi)$. For the $\Lambda(t)$ cosmology
with a prescribed form of $\Lambda(t)$ we have found the exact solution in the
form of Bessel functions.
We have also constrained the model parameters for this class of models using
the astronomical data such as SNIa data, BAO, CMB, measurements of $H(z)$ and
the Alcock-Paczy{\'n}ski test. In this context we formulate a simple criterion
of variability of $\Lambda$ with respect to $t$ in terms of variability of the
jerk or sign of estimator $(1-\Omega_{\text{m},0}-\Omega_{\Lambda,0})$. For all
fit results, we have find that the value of $\alpha^2 = 0$, measuring deviation
from $\Lambda$CDM model is always consistent with data.
AIMS: In their 1st extension to the Milky Way Star Clusters (MWSC) survey, Schmeja et al. applied photometric filters to the 2MASS to find new cluster candidates that were subsequently confirmed or rejected by the MWSC pipeline. To further extend the MWSC census, we aimed at discovering new clusters by conducting an almost global search in proper motion catalogues as a starting point. METHODS: We first selected high-quality samples from the PPMXL and UCAC4 for comparison and verification of the proper motions. For 441 circular proper motion bins (radius 15 mas/yr) within $\pm$50 mas/yr, the sky outside a thin Galactic plane zone ($|b|$$<$5$^{\circ}$) was binned in small areas ('sky pixels') of 0.25$\times$0.25 deg$^2$. Sky pixels with enhanced numbers of stars with a certain common proper motion in both catalogues were considered as cluster candidates. After visual inspection of the sky images, we built an automated procedure that combined these representations of the sky for neighbouring proper motion subsamples after a background correction. RESULTS: About half of our 692 candidates overlapped with known clusters (46 globular and 68 open clusters in the Galaxy, about 150 known clusters of galaxies) or the Magellanic Clouds. About 10% of our candidates turned out to be 63 new open clusters confirmed by the MWSC pipeline. They occupy predominantly the two inner Galactic quadrants and have apparent sizes and numbers of high-probable members slightly larger than those of the typically small MWSC clusters, whereas their other parameters (ages, distances, tidal radii) fall in the typical ranges. As our search aimed at finding compact clusters, we did not find new very nearby (extended) clusters. (abridged)
In this work we have estimated upper and lower limits to the strength of the magnetic dipole moment of all 14 accreting millisecond X-ray pulsars observed with the Rossi X-ray Timing Explorer (RXTE). For each source we searched the archival RXTE data for the highest and lowest flux levels with a significant detection of pulsations. We assume these flux levels to correspond to the closest and farthest location of the inner edge of the accretion disk at which channelled accretion takes place. By estimating the accretion rate from the observed luminosity at these two flux levels, we place upper and lower limits on the magnetic dipole moment of the neutron star, using assumptions from standard magnetospheric accretion theory. Finally, we discuss how our field strength estimates can be further improved as more information on these pulsars is obtained.
Gamma-ray bursts (GRBs) are believed to be powered by the electromagnetic extraction of spin energy from a black hole endowed with a magnetic field supported by electric currents in a surrounding disk (Blandford & Znajek 1977). A generic feature of this mechanism is that, under certain fairly general assumptions, the energy loss rate decays exponentially. In this work, we are looking precisely for such exponential decay in the lightcurves of long duration GRBs observed with the XRT instrument on the Swift satellite. We found out that almost 30 % of XRT lightcurves show such behavior before they reach the afterglow plateau. According to Blandford & Znajek, the duration of the burst depends on the magnetic flux accumulated on the event horizon. This allows us to estimate the surface magnetic field of a possible progenitor. Our estimations are consistent with magnetic fields observed in Wolf-Rayet stars.
Novae have been reported as transients for more than two thousand years. Their bright optical outbursts are the result of explosive nuclear burning of gas accreted from a binary companion onto a white dwarf. Novae containing a white dwarf close to the Chandrasekhar mass limit and accreting at a high rate are potentially the unknown progenitors of the type Ia supernovae used to measure the acceleration of the Universe. Swift X-ray observations have radically transformed our view of novae by providing dense monitoring throughout the outburst, revealing new phenomena in the super-soft X-rays from the still-burning white dwarf such as early extreme variability and half- to one-minute timescale quasi-periodic oscillations. The distinct evolution of this emission from the harder X-ray emission due to ejecta shocks has been clearly delineated. Soft X-ray observations allow the mass of the white dwarf, the mass burned and the mass ejected to be estimated. In combination with observations at other wavelengths, including the high spectral resolution observations of the large X-ray observatories, high resolution optical and radio imaging, radio monitoring, optical spectroscopy, and the detection of GeV gamma-ray emission from recent novae, models of the explosion have been tested and developed. I review nine novae for which Swift has made a significant impact; these have shown the signature of the components in the interacting binary system in addition to the white dwarf: the re-formed accretion disk, the companion star and its stellar wind.
We measure the angular two-point correlation and angular power spectrum from the NRAO VLA Sky Survey (NVSS) of radio galaxies. Contrary to previous claims in the literature, we show that it is consistent with primordial Gaussianity on all angular scales and it is consistent with the best-fit cosmological model from the Planck analysis, as well as the redshift distribution obtained from the Combined EIS-NVSS Survey Of Radio Sources (CENSORS). Our analysis is based on an optimal estimation of the two-point correlation function and makes use of a new mask, which takes into account direction dependent effects of the observations, side lobe effects of bright sources and galactic foreground. We also use a lower flux threshold and take the cosmic radio dipole into account. The latter turns out to be an essential step in the analysis. This improved cosmological analysis of the NVSS stresses the importance of a flux calibration that is robust and stable on large angular scales for future radio continuum surveys.
Nuclear stellar discs (NSDs) can help to constrain the assembly history of their host galaxies, as long as we can assume them to be fragile structures that are disrupted during merger events. In this work we investigate the fragility of NSDs by means of N-body simulations reproducing the last phases of a galaxy encounter, when the nuclear regions of the two galaxies merge. For this, we exposed a NSD set in the gravitational potential of the bulge and supermassive black hole of a primary galaxy to the impact of the supermassive black hole from a secondary galaxy. We explored merger events of different mass ratios, from major mergers with a 1:1 mass ratio to intermediate and minor interactions with 1:5 and 1:10 ratios, while considering various impact geometries. We analyse the end results of such mergers from different viewing angles and looked for possible photometric and kinematic signatures of the presence of a disc in the remnant surface density and velocity maps, while adopting detection limits from real observations. Our simulations show that indeed NSDs are fragile against major mergers, which leave little trace of NSDs both in images and velocity maps, while signatures of a disc can be found in the majority of the intermediate to minor-merger remnants and in particular when looking at their kinematics. These results show that NSDs could allow to distinguish between these two modes of galaxy assembly, which may indeed pertain to different kinds of galaxies or galactic environments.
Standing sausage modes are important in interpreting quasi-periodic pulsations in the lightcurves of solar flares. Their periods and damping times play an important role in seismologically diagnosing key parameters like the magnetic field strength in regions where flare energy is released. Usually such applications are based on theoretical results neglecting unresolved fine structures in magnetized loops. However, the existence of fine structuring is suggested on both theoretical and observational grounds. Adopting the framework of cold magnetohydrodynamics (MHD), we model coronal loops as magnetized cylinders with a transverse equilibrium density profile comprising a monolithic part and a modulation due to fine structuring in the form of concentric shells. The equation governing the transverse velocity perturbation is solved with an initial-value-problem approach, and the effects of fine structuring on the periods $P$ and damping times $\tau$ of global, leaky, standing sausage modes are examined. A parameter study shows that fine structuring, be it periodically or randomly distributed, brings changes of only a few percent to $P$ and $\tau$ when there are more than about ten shells. The monolithic part, its steepness in particular, plays a far more important role in determining $P$ and $\tau$. We conclude that when measured values of $P$ and $\tau$ of sausage modes are used for seismological purposes, it is justified to use theoretical results where the effects due to fine structuring are neglected.
We present the X-shooter Lens Survey (XLENS) data. The main goal of XLENS is to disentangle the stellar and dark matter content of massive early-type galaxies (ETGs), through combined strong gravitational lensing, dynamics and spectroscopic stellar population studies. The sample consists of 11 lens galaxies covering the redshift range from $0.1$ to $0.45$ and having stellar velocity dispersions between $250$ and $380\,\mathrm{km}\,\mathrm{s}^{-1}$. All galaxies have multi-band, high-quality HST imaging. We have obtained long-slit spectra of the lens galaxies with X-shooter on the VLT. We are able to disentangle the dark and luminous mass components by combining lensing and extended kinematics data-sets, and we are also able to precisely constrain stellar mass-to-light ratios and infer the value of the low-mass cut-off of the IMF, by adding spectroscopic stellar population information. Our goal is to correlate these IMF parameters with ETG masses and investigate the relation between baryonic and non-baryonic matter during the mass assembly and structure formation processes. In this paper we provide an overview of the survey, highlighting its scientific motivations, main goals and techniques. We present the current sample, briefly describing the data reduction and analysis process, and we present the first results on spatially resolved kinematics.
We present the results of ammonia observations towards 66 massive star forming regions identified by the Red MSX source survey. We have used the Green Bank Telescope and the K-band focal plane array to map the ammonia NH3 (1,1) and (2,2) inversion emission at a resolution of 30 arcsec in 8 arcmin regions towards the positions of embedded massive star formation. We have identified a total of 115 distinct clumps, approximately two-thirds of which are associated with an embedded massive young stellar object or compact HII region, while the others are classified as quiescent. There is a strong spatial correlation between the peak NH3 emission and the presence of embedded objects. We derive the spatial distribution of the kinetic gas temperatures, line widths, and NH$_3$ column densities from these maps, and by combining these data with dust emission maps we estimate clump masses, H$_2$ column densities and ammonia abundances. The clumps have typical masses of ~1000 Msun and radii ~0.5 pc, line widths of ~2 km/s and kinetic temperatures of ~16-20 K. We find no significant difference between the sizes and masses of the star forming and quiescent subsamples; however, the distribution maps reveal the presence of temperature and line width gradients peaking towards the centre for the star forming clumps while the quiescent clumps show relatively uniform temperatures and line widths throughout. Virial analysis suggests that the vast majority of clumps are gravitationally bound and are likely to be in a state of global free fall in the absence of strong magnetic fields. The similarities between the properties of the two subsamples suggest that the quiescent clumps are also likely to form massive stars in the future, and therefore provide a excellent opportunity to study the initial conditions of massive pre-stellar and protostellar clumps.
We study the mapping from Lagrangian to Eulerian space in the context of the Effective Field Theory (EFT) of Large Scale Structure. We compute Lagrangian displacements with Lagrangian Perturbation Theory (LPT) and perform the full non-perturbative transformation from displacement to density. When expanded up to a given order, this transformation reproduces the standard Eulerian Perturbation Theory (SPT) at the same order. However, the full transformation from displacement to density also includes higher order terms. These terms explicitly resum long wavelength motions, thus making the resulting density field better correlated with the true non-linear density field. As a result, the regime of validity of this approach is expected to extend that of the Eulerian EFT, and match that of the IR-resummed Eulerian EFT. This approach thus effectively enables a test of the IR-resummed EFT at the field level. We estimate the size of stochastic, non-perturbative contributions to the matter density power spectrum. We find that in our highest order calculation, at redshift z=0 the power spectrum of the density field is reproduced with an accuracy of 1 % (10 %) up to k=0.25 h/Mpc (k=0.46 h/Mpc). We believe that the dominant source of the remaining error is the stochastic contribution. Unfortunately, on these scales the stochastic term does not yet scale as $k^4$ as it does in the very low-k regime. Thus, modeling this contribution might be challenging.
We study the Effective Field Theory of Large Scale Structure for cosmic density and momentum fields. We show that the finite part of the two-loop calculation and its counterterms introduce an apparent scale dependence for the leading order parameter $c_\text{s}^2$ of the EFT starting at k=0.1 h/Mpc. These terms limit the range over which one can trust the one-loop EFT calculation at the 1 % level to k<0.1 h/Mpc at redshift z=0. We construct a well motivated one parameter ansatz to fix the relative size of the one- and two-loop counterterms using their high-k sensitivity. Although this one parameter model is a very restrictive choice for the counterterms, it explains the apparent scale dependence of $c_\text{s}^2$ seen in simulations. It is also able to capture the scale dependence of the density power spectrum up to k$\approx$ 0.3 h/Mpc at the 1 % level at redshift $z=0$. Considering a simple scheme for the resummation of large scale motions, we find that the two loop calculation reduces the need for this IR-resummation at k<0.2 h/Mpc. Finally, we extend our calculation to momentum statistics and show that the same one parameter model can also describe density-momentum and momentum-momentum statistics.
We report the discovery of three large (R29 >~ 1 arcminute) extremely low surface brightness (mu_(V,0) ~ 27.0) galaxies identified using our deep, wide-field imaging of the Virgo Cluster from the Burrell Schmidt telescope. Complementary data from the Next Generation Virgo Cluster Survey do not resolve red giant branch stars in these objects down to i=24, yielding a lower distance limit of 2.5 Mpc. At the Virgo distance, these objects have half-light radii 3-10 kpc and luminosities L_V=2-9x10^7 Lsun. These galaxies rival the most extreme LSB galaxies recently identified in the Coma cluster and are located well within Virgo's virial radius; two are projected directly on the cluster core. One object appears to be a nucleated LSB in the process of being tidally stripped to form a new Virgo ultracompact dwarf galaxy. The others show no sign of tidal disruption, despite the fact that such objects should be most vulnerable to tidal destruction in the cluster environment. The relative proximity of Virgo makes these objects amenable to detailed studies of their structural properties and stellar populations. They thus provide an important new window onto the connection between cluster environment and galaxy evolution at the extremes.
We investigate even-even two-proton borromean systems at prominent intermediate heavy waiting points for the rapid proton capture process. The most likely single-particle levels are used to calculate three-body energy and structure as a function of proton-core resonance energy. We establish a linear dependence between two- and three-body energies with the same slope, but the absolute value slightly dependent on partial wave structure. Using these relations we predict low-lying excited states in the isotones following the critical waiting points. The capture rate for producing a borromean bound state is described both based on a full three-body calculation and on a very simple analytic rate expression for temperatures about $1-5$~GK. This rate is valid for both direct and sequential capture paths, and it only depends on the three-body resonance energy. As a result the relevant path of the radiative capture reactions can be determined. We present numerical results for $E1$ and $E2$ photon emission, and discuss occurrence preferences in general as well as relative sizes of these most likely processes. Finally, we present narrow estimated intervals for the proton capture rates relevant for the critical waiting points.
The quasinormal resonance spectrum $\{\omega_n(\mu,q,M,Q)\}_{n=0}^{n=\infty}$ of charged massive scalar fields in the charged Reissner-Nordstr\"om black-hole spacetime is studied {\it analytically} in the large-coupling regime $qQ\gg M\mu$ (here $\{\mu, q\}$ are respectively the mass and charge coupling constant of the field, and $\{M,Q\}$ are respectively the mass and electric charge of the black hole). This physical system provides a striking illustration for the validity of the universal relaxation bound $\tau \times T \geq \hbar/\pi$ in black-hole physics (here $\tau\equiv 1/\Im\omega_0$ is the characteristic relaxation time of the composed black-hole-scalar-field system, and $T$ is the Bekenstein-Hawking temperature of the black hole). In particular, it is shown that the relaxation dynamics of charged massive scalar fields in the charged Reissner-Nordstr\"om black-hole spacetime may {\it saturate} this quantum time-times-temperature inequality. Interestingly, we prove that potential violations of the bound by light scalar fields are excluded by the Schwinger-type pair-production mechanism (a vacuum polarization effect), a {\it quantum} phenomenon which restricts the physical parameters of the composed black-hole-charged-field system to the regime $qQ\ll M^2\mu^2/\hbar$.
In recent years Planck-scale modifications of the dispersion relation have been attracting increasing interest also from the viewpoint of possible applications in astrophysics and cosmology, where spacetime curvature cannot be neglected. Nonetheless the interplay between Planck-scale effects and spacetime curvature is still poorly understood, particularly in cases where curvature is not constant. These challenges have been so far postponed by relying on an ansatz, first introduced by Jacob and Piran. We here propose a general strategy of analysis of the effects of modifications of dispersion relation in FRW spacetimes, applicable both to cases where the relativistic equivalence of frames is spoiled ("preferred-frame scenarios") and to the alternative possibility of "DSR-relativistic theories", theories that are fully relativistic but with relativistic laws deformed so that the modified dispersion relation is observer independent. We show that the Jacob-Piran ansatz implicitly assumes that spacetime translations are not affected by the Planck-scale, while under rather general conditions the same Planck-scale quantum-spacetime structures producing modifications of the dispersion relation also affect translations. Through the explicit analysis of one of the effects produced by modifications of the dispersion relation, an effect amounting to Planck-scale corrections to travel times, we show that our concerns are not merely conceptual but rather can have significant quantitative implications.
Evolution of density and metric perturbations in the background of high frequency oscillations of curvature in F(R) gravity is considered. In addition to the usual Jeans-like instability new effects of amplification of perturbations, associated with parametric resonance and antifriction phenomena, are found.
There is no doubt that the field of Fundamental Constants in Physics and Their Time Variation is one of the hottest subjects in modern theoretical and experimental physics, with potential implications in all fundamental areas of physics research, such as particle physics, gravitation, astrophysics and cosmology. In this Special Issue, the state-of-the-art in the field is presented in detail.
In this paper, we consider a spatially flat FLRW cosmological model with matter obeying a barotropic equation of state $p = w \mu$, $-1<w\leq1$, and a cosmological constant, $\Lambda$. We use Osgood's criterion to establish three cases when such models admit finite-time singularities. The first case is for an arbitrary initial condition, with a negative cosmological constant, and phantom energy $w < -1$. We show that except for a very fine-tuned choice of the initial condition $\theta_{0}$, the universe will develop a finite-time singularity. The second case we consider is for a nonnegative cosmological constant, phantom energy, and the expansion scalar being larger than that of the flat-space de Sitter solution, and show that such models only expand forever for $\Lambda = 0$. In all other cases, the universe model develops a finite-time singularity. The final case we consider is for a nonnegative cosmological constant, a matter source with $-1 < w \leq 1$, and an expansion scalar that is asymptotically that of the de Sitter universe. We show that such models will only expand forever when $\Lambda = 0$, otherwise, they will develop a finite-time singularity. This is significant, since the inflationary epoch is a subset of this domain. However, as we show, the inclusion of a bulk viscosity term in the Einstein field equations eliminates this singularity, and the universe expands forever. This could have interesting implications for the role of bulk viscosity in dynamical models of the universe.
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Nitric acid is a possible biomarker in the atmospheres of exoplanets. An accurate line list of rotational and rotational-vibrational transitions is computed for nitric acid (HNO$_3$). This line list covers wavelengths longer than 1.42 $\mu$m (0 - 7000 cm$^{-1}$) and temperatures up to 500 K. The line list is computed using a hybrid variational -- perturbation theory and empirically tuned potential energy and dipole surfaces. It comprises almost 7 billion transitions involving rotations up to $J=100$. Comparisons with spectra from the HITRAN and PNNL databases demonstrate the accuracy of our calculations. Synthetic spectra of water - nitric acid mixturessuggest that nitric acid has features at 7.5 and 11.25 $\mu$m that are capable of providing a clear signature for HNO$_3$; the feature at 11.25 $\mu$m is particularly promising. Partition functions plus full line lists of transitions are made available in an electronic form as supplementary data to the article and at www.exomol.com.
The optical Galactic Plane H$\alpha$ surveys IPHAS and VPHAS+ are dramatically improving our understanding of Galactic stellar populations and stellar evolution by providing large samples of stars in short lived, but important, evolutionary phases, and high quality homogeneous photometry and images over the entire Galactic Plane. Here I summarise some of the contributions these surveys have already made to our understanding of a number of key areas of stellar and Galactic astronomy.
We study the distribution of cold dark matter (CDM) in cosmological zoom-in simulations from the Feedback in Realistic Environments (FIRE) project, for a range of halo mass (10^9-10^12 Msun) and stellar mass (10^4-10^11 Msun). The FIRE simulations incorporate explicit stellar feedback within the multi-phase ISM. We find that stellar feedback, without any "fine-tuned" parameters, can greatly alleviate small-scale problems in CDM. Feedback causes bursts of star formation and outflows, altering the DM distribution. As a result, the inner slope of the DM halo profile "alpha" shows a strong mass dependence: profiles are shallow at M_h ~ 10^10-10^11 Msun and steepen at higher/lower masses. The resulting core sizes and slopes are consistent with observations. This is broadly consistent with previous work using simpler feedback schemes, but we find steeper mass dependence of "alpha," and relatively late growth of cores. Because the star formation efficiency is strongly halo mass dependent, a rapid change in the central slope occurs at M_h ~10^10 Msun, as sufficient feedback energy becomes available to perturb the DM. We show that large cores are not established during the period of rapid growth of halos because of ongoing DM mass accumulation. Instead, cores require several bursts of star formation after the rapid buildup has completed. The same effects dramatically reduce circular velocities in the inner kpc of massive dwarfs; this could be sufficient to explain the "Too Big To Fail" problem without invoking non-standard DM. Finally, we study baryonic contraction in Milky Way-mass halos. The net result of stellar feedback and baryonic contraction is to produce DM profiles slightly shallower than the NFW profile, as required by the normalization of the Tully-Fisher relation.
Content: We present the results from $Suzaku$ observations of the merging cluster of galaxies CIZA J2242.8+5301 at $z$=0.192. Aims. To study the physics of gas heating and particle acceleration in cluster mergers, we investigated the X-ray emission from CIZA J2242.8+5301, which hosts two giant radio relics in the northern/southern part of the cluster. Methods. We analyzed data from three-pointed Suzaku observations of CIZA J2242.8+5301 to derive the temperature distribution in four different directions. Results: The Intra-Cluster Medium (ICM) temperature shows a remarkable drop from 8.5$_{-0.6}^{+0.8}$ keV to 2.7$_{-0.4}^{+0.7}$ keV across the northern radio relic. The temperature drop is consistent with a Mach number ${\cal M}_n=2.7^{+0.7}_{-0.4}$ and a shock velocity $v_{shock:n}=2300_{-400}^{+700}\rm\,km\,s^{-1}$. We also confirm the temperature drop across the southern radio relic. However, the ICM temperature beyond this relic is much higher than beyond the northern one, which gives a Mach number ${\cal M}_s=1.7^{+0.4}_{-0.3}$ and shock velocity $v_{shock:s}=2040_{-410}^{+550}\rm \,km\,s^{-1}$. These results agree with other systems showing a relationship between the radio relics and shock fronts which are induced by merging activity. We compare the X-ray derived Mach numbers with the radio derived Mach numbers from the radio spectral index under the assumption of diffusive shock acceleration in the linear test particle regime. For the northern radio relic, the Mach numbers derived from X-ray and radio observations agree with each other. Based on the shock velocities, we estimate that CIZA J2242.8+5301 is observed approximately 0.6 Gyr after core passage. The magnetic field pressure at the northern relic is estimated to be 9% of the thermal pressure.
The initial distribution of spin rates of massive stars is a fingerprint of their elusive formation process. It also sets a key initial condition for stellar evolution and is thus an important ingredient in stellar population synthesis. So far, most studies have focused on single stars. Most O stars are however found in multiple systems. By establishing the spin-rate distribution of a sizeable sample of O-type spectroscopic binaries and by comparing the distributions of binary sub-populations with one another as well as with that of presumed single stars in the same region, we aim to constrain the initial spin distribution of O stars in binaries, and to identify signatures of the physical mechanisms that affect the evolution of the massive stars spin rates. We use ground-based optical spectroscopy obtained in the framework of the VLT-FLAMES Tarantula Survey (VFTS) to establish the projected equatorial rotational velocities (\vrot) for components of 114 spectroscopic binaries in 30 Doradus. The \vrot\ values are derived from the full-width at half-maximum (FWHM) of a set of spectral lines, using a FWHM vs. \vrot\ calibration that we derive based on previous line analysis methods applied to single O-type stars in the VFTS sample. The overall \vrot\ distribution of the primary stars resembles that of single O-type stars in the VFTS, featuring a low-velocity peak (at $\vrot < 200$ kms) and a shoulder at intermediate velocities ($200 < \vrot < 300$ kms). The distributions of binaries and single stars however differ in two ways. First, the main peak at $\vrot \sim$100 kms is broader and slightly shifted toward higher spin rates in the binary distribution compared to that of the presumed-single stars. Second, the \vrot distribution of primaries lacks a significant population of stars spinning faster than 300 kms while such a population is clearly present in the single star sample.
We present a new algorithm for detecting filamentary structure FilFinder. The algorithm uses the techniques of mathematical morphology for filament identification, presenting a complementary approach to current algorithms which use matched filtering or critical manifolds. Unlike other methods, FilFinder identifies filaments over a wide dynamic range in brightness. We apply the new algorithm to far infrared imaging data of dust emission released by the Herschel Gould Belt Survey team. Our preliminary analysis characterizes both filaments and fainter striations. We find a typical filament width of 0.09 pc across the sample, but the brightness varies from cloud to cloud. Several regions show a bimodal filament brightness distribution, with the bright mode (filaments) being an order of magnitude brighter than the faint mode (striations). Using the Rolling Hough Transform, we characterize the orientations of the striations in the data, finding preferred directions that agree with magnetic field direction where data are available. There is a suggestive but noisy correlation between typical filament brightness and literature values of the star formation rates for clouds in the Gould Belt.
Supermassive black holes (SMBHs) and their host galaxies are generally thought to coevolve, so that the SMBH achieves up to about 0.2 to 0.5% of the host galaxy mass in the present day. The radiation emitted from the growing SMBH is expected to affect star formation throughout the host galaxy. The relevance of this scenario at early cosmic epochs is not yet established. We present spectroscopic observations of a galaxy at redshift z = 3.328, which hosts an actively accreting, extremely massive BH, in its final stages of growth. The SMBH mass is roughly one-tenth the mass of the entire host galaxy, suggesting that it has grown much more efficiently than the host, contrary to models of synchronized coevolution. The host galaxy is forming stars at an intense rate, despite the presence of a SMBH-driven gas outflow.
Massive quiescent galaxies at high redshift have been observed to have much smaller physical sizes than their local counterparts. Several mechanisms have been invoked to explain the strong evolution of galaxy size with redshift, including progenitor bias, major and minor mergers, adiabatic expansion, and renewed star formation. However, it is difficult to connect galaxy populations between cosmological epochs to test these theories observationally. Herein, we select a sample of 35 massive, compact galaxies ($M_*$ = 1-3 x $10^{11}$ M$_\odot$, $M_*/R^{1.5}$ > $10^{10.5}$ M$_\odot$/kpc$^{1.5}$) at z=2 in the cosmological hydrodynamical simulation Illustris and trace them forward to z=0 to uncover how they evolve to the present day. By z=0, the original factor of 3 difference in stellar mass has spread to a factor of 20. The spread in dark matter halo mass similarly increases from a factor of 5 to a factor of 40. The compact galaxies' evolutionary paths are diverse: about half acquire an ex-situ envelope and exist as the core of a more massive descendant, 30% survive undisturbed and gain very little mass, 15% are entirely consumed and destroyed in a merger with a more massive galaxy, and the remainder are thoroughly mixed by major mergers. Nearly all the galaxies grow in size as well as mass, so that only about 10% of the z=2 compact galaxies still satisfy our compactness criterion by z=0. The majority of the size growth is driven by the acquisition of additional stellar mass at large radii through mergers and accretion. We find a relationship between a galaxy's z=0 stellar mass and its progenitors' maximum past compactness: more massive galaxies are more likely to have had a compact progenitor. However, this trend possesses significant dispersion which precludes a direct linkage between massive galaxies at z=0 and compact galaxies at z=2. (Abridged)
We report the detection of CO(1-0) emission from NGC 1277, a lenticular galaxy in the Perseus Cluster, which has been proposed to host a $(1.3-1.7) \times 10^{10}\ M_\odot$ black hole (BH) based on stellar kinematic measurements. The CO(1-0) emission, observed with the IRAM Plateau de Bure Interferometer (PdBI) using both, a more extended (~1-arcsec resolution) and a more compact (~2.5-arcsec resolution) configuration, is likely to originate from the dust lane encompassing the galaxy nucleus at a distance of 0.9 arcsec (~320 pc). The spatially-unresolved double-horned CO(1-0) profile found at 2.5-arcsec resolution is likely to trace gas orbiting in the dust lane with rotational velocities of ~520 km s$^{-1}$, indicative of an enclosed mass of ~$2 \times 10^{10}\ M_\odot$. Based on models with realistic mass distributions, the CO(1-0) kinematics is found to be consistent with a ~$1.7 \times 10^{10}\ M_\odot$ BH, while a less massive BH is still possible assuming a large stellar mass-to-light ratio. The strongest CO(1-0) component, centred at ~+500 km s$^{-1}$, is detected at 1-arcsec resolution. It shows an offset from the underlying continuum peak and may originate from a gas clump near the eastern orbital node of the dust lane. The extended 2.6-mm continuum emission is likely associated with a weak AGN, possibly characterized by an inverted radio-to-millimetre spectral energy distribution. Literature radio and X-ray data indicate that an ultra-massive BH in NGC 1277 would not only be over-massive with respect to the BH scaling relations, but also with respect to the fundamental plane of BH activity.
With [Fe/H] ~ -3.3, CD -24_17504 is a canonical metal-poor main sequence turn-off star. Though it has appeared in numerous literature studies, the most comprehensive abundance analysis for the star based on high resolution, high signal-to-noise spectra is nearly 15 years old. We present a new detailed abundance analysis for 21 elements based on combined archival Keck-HIRES and VLT-UVES spectra of the star that is higher in both spectral resolution and signal-to-noise than previous data. Our results for many elements are very similar to those of an earlier comprehensive study of the star, but we present for the first time a carbon abundance from the CH G-band feature as well as improved upper limits for neutron-capture species such as Y, Ba and Eu. In particular, we find that CD -24_17504 has [Fe/H] = -3.41, [C/Fe] = +1.10, [Sr/H] = -4.68 and [Ba/H] <= -4.46, making it a carbon enhanced metal-poor star with neutron-capture element abundances among the lowest measured in Milky Way halo stars.
Conditions in the protosolar nebula have left their mark in the composition
of cometary volatiles, thought to be some of the most pristine material in the
solar system. Cometary compositions represent the end point of processing that
began in the parent molecular cloud core and continued through the collapse of
that core to form the protosun and the solar nebula, and finally during the
evolution of the solar nebula itself as the cometary bodies were accreting.
Disentangling the effects of the various epochs on the final composition of a
comet is complicated. But comets are not the only source of information about
the solar nebula. Protostellar disks around young stars similar to the protosun
provide a way of investigating the evolution of disks similar to the solar
nebula while they are in the process of evolving to form their own solar
systems. In this way we can learn about the physical and chemical conditions
under which comets formed, and about the types of dynamical processing that
shaped the solar system we see today.
This paper summarizes some recent contributions to our understanding of both
cometary volatiles and the composition, structure and evolution of protostellar
disks.
Empirical calibrations of the stellar age-rotation-activity relation (ARAR) rely on observations of the co-eval populations of stars in open clusters. We used the Chandra X-ray Observatory to study M37, a 500-Myr-old open cluster that has been extensively surveyed for rotation periods ($P_{\rm rot}$). M37 was observed almost continuously for five days, for a total of 440.5 ksec, to measure stellar X-ray luminosities ($L_{\mathrm{X}}$), a proxy for coronal activity, across a wide range of masses. The cluster's membership catalog was revisited to calculate updated membership probabilities from photometric data and each star's distance to the cluster center. The result is a comprehensive sample of 1699 M37 members: 426 with $P_{\rm rot}$, 278 with X-ray detections, and 76 with both. We calculate Rossby numbers, $R_o = P_{\rm rot}/\tau$, where $\tau$ is the convective turnover time, and ratios of the X-ray-to-bolometric luminosity, $L_{\rm X}/L_{\rm bol}$, to minimize mass dependencies in our characterization of the rotation-coronal activity relation at 500 Myr. We find that fast rotators, for which $R_o<0.09\pm0.01$, show saturated levels of activity, with log($L_{\rm X}/L_{\rm bol}$)$=-3.06\pm0.04$. For $R_o\geq0.09\pm0.01$, activity is unsaturated and follows a power law of the form $R_o^\beta$, where $\beta$=$-2.03_{-0.14}^{+0.17}$. This is the largest sample available for analyzing the dependence of coronal emission on rotation for a single-aged population, covering stellar masses in the range 0.4$-$1.3 $M_{\odot}$, $P_{\rm rot}$ in the range 0.4$-$12.8 d, and $L_{\rm X}$ in the range 10$^{28.4-30.5}$ erg s$^{-1}$. Our results make M37 a new benchmark open cluster for calibrating the ARAR at ages of $\approx$500 Myr.
We reported the optical observations of GRB 121011A by 0.8-m TNT telescope at Xinglong observatory, China. The light curve of optical afterglow shows a smooth and featureless bump during the epoch of $\sim$130 sec and $\sim$5000 sec with a rising index of $1.57\pm0.28$ before the break time of $539\pm44$ sec, and a decaying index of about $1.29\pm0.07$ up to the end of our observations. Meanwhile, the X-ray light curve decays in a single power-law with a slop of about $1.51\pm0.03$ observed by $XRT$ onboard ${\rm} Swift$ from 100 sec to about 10000 sec after the burst trigger. The featureless optical light curve could be understood as an onset process under the external-shock model. The typical frequency has been below or near the optical one before the deceleration time, and the cooling frequency is located between the optical and X-ray wavelengths. The external medium density has a transition from a mixed stage of ISM and wind-type medium before the peak time to the ISM at the later phase. The joint-analysis of X-ray and optical light curves shows that the emission from both frequencies are consistent with the prediction of the standard afterglow model without any energy injections, indicating that the central engine has stopped its activity and does not restart anymore after the prompt phase.
The formation, evolution and death of massive stars release large quantities of energy and momentum into the gas surrounding the sites of star formation. This process, generically termed 'feedback', inhibits further star formation either by removing gas from the galaxy, or by heating it to temperatures that are too high to form new stars. Observations reveal feedback in the form of galactic-scale outflows of gas in galaxies with high rates of star formation, especially in the early Universe. Feedback in faint, low-mass galaxies probably facilitated the escape of ionizing radiation from galaxies when the Universe was about 500 million years old, so that the hydrogen between galaxies changed from neutral to ionized--the last major phase transition in the Universe.
Using the data from the Proportional Counter Array (PCA) and the High-Energy X-ray Timing Experiment (HEXTE) on board it Rossi X-Ray Timing Explorer for Z source GX 17+2, we investigate the evolution of its PCA spectra and HEXTE spectra along a "Z" track on its hardness-intensity diagram. A hard X-ray tail is detected in the HEXTE spectra. The detected hard X-ray tails are discontinuously scattered throughout the "Z" track. The found hard tail hardens from the horizontal branch, through the normal branch, to the flaring branch in principle and it contributes ~(20-50)% of the total flux in 20-200 keV. Our joint fitting results of the PCA+HEXTE spectra in 3-200 keV show that the portion of Comptonization in the bulk-motion Comptonization (BMC) model accounts for the hard X-ray tail, which indicates that the BMC process could be responsible for the detected hard tail. The temperature of the seed photons for BMC is ~2.7 keV, implying that these seed photons might be emitted from the surface of the neutron star (NS) or the boundary layer between the NS and the disk and, therefore, this process could take place around the NS or in the boundary layer.
The co-planarity of solar-system planets led Kant to suggest that they formed from an accretion disk, and the discovery of hundreds of such disks around young stars as well as hundreds of co-planar planetary systems by the {\it Kepler} satellite demonstrate that this formation mechanism is extremely widespread. Many moons in the solar system, such as the Galilean moons of Jupiter, also formed out of the accretion disks that coalesced into the giant planets. We report here the discovery of an intermediate system OGLE-2013-BLG-0723LB/Bb composed of a Venus-mass planet orbiting a brown dwarf, which may be viewed either as a scaled down version of a planet plus star or as a scaled up version of a moon plus planet orbiting a star. The latter analogy can be further extended since they orbit in the potential of a larger, stellar body. For ice-rock companions formed in the outer parts of accretion disks, like Uranus and Callisto, the scaled masses and separations of the three types of systems are similar, leading us to suggest that formation processes of companions within accretion disks around stars, brown dwarfs, and planets are similar.
We study the dependence of angular two-point correlation functions on stellar mass ($M_{*}$) and specific star formation rate (sSFR) of $M_{*}>10^{10}M_{\odot}$ galaxies at $z\sim1$. The data from UKIDSS DXS and CFHTLS covering 8.2 deg$^{2}$ sample scales larger than 100 $h^{-1}$Mpc at $z\sim1$, allowing us to investigate the correlation between clustering, $M_{*}$, and star formation through halo modeling. Based on halo occupation distributions (HODs) of $M_{*}$ threshold samples, we derive HODs for $M_{*}$ binned galaxies, and then calculate the $M_{*}/M_{\rm halo}$ ratio. The ratio for central galaxies shows a peak at $M_{\rm halo}\sim10^{12}h^{-1}M_{\odot}$, and satellites predominantly contribute to the total stellar mass in cluster environments with $M_{*}/M_{\rm halo}$ values of 0.01--0.02. Using star-forming galaxies split by sSFR, we find that main sequence galaxies ($\rm log\,sSFR/yr^{-1}\sim-9$) are mainly central galaxies in $\sim10^{12.5} h^{-1}M_{\odot}$ haloes with the lowest clustering amplitude, while lower sSFR galaxies consist of a mixture of both central and satellite galaxies where those with the lowest $M_{*}$ are predominantly satellites influenced by their environment. Considering the lowest $M_{\rm halo}$ samples in each $M_{*}$ bin, massive central galaxies reside in more massive haloes with lower sSFRs than low mass ones, indicating star-forming central galaxies evolve from a low $M_{*}$--high sSFR to a high $M_{*}$--low sSFR regime. We also find that the most rapidly star-forming galaxies ($\rm log\,sSFR/yr^{-1}>-8.5$) are in more massive haloes than main sequence ones, possibly implying galaxy mergers in dense environments are driving the active star formation. These results support the conclusion that the majority of star-forming galaxies follow secular evolution through the sustained but decreasing formation of stars.
A linear [Fe/H]-[O/H] relation is found for different stellar populations in the Galaxy (halo, thick disk, thin disk) from a data sample obtained in a recent investigation (Ram{\'\i}rez et al. 2013). These correlations support previous results inferred from poorer samples: stars display a "main sequence" expressed as [Fe/H] = $a$[O/H$]+b\mp\Delta b$ where a unit slope, $a=1$, implies a constant [O/Fe] abundance ratio. Oxygen and iron empirical abundance distributions are then determined for different subsamples, which are well explained by the theoretical predictions of multistage closed-(box+reservoir) (MCBR) chemical evolution models by taking into account the found correlations. The interpretation of these distributions in the framework of MCBR models gives us clues about inflow/outflow rates in these different Galactic regions and their corresponding evolution. Outflow rate for the thick and the thin disks are lower than the halo outflow rate. Moreover if the thin disk built up from the thick disk, both systems result of comparable masses. Besides that, the iron-to-oxygen yield ratio and the primary to not primary contribution ratio for the iron production are obtained from the data, resulting consistent with SNII progenitor nucleosynthesis and with the iron production from SNIa supernova events.
Aims: We aim to study the temporal and spectral behaviour of the eclipsing
polar CSS081231:071126+440405 from the infrared to the X-ray regime.
Methods: We obtained phase-resolved XMM-Newton X-ray observations on two
occasions in 2012 and 2013 in different states of accretion. In 2013 the
XMM-Newton X-ray and UV data were complemented by optical photometric and
spectroscopic observations.
Results: CSS081231 displays two-pole accretion in the high state. The
magnetic fields of the two poles are 36 and 69 MG, indicating a non-dipolar
field geometry. The X-ray spectrum of the main accreting pole with the lower
field comprises a hot thermal component from the cooling accretion plasma,
$kT_{plas}$ of a few tens of keV, and a blackbody-like component from the
accretion area with $kT_{rm bb} \sim$ 50-100\,eV. The high-field pole which was
located opposite to the mass-donating star accretes at a low rate and has a
plasma temperature of about 4\,keV. At both occasions the X-ray eclipse
midpoint precedes the optical eclipse midpoint by 3.2 seconds. The center of
the X-ray bright phase shows accretion-rate dependent longitudinal motion of
$\sim$20 degrees.
Conclusions: CSS081231 is a bright polar that escaped detection in the RASS
survey because it was in a low accretion state. Even in the high state it lacks
the prominent soft component previously thought ubiquitous in polars. Such an
excess may still be present in the unobserved extreme ultraviolet. All polars
discovered in the XMM-Newton era lack the prominent soft component. The
intrinsic spectral energy distribution of polars still await characterisation
by future X-ray surveys such as eROSITA. The trajectory taken by material to
reach the second pole is still uncertain.
The possibility that there is some Cosmic Polarization Rotation (CPR), i.e. that the polarization angle rotates while a photon travels in vacuum over large distances, is important for at least two reasons: first, the polarization angle seems to be the most permanent characteristic of photons and, second, CPR would be associated with violations of fundamental physical principles, like the Einstein Equivalence Principle on which all metric theories of gravity are based, including General Relativity, for which we celebrate the Centennial this year 2015. We review here the astrophysical tests which have been carried out to check if CPR exists. These are using the radio and ultraviolet polarization of radio galaxies and the polarization of the cosmic microwave background (both E-mode and B-mode). These tests so far have been negative, leading to upper limits of a couple of degrees on any CPR angle, thereby increasing our confidence in those physical principles and in the resulting theories, including General Relativity. We discuss future prospects in detecting CPR or improving the constraints on it.
Optical spectroscopic monitoring has been conducted of two O stars in the Small and one in the Large Magellanic Cloud, the spectral characteristics of which place them in the Of?p category, which has been established in the Galaxy to consist of oblique magnetic rotators. All of these Magellanic stars show systematic spectral variations typical of the Of?p class, further strengthening their magnetic candidacy to the point of virtual certainty. The spectral variations are related to photometric variations derived from OGLE data by Naze et al. (2015) in a parallel study, which yields rotational periods for two of them. Now circular spectropolarimetry is required to measure their fields, and ultraviolet spectroscopy to further characterize their low-metallicity, magnetically confined winds, in support of hydrodynamical analyses.
From the rate of hydrogen ionization and the gamma ray flux, we derived the spectrum of relativistic and subrelativistic cosmic rays (CRs) nearby and inside the molecular cloud Sgr B2 near the Galactic Center (GC). We studied two cases of CR propagation in molecular clouds: free propagation and scattering of particles by magnetic fluctuations excited by the neutral gas turbulence. We showed that in the latter case CR propagation inside the cloud can be described as diffusion with the coefficient $\sim 3\times 10^{27}$ cm$^2$ s$^{-1}$. For the case of hydrogen ionization by subrelativistic protons, we showed that their spectrum outside the cloud is quite hard with the spectral index $\delta>-1$. The energy density of subrelativistic protons ($>50$ eV cm$^{-3}$) is one order of magnitude higher than that of relativistic CRs. These protons generate the 6.4 keV emission from Sgr B2, which was about 30\% of the flux observed by Suzaku in 2013. Future observations for the period after 2013 may discover the background flux generated by subrelativistic CRs in Sgr B2. Alternatively hydrogen ionization of the molecular gas in Sgr B2 may be caused by high energy electrons. We showed that the spectrum of electron bremsstrahlung is harder than the observed continuum from Sgr B2, and in principle this X-ray component provided by electrons could be seen from the INTEGRAL data as a stationary high energy excess above the observed spectrum $E_x^{-2}$.
Here we present a new approach for constraining luminous blazars, incorporating fully time-dependent and self-consistent modeling of bright gamma-ray flares of PKS1510-089 resolved with Fermi-LAT, in the framework of the internal shock scenario. The results of our modeling imply the location of the gamma-ray flaring zone outside of the broad-line region, namely around 0.3pc from the core for a free-expanding jet with the opening angle Gamma, \theta_\mathrm{jet} \simeq 1 (where Gamma is the jet bulk Lorentz factor), up to \simeq 3pc for a collimated outflow with Gamma, \theta_\mathrm{jet} \simeq 0.1. Moreover, under the Gamma, \theta_\mathrm{jet} \simeq 1 condition, our modeling indicates the maximum efficiency of the jet production during the flares, with the total jet energy flux strongly dominated by protons and exceeding the available accretion power in the source. This is in contrast to the quiescence states of the blazar, characterized by lower jet kinetic power and an approximate energy equipartition between different plasma constituents. We demostrate how strictly simultaneous observations of flaring PKS1510-089 at optical, X-ray, and GeV photon energies on hourly timescales, augmented by extensive simulations as presented in this paper, may help to impose further precise constraints on the magnetization and opening angle of the emitting region. Our detailed modeling implies in addition that a non-uniformity of the Doppler factor across the jet, caused by the radial expansion of the outflow, may lead to a pronounced time distortion in the observed gamma-ray light curves, resulting in particular in asymmetric flux profiles with substantially extended decay phases.
We have measured the angular sizes of radio cores of active galactic nuclei (AGN) and analyzed their sky distributions and frequency dependencies to study synchrotron opacity in AGN jets and the strength of angular broadening in the interstellar medium. We have used archival very long baseline interferometry (VLBI) data of more than 3000 compact extragalactic radio sources observed at frequencies, $\nu$, from 2 to 43 GHz to measure the observed angular size of VLBI cores. We have found a significant increase in the angular sizes of the extragalactic sources seen through the Galactic plane ($|b|\lesssim10^\circ$) at 2, 5 and 8 GHz, about 1/3 of which show significant scattering. These sources are mainly detected in directions to the Galactic bar, the Cygnus region, and a region with galactic longitudes $220^\circ\lesssim l\lesssim260^\circ$ (the Fitzgerald window). The strength of interstellar scattering of the AGNs is found to correlate with the Galactic H$\alpha$ intensity, free-electron density, and Galactic rotation measure. The dependence of scattering strengths on source redshift is insignificant, suggesting that the dominant scattering screens are located in our Galaxy. The observed angular size of Sgr A$^\ast$ is found to be the largest among thousands of AGN observed over the sky; we discuss possible reasons of this strange result. Excluding extragalactic radio sources with significant scattering, we find that angular size of opaque cores in AGN scales typically as $\nu^{-1}$ confirming predictions of a conical synchrotron jet model with equipartition.
A novel model of particle acceleration in the rotating magnetospheres of active galactic nuclei (AGN) and pulsars is constructed. The particle energies may be boosted up to enormous energies in a several step mechanism. In the first stage, the Langmuir waves are centrifugally excited and amplified by means of a parametric process that efficiently pumps rotational energy to excite electrostatic fields. By considering the pulsars it is shown that the Langmuir waves very soon Landau damp on the relativistic electrons already present in a magnetosphere. It has been found that the process is so efficient that no energy losses might affect the mechanism of particle acceleration. Applying typical parameters for young pulsars we have shown that by means of this process the electrons might achieve energies of the order of $10^{18}$ eV. The situation in AGN magnetospheres is slightly different. In the second stage, the process of "Langmuir collapse" develops, creating appropriate conditions for transferring electric energy to boost up already high proton energies to much higher values. As in the previous case, one can show that various energy losses are relatively weak, and do not impose any significant constraints on maximum achievable proton energies of the order of $10^{21}$ eV.
Near-IR spectroscopy is presented for Nova Scorpii 2014. It is shown that the outburst occurred in a symbiotic binary system - an extremely rare configuration for a classical nova outburst to occur in but appropriate for the eruption of a recurrent nova of the T CrB class. We estimate the spectral class of secondary as M5III $\pm$ (two sub-classes). The maximum magnitude versus rate of decline (MMRD) relations give an unacceptably large value of 37.5 kpc for the distance. The spectra are typical of the He/N class of novae with strong HeI and H lines. The profiles are broad and flat topped with full width at zero intensities (FWZIs) approaching 9000-10000 km s$^{-1}$ and also have a sharp narrow component superposed which is attributable to emission from the giant's wind. Hot shocked gas, accompanied by X-rays and $\gamma$ rays, is expected to form when the high velocity ejecta from the nova plows into the surrounding giant wind. Although X-ray emission was observed no $\gamma$-ray emission was reported. It is also puzzling that no signature of a decelerating shock is seen in the near-infrared (NIR), seen in similar systems like RS Oph, V745 Sco and V407 Cyg, as rapid narrowing of the line profiles. The small outburst amplitude and the giant secondary strongly suggest that Nova Sco 2014 could be a recurrent nova.
The light curves of Type Ia supernovae (SNe Ia) are powered by the radioactive decay of $^{56}$Ni to $^{56}$Co at early times, and the decay of $^{56}$Co to $^{56}$Fe from ~60 days after explosion. We examine the evolution of the [Co III] 5892 A emission complex during the nebular phase for SNe Ia with multiple nebular spectra and show that the line flux follows the square of the mass of $^{56}$Co as a function of time. This result indicates both efficient local energy deposition from positrons produced in $^{56}$Co decay, and long-term stability of the ionization state of the nebula. We compile 77 nebular spectra of 25 SN Ia from the literature and present 17 new nebular spectra of 7 SNe Ia, including SN2014J. From these we measure the flux in the [Co III] 5892 A line and remove its well-behaved time dependence to infer the initial mass of $^{56}$Ni ($M_{Ni}$) produced in the explosion. We then examine $^{56}$Ni yields for different SN Ia ejected masses ($M_{ej}$ - calculated using the relation between light curve width and ejected mass) and find the $^{56}$Ni masses of SNe Ia fall into two regimes: for narrow light curves (low stretch s~0.7-0.9), $M_{Ni}$ is clustered near $M_{Ni}$ ~ 0.4$M_\odot$ and shows a shallow increase as $M_{ej}$ increases from ~1-1.4$M_\odot$; at high stretch, $M_{ej}$ clusters at the Chandrasekhar mass (1.4$M_\odot$) while $M_{Ni}$ spans a broad range from 0.6-1.2$M_\odot$. This could constitute evidence for two distinct SN Ia explosion mechanisms.
We solve the set of hydrodynamic (HD) equations for optically thin Advection Dominated Accretion Flows (ADAFs) by assuming radially self-similar in spherical coordinate system $ (r, \theta, \phi) $. The disk is considered to be steady state and axi-symmetric. We define the boundary conditions at the pole and the equator of the disk and to avoid singularity at the rotation axis, the disk is taken to be symmetric with respect to this axis. Moreover, only the $ \tau_{r \phi} $ component of viscous stress tensor is assumed and we have set $ v_{\theta} = 0 $. The main purpose of this study is to investigate the variation of dynamical quantities of the flow in the vertical direction by finding an analytical solution. As a consequence, we found that the advection parameter, $ f^{adv} $, varies along the $ \theta $ direction and reaches to its maximum near the rotation axis. Our results also show that, in terms of no-outflow solution, thermal equilibrium still exists and consequently advection cooling can balance viscous heating.
Aside from numerical algorithms and problem setup, large-scale physics simulations on distributed-memory supercomputers require more basic utilitarian functionality, such as physical units and constants; display to the screen or standard output device; message passing; I/O to disk; and runtime parameter management and usage statistics. Here we describe and make available Fortran 2003 classes furnishing extensible object-oriented implementations of this sort of rudimentary functionality, along with individual `unit test' programs and larger example problems demonstrating their use. These classes compose the Basics division of our developing astrophysics simulation code GenASiS (General Astrophysical Simulation System), but their fundamental nature makes them useful for physics simulations in many fields.
Aiming at exploring the nature of dark energy, we use thirty-six observational Hubble parameter data (OHD) in the redshift range $0 \leqslant z \leqslant 2.36$ to make a cosmological model-independent test of the two-point $Omh^2(z_{2};z_{1})$ diagnostic. In $\Lambda$CDM, we have $Omh^2 \equiv \Omega_{m}h^2$, where $\Omega_{m}$ is the matter density parameter at present. We bin all the OHD into four data points to mitigate the observational contaminations. By comparing with the value of $\Omega_{m}h^2$ which is constrained tightly by the Planck observations, our results show that in all six testing pairs of $Omh^2$ there are two testing pairs are consistent with $\Lambda$CDM at $1\sigma$ confidence level (CL), whereas for another two of them $\Lambda$CDM can only be accommodated at $2\sigma$ CL. Particularly, for remaining two pairs, $\Lambda$CDM is not compatible even at $2\sigma$ CL. Therefore it is reasonable that although deviations from $\Lambda$CDM exist for some pairs, cautiously, we cannot rule out the validity of $\Lambda$CDM. We further apply two methods to derive the value of Hubble constant $H_0$ utilizing the two-point $Omh^2(z_{2};z_{1})$ diagnostic. We obtain $H_0 = 71.23\pm1.54$ ${\mathrm{km \ s^{-1} \ Mpc^{-1}}}$ from inverse variance weighted $Omh^2$ value (method (I)) and $H_0 = 69.37\pm1.59$ ${\mathrm{km \ s^{-1} \ Mpc^{-1}}}$ that the $Omh^2$ value originates from Planck measurement (method (II)), both at $1\sigma$ CL. Finally, we explore how the error in OHD propagate into $w(z)$ at certain redshift during the reconstruction of $w(z)$. We argue that the current precision on OHD is not sufficient small to ensure the reconstruction of $w(z)$ in an acceptable error range, especially at the low redshift
Career opportunities are often a matter of chance, but also of willingness to cross interdisciplinary boundaries.
Current Imaging Atmospheric Cherenkov Telescopes for Very High Energy $\gamma$-ray astrophysics are pointing instruments with a Field of View up to a few tens of sq deg. We propose to build an array of two non-steerable (drift) telescopes. Each of the telescopes would have a camera with a FOV of 5$\times$60 sq deg oriented along the meridian. About half of the sky drifts through this FOV in a year. We have performed a Montecarlo simulation to estimate the performance of this instrument. We expect it to survey this half of the sky with an integral flux sensitivity of $\sim$0.77\% of the steady flux of the Crab Nebula in 5 years, an analysis energy threshold of $\sim$150 GeV and an angular resolution of $\sim$0.1$^{\circ}$. For astronomical objects that transit over the telescope for a specific night, we can achieve an integral sensitivity of 12\% of the Crab Nebula flux in a night, making it a very powerful tool to trigger further observations of variable sources using steerable IACTs or instruments at other wavelengths.
A recent study by Ramirez et al. (2014) demonstrated that an atmosphere with 1.3-4 bar of CO2 and H2O, in addition to 5-20% H2, could have raised the mean annual and global surface temperature of early Mars above the freezing point of water. Such warm temperatures appear necessary to generate the rainfall (or snowfall) amounts required to carve the ancient martian valleys. Here, we use our best estimates for early martian outgassing rates, along with a 1-D photochemical model, to assess the conversion efficiency of CO, CH4, and H2S to CO2, SO2, and H2. Our outgassing estimates assume that Mars was actively recycling volatiles between its crust and interior, as Earth does today. H2 production from serpentinization and deposition of banded iron-formations is also considered. Under these assumptions, maintaining an H2 concentration of ~1-2% by volume is achievable, but reaching 5% H2 requires additional H2 sources or a slowing of the hydrogen escape rate below the diffusion limit. If the early martian atmosphere was indeed H2-rich, we might be able to see evidence of this in the rock record. The hypothesis proposed here is consistent with new data from the Curiosity Rover, which show evidence for a long-lived lake in Gale Crater near Mt. Sharp. It is also consistent with measured oxygen fugacities of martian meteorites, which show evidence for progressive mantle oxidation over time.
High-quality white-light images from the SECCHI/HI-1 telescope onboard STEREO-B reveal high-velocity evanescent clumps [HVECs] expelled from the coma of the C/2011 L4 [Pan-STARRS] comet. Animated images provide evidence of highly dynamic ejecta moving near-radially in the anti-sunward direction. The bulk speed of the clumps at their initial detection in the HI1-B images range from $200-400$ km s$^{-1}$ followed by an appreciable acceleration up to speeds of $450-600$ km s$^{-1}$, which are typical of slow to intermediate solar wind speeds. The clump velocities do not exceed these limiting values and seem to reach a plateau. The images also show that the clumps do not expand as they propagate. Order of magnitude calculations show that ionized single atoms or molecules accelerate too quickly compared to observations, while dust grains micron sized or larger accelerate too slowly. We find that neutral Na, Li, K, or Ca atoms with $\beta>50$ could possibly fit the observations. Just as likely, we find that an interaction with the solar wind and the heliospheric magnetic field (HMF) can cause the observed clump dynamical evolution, accelerating them quickly up to solar wind velocities. We thus speculate that the HVECs are composed of charged particles (dust particles) or neutral atoms accelerated by radiation pressure at $\beta>50$ values. In addition, the data suggest that clump ejecta initially move along near-radial, bright structures, which then separate into HVECs and larger dust grains that steadily bend backwards relative to the comet's orbital motion due to the effects of solar radiation and gravity. These structures gradually form new striae in the dust tail. The near-periodic spacing of the striae may be indicative of outgassing activity modulation due to the comet nucleus' rotation. It is, however, unclear whether all striae are formed as a result of this process.
Here we report the results of searching millisecond pulsar (MSP) candidates from the Fermi LAT second source catalog (2FGL). Seven unassociated $\gamma-$ray sources in this catalog are identified as promising MSP candidates based on their $\gamma$-ray properties. Through the X-ray analysis, we have detected possible X-ray counterparts, localized to an arcsecond accuracy. We have systematically estimated their X-ray fluxes and compared with the corresponding $\gamma$-ray fluxes. The X-ray to $\gamma$-ray flux ratios for 2FGL J1653.6-0159 and 2FGL J1946.4-5402 are comparable with the typical value for pulsars. For 2FGL J1625.2-0020, 2FGL J1653.6-0159 and 2FGL J1946.4-5402, their candidate X-ray counterparts are bright enough for performing a detailed spectral and temporal analysis to discriminate their thermal/non thermal nature and search for the periodic signal. We have also searched for possible optical/IR counterparts at the X-ray positions. For the optical/IR source coincident with the brightest X-ray object that associated with 2FGL J1120.0-2204, its spectral energy distribution is comparable with a late-type star. Evidence for the variability has also been found by examining its optical light curve. All the aforementioned 2FGL sources resemble a pulsar in one or more aspects, which make them as the promising targets for follow-up investigations.
A variety of beyond the standard model scenarios contain very light hidden sector U(1) gauge bosons undergoing kinetic mixing with the photon. The resulting oscillation between ordinary and hidden photons leads to spectral distortions of the cosmic microwave background. We update the bounds on the mixing parameter $\chi_0$ and the mass of the hidden photon $m_{\gamma'}$ for future experiments measuring CMB spectral distortions, such as PIXIE and PRISM/COrE. For $10^{-14}\;{\rm eV}\lesssim m_{\gamma'}\lesssim 10^{-13}\;{\rm eV}$, we find the kinetic mixing angle $\chi_0$ has to be less than $10^{-8}$ at 95\% CL. These bounds are more than an order of magnitude stronger than those derived from the COBE/FIRAS data.
There is ample evidences of twisted magnetic structures in the corona. This motivates us to consider the magnetic twist as the cause of Alfven frequency continuum in coronal loops, which can support the resonant absorption as the rapid damping mechanism for the observed coronal kink MHD oscillations. For a straight cylindrical compressible zero-beta thin flux tube with a magnetic twist in a thin boundary and straight magnetic field in the interior and exterior regions as well as a step-like radial density profile, we derive the dispersion relation and solve it analytically. Consequently, we obtain the frequencies and damping rates of the fundamental (l=1) and first/second overtones (l=2,3) kink (m=1) MHD modes. We conclude that the resonant absorption by the magnetic twist can justify the rapid damping of kink MHD waves observed in coronal loops. Furthermore, the magnetic twist in the inhomogeneous layer can achieve deviations from P_1/P_2=2 and P_1/P_3=3 of the same order of magnitude as in the observations.
In this white paper, we assess the potential for JWST to characterize the atmospheres of super-Earth exoplanets, by simulating a range of transiting spectra with different masses and temperatures. Our results are based on a JWST simulator tuned to the expected performance of the workhorse spectroscopic instrument NIRSpec, and is based on the latest exoplanet transit models by Howe & Burrows (2012). This study is especially timely since the observing modes for the science instruments on JWST are finalized (Clampin 2010) and because NASA has selected the TESS mission as an upcoming Explorer. TESS is expected to identify more than 1000 transiting exoplanet candidates, including a sample of about 100 nearby (<50 pc) super- Earths (Ricker et al. 2010).
Ongoing and future imaging surveys represent significant improvements in depth, area and seeing compared to current data-sets. These improvements offer the opportunity to discover up to three orders of magnitude more galaxy-galaxy strong lenses than are currently known. In this work we forecast the number of lenses discoverable in forthcoming surveys and simulate their properties. We generate a population of statistically realistic strong lenses and simulate observations of this population for the Dark Energy Survey (DES), Large Synoptic Survey Telescope (LSST) and Euclid surveys. We verify our model against the galaxy-scale lens search of the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS), predicting 250 discoverable lenses compared to 220 found by Gavazzi et al (2014). The predicted Einstein radius distribution is also remarkably similar to that found by Sonnenfeld et al (2013). For future surveys we find that, assuming Poisson limited lens galaxy subtraction, searches in DES, LSST and Euclid datasets should discover 2400, 120000, and 170000 galaxy-galaxy strong lenses respectively. Finders using blue minus red (g-i) difference imaging for lens subtraction can discover 1300 and 62000 lenses in DES and LSST. The uncertainties on the model are dominated by the high redshift source population which typically gives fractional errors on the discoverable lens number at the tens of percent level. We find that doubling the signal-to-noise ratio required for a lens to be detectable, approximately halves the number of detectable lenses in each survey, indicating the importance of understanding the selection function and sensitivity of future lens finders in interpreting strong lens statistics. We make our population forecasting and simulated observation codes publicly available so that the selection function of strong lens finders can easily be calibrated.
The gravitational influence of a planet on a nearby disk provides a powerful tool for detecting and studying extrasolar planetary systems. Here we demonstrate that gaps can be opened in dynamically cold debris disks at the mean-motion resonances of an orbiting planet. The gaps are opened away from the orbit of the planet itself, revealing that not all disk gaps need contain a planetary body. These gaps are large and deep enough to be detectable in resolved disk images for a wide range of reasonable disk-planet parameters, though we are not aware of any such gaps detected to date. The gap shape and size are diagnostic of the planet location, eccentricity and mass, and allow one to infer the existence of unseen planets, as well as many important parameters of both seen and unseen planets in these systems. We present expressions to allow the planetary mass to be calculated from observed gap width and location.
Lithium rich giant stars are rare objects. For some of them, Li enrichment exceeds abundance of this element found in solar system meteorites, suggesting that these stars have gone through a Li enhancement process. We identified a Li rich giant HD 107028 with A(Li) > 3.3 in a sample of evolved stars observed within the PennState Torun Planet Search. In this work we study different enhancement scenarios and we try to identify the one responsible for Li enrichment for HD 107028. We collected high resolution spectra with three different instruments, covering different spectral ranges. We determine stellar parameters and abundances of selected elements with both equivalent width measurements and analysis, and spectral synthesis. We also collected multi epoch high precision radial velocities in an attempt to detect a companion. Collected data show that HD 107028 is a star at the base of Red Giant Branch. Except for high Li abundance, we have not identified any other anomalies in its chemical composition, and there is no indication of a low mass or stellar companion. We exclude Li production at the Luminosity Function Bump on RGB, as the effective temperature and luminosity suggest that the evolutionary state is much earlier than RGB Bump. We also cannot confirm the Li enhancement by contamination, as we do not observe any anomalies that are associated with this scenario. After evaluating various scenarios of Li enhancement we conclude that the Li-overabundance of HD 107028 originates from Main Sequence evolution, and may be caused by diffusion process.
We performed a detailed analysis of elemental abundances, dust features, and polycyclic aromatic hydrocarbons (PAHs) in the C-rich planetary nebula (PN) Wray16-423 in the Sagittarius dwarf spheroidal galaxy, based on a unique dataset taken from the Subaru/HDS, MPG/ESO FEROS, HST/WFPC2, and Spitzer/IRS. We performed the first measurements of Kr, Fe, and recombination O abundance in this PN. The extremely small [Fe/H] implies that most Fe atoms are in the solid phase, considering into account the abundance of [Ar/H]. The Spitzer/IRS spectrum displays broad 16-24 um and 30 um features, as well as PAH bands at 6-9 um and 10-14 um. The unidentified broad 16-24 um feature may not be related to iron sulfide (FeS), amorphous silicate, or PAHs. Using the spectral energy distribution model, we derived the luminosity and effective temperature of the central star, and the gas and dust masses. The observed elemental abundances and derived gas mass are in good agreement with asymptotic giant branch nucleosynthesis models for an initial mass of 1.90 Msun and a metallicity of Z=0.004. We infer that respectively about 80 %, 50 %, and 90 % of the Mg, S, and Fe atoms are in the solid phase. We also assessed the maximum possible magnesium sulfide (MgS) and iron-rich sulfide (Fe50S) masses and tested whether these species can produce the band flux of the observed 30 um feature. Depending on what fraction of the sulfur is in sulfide molecules such as CS, we conclude that MgS and Fe50S could be possible carriers of the 30 um feature in this PN.
Previously proposed mechanisms have difficulty explaining the disruption of Comet C/2012 S1 (ISON) as it approached the Sun. We describe a novel cometary disruption mechanism whereby comet nuclei fragment and disperse through dynamic sublimation pressure, which induces differential stresses within the interior of the nucleus. When these differential stresses exceed its material strength, the nucleus breaks into fragments. We model the sublimation process thermodynamically and propose that it is responsible for the disruption of Comet ISON. We estimate the bulk unconfined crushing strength of Comet ISON's nucleus and the resulting fragments to be 0.5 Pa and 1-9 Pa respectively, assuming typical Jupiter Family Comet (JFC) albedos. However, if Comet ISON has an albedo similar to Pluto, this strength estimate drops to 0.2 Pa for the intact nucleus and 0.6-4 Pa for its fragments. Regardless of assumed albedo, these are similar to previous strength estimates of JFCs. This suggests that, if Comet ISON is representative of dynamically new comets, then low bulk strength is a primordial property of some comet nuclei, and not due to thermal processing during migration into the Jupiter Family.
An appealing explanation for the Planck data is provided by inflationary models with a non-canonical kinetic term: a Laurent expansion of the kinetic function translates into a potential with a nearly shift-symmetric plateau in canonical fields. The shift symmetry can be broken at large field values by including higher-order poles. We show that the resulting corrections to the inflationary dynamics and predictions are universal at lowest order, and can induce power loss at large angular scales. At lowest order there are no corrections from a pole of one order higher; this is referred to as extended no-scale in string theory and we explain why this is a general phenomenon. Finally, we outline which other corrections may arise as string loop corrections.
The opportunity to study physics at supra-nuclear densities through X-ray observations of neutron stars has led to in-depth investigations of certain approximately universal relations that can remove degeneracies in pulse profile models. One such set of relations determines all of the multipole moments of a neutron star just from the first three (the mass monopole, the current dipole and the mass quadrupole moment) approximately independently of the equation of state. These three-hair relations were found to hold in neutron stars that rotate rigidly, as is the case in old pulsars, but neutron stars can also rotate differentially, as is the case for proto-neutron stars and hypermassive transient remnants of binary mergers. We here extend the three-hair relations to differentially rotating stars for the first time with a generic rotation law using two approximations: a weak-field scheme (an expansion in powers of the neutron star compactness) and a perturbative differential rotation scheme (an expansion about rigid rotation). These approximations allow us to analytically derive approximately universal relations that allow us to determine all of the multipole moments of a (perturbative) differentially rotating star in terms of only the first four moments. These new four-hair relations for differentially rotating neutron stars are found to be approximately independent of the equation of state to a higher degree than the three-hair relations for uniformly rotating stars. Our results can be instrumental in the development of four-hair relations for rapidly differentially rotating stars in full General Relativity using numerical simulations.
The motion of a particle in a spatially harmonic magnetic field is a basic problem involved, for example, in the mechanism of formation of a collisionless shock. In such settings, it is generally reasoned that particles entering a Weibel generated turbulence are trapped inside it, provided their Larmor radius in the peak field is smaller than the field coherence length. The goal of this work is to put this heuristic conclusion on firm ground by studying, both analytically and numerically, such motion. A toy model is analyzed, consisting of a relativistic particle entering a region of space occupied by a spatially harmonic field. The particle penetrates the magnetic structure in a direction aligned with the magnetic filaments. Although the conclusions are not trivial, the main result is confirmed.
A systematic self-consistent procedure is provided to describe by means of the Szekeres dust models the evolution of multiple self-gravitating cold dark matter structures (over-densities and density voids), whose spatial location can be prescribed beforehand for all times by suitable initial conditions that define the free parameters of the models. Following this procedure makes it possible to obtain a fully relativistic non-perturbative coarse grained description of actually existing cosmic structure at various scales. We discuss possible astrophysical and cosmological applications.
Supersymmetry is the most natural framework for physics above the TeV scale, and the corresponding framework for early-Universe cosmology, including inflation, is supergravity. No-scale supergravity emerges from generic string compactifications and yields a non-negative potential, and is therefore a plausible framework for constructing models of inflation. No-scale inflation yields naturally predictions similar to those of the Starobinsky model based on $R + R^2$ gravity, with a tilted spectrum of scalar perturbations: $n_s \sim 0.96$, and small values of the tensor-to-scalar perturbation ratio $r < 0.1$, as favoured by Planck and other data on the cosmic microwave background (CMB). Detailed measurements of the CMB may provide insights into the embedding of inflation within string theory as well as its links to collider physics.
We consider a generalization of the standard model which respects quantum conformal invariance. This model leads to identically zero vacuum energy. We show how non-relativistic matter and dark energy arises in this model. Hence the model is shown to be consistent with observations.
We have recently compiled a database with all doctoral dissertations (PhDs) completed in modern Greece (1837-2014), in the general area of astronomy and astrophysics, as well as in space and ionospheric physics. A preliminary statistical analysis of the data is presented, along with a discussion of the general trends observed.
We discuss the possibility that suitable modifications of gravity could account for some amount of the radiation we observe today, in addition to the possibility of explaining the present speed up of the universe. We start introducing and reviewing cosmological reconstruction methods for metric $f(R)$ theories of gravity that can be considered as one of the straightforward modifications of Einstein's gravity as soon as $f(R)\neq R$. We then take into account two possible $f(R)$ models which could give rise to (dark) radiation. Constraints on the models are found by using the Planck Collaboration 2015 data within a cosmographic approach and by obtaining the matter power spectrum of those models. The conclusion is that $f(R)$ gravity can only contribute minimally to the (dark) radiation to avoid departures from the observed matter power spectrum at the smallest scales (of the order of $0.01$Mpc$^{-1}$), i.e., precisely those scales that exited the horizon at the radiation dominated epoch. This result could strongly contribute to select reliable $f(R)$ models.
We perform a comprehensive analysis of a number of scalar field theories in a attempt to find analytically 5-dimensional, localised-on-the-brane, black-hole solutions. Extending a previous analysis, we assume a generalised Vaidya ansatz for the 5-dimensional metric tensor that allows for time-dependence, non-trivial profile of the mass function in terms of the bulk coordinate and a deviation from the over-restricting Schwarzschild-type solution on the brane. In order to support such a solution, we study a variety of theories including single or multiple scalar fields, with canonical or non-canonical kinetic terms, minimally or non-minimally coupled to gravity. We demonstrate that for such a metric ansatz and for a carefully chosen, non-isotropic in 5 dimensions, energy-momentum tensor, solutions that have the form of a Schwarzschild-(Anti)de Sitter or Reissner-Nordstrom type of solution do emerge, however, the resulting profile of the mass-function along the bulk coordinate, when allowed, is not the correct one to eliminate the bulk singularities.
We examine the Einstein equation coupled to the Klein-Gordon equation for a
complex-valued scalar field. These two equations together are known as the
Einstein-Klein-Gordon system. In the low-field, non-relativistic limit, the
Einstein-Klein-Gordon system reduces to the Poisson-Schr\"odinger system. We
describe the simplest solutions of these systems in spherical symmetry, the
spherically symmetric static states, and some scaling properties they obey. We
also describe some approximate analytic solutions for these states.
The EKG system underlies a theory of wave dark matter, also known as scalar
field dark matter (SFDM), boson star dark matter, and Bose-Einstein condensate
(BEC) dark matter. We discuss a possible connection between the theory of wave
dark matter and the baryonic Tully-Fisher relation, which is a scaling relation
observed to hold for disk galaxies in the universe across many decades in mass.
We show how fixing boundary conditions at the edge of the spherically symmetric
static states implies Tully-Fisher-like relations for the states. We also
catalog other "scaling conditions" one can impose on the static states and show
that they do not lead to Tully-Fisher-like relations--barring one exception
which is already known and which has nothing to do with the specifics of wave
dark matter.
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