We describe the selection of a sample of photometrically-defined Lyman break galaxies (LBGs) at z~5 using the multi-wavelength imaging data of the ESO Remote Galaxy Survey (ERGS). The data is drawn from ten widely-separated fields covering a total sky area of 275 arcmin squared. Starting with a simple colour (R-I>1.3) and magnitude (I<26.3) cut to isolate the Lyman break and then refining the sample by applying further optical and near-infrared photometric criteria we identify a sample of 253 LBG candidates. We carefully model the completeness of this sample and the factors that affect its reliability. There is considerable overlap between this sample and a spectroscopically-confirmed sample drawn from the same survey and this allows us to determine the reliability of the optical photometric selection (~60 per cent) and to show that the reliability can be significantly improved (to ~80 per cent) by applying near-infrared waveband criteria to exclude very red contaminants. Even this high level of reliability may compromise some statistical studies of LBG properties. We show that over 30 per cent of the highest reliability candidates have multiple UV-luminous components and/or disturbed morphology in HST imaging, though it is unclear whether this represents multiple interacting/merging sources or individual large sources with multiple UV bright regions. Using this sample we confirm that the normalisation of the bright end of the z=5 UV luminosity function (down to M*) is lower than the same at z=4 by a factor of 3. Using a Schechter fit we determine M*uv=-20.9+/-0.2. We discuss whether it is reasonable to expect the UV luminosity function to follow a Schechter function, given the UV emission is short-lived and stochastic, and does not necessarily trace the underlying mass of the galaxy.
We consider the problem of fitting a parametric model to time-series data that are afflicted by correlated noise. The noise is represented by a sum of two stationary Gaussian processes: one that is uncorrelated in time, and another that has a power spectral density varying as $1/f^\gamma$. We present an accurate and fast [O(N)] algorithm for parameter estimation based on computing the likelihood in a wavelet basis. The method is illustrated and tested using simulated time-series photometry of exoplanetary transits, with particular attention to estimating the midtransit time. We compare our method to two other methods that have been used in the literature, the time-averaging method and the residual-permutation method. For noise processes that obey our assumptions, the algorithm presented here gives more accurate results for midtransit times and truer estimates of their uncertainties.
We examine the properties of a recently proposed observationally viable alternative to homogeneous cosmology with smooth dark energy, the timescape cosmology. In the timescape model cosmic acceleration is realized as an apparent effect related to the calibration of clocks and rods of observers in bound systems relative to volume-average observers in an inhomogeneous geometry in ordinary general relativity. The model is based on an exact solution to a Buchert average of the Einstein equations with backreaction. The present paper examines a number of observational tests which will enable the timescape model to be distinguished from homogeneous cosmologies with a cosmological constant or other smooth dark energy, in current and future generations of dark energy experiments. Predictions are presented for: comoving distance measures; H(z); the equivalent of the dark energy equation of state, w(z); the Om(z) measure of Sahni, Shafieloo and Starobinsky; the Alcock-Paczynski test; the baryon acoustic oscillation measure, D_v; the inhomogeneity test of Clarkson, Bassett and Lu; and the time drift of cosmological redshifts. Where possible, the predictions are compared to recent independent studies of similar measures in homogeneous cosmologies with dark energy. Three separate tests with indications of results in possible tension with the Lambda CDM model are found to be consistent with the expectations of the timescape cosmology.
We study the relation between size and star formation activity in a complete sample of 225 massive (M > 5 x 10^10 Msun) galaxies at 1.5<z<2.5, selected from the FIREWORKS UV-IR catalog of the CDFS. Based on stellar population synthesis model fits to the observed restframe UV-NIR SEDs, and independent MIPS 24 micron observations, 65% of galaxies are actively forming stars, while 35% are quiescent. Using sizes derived from 2D surface brightness profile fits to high resolution (FWHM_{PSF}~0.45 arcsec) groundbased ISAAC data, we confirm and improve the significance of the relation between star formation activity and compactness found in previous studies, using a large, complete mass-limited sample. At z~2, massive quiescent galaxies are significantly smaller than massive star forming galaxies, and a median factor of 0.34+/-0.02 smaller than galaxies of similar mass in the local universe. 13% of the quiescent galaxies are unresolved in the ISAAC data, corresponding to sizes <1 kpc, more than 5 times smaller than galaxies of similar mass locally. The quiescent galaxies span a Kormendy relation which, compared to the relation for local early types, is shifted to smaller sizes and brighter surface brightnesses and is incompatible with passive evolution. The progenitors of the quiescent galaxies, were likely dominated by highly concentrated, intense nuclear star bursts at z~3-4, in contrast to star forming galaxies at z~2 which are extended and dominated by distributed star formation.
In a variety of inflation models the motion of the inflaton may trigger the production of some non-inflaton particles during inflation, for example via parametric resonance or a phase transition. Particle production during inflation leads to observables in the cosmological fluctuations, such as features in the primordial power spectrum and also nongaussianities. Here we focus on a prototype scenario with inflaton, \phi, and iso-inflaton, \chi, fields interacting during inflation via the coupling g^2 (\phi-\phi_0)^2\chi^2. Since several previous investigations have hinted at the presence of localized "glitches" in the observed primordial power spectrum, which are inconsistent with the simplest power-law model, it is interesting to determine the extent to which such anomalies can be explained by this simple and well-motivated model. Our prototype scenario predicts a bump-like feature in the primordial power spectrum, rather than an oscillatory "ringing" pattern as has previously been assumed. We discuss the observational constraints on such features. We find that bumps with amplitude as large as 10% of the usual scale invariant fluctuations from inflation are allowed on scales relevant for Cosmic Microwave Background experiments. Our results imply an upper limit on the coupling g^2 which is crucial for assessing the detectability of the nongaussianity produced by inflationary particle production. We also discuss more complicated features that result from superposing multiple instances of particle production. Finally, we point to a number of microscopic realizations of this scenario in string theory and supersymmetry and discuss the implications of our constraints for the popular brane/axion monodromy inflation models.
We test the hypothesis that prompt gamma-ray burst pulse emission starts simultaneously at all energies (the Pulse Start Conjecture). Our analysis, using a sample of BATSE bursts observed with four channel, 64-ms data and performed using a pulse fit model, generally supports this hypothesis for the Long GRB class, although a few discrepant pulses belong to bursts observed during times characterized by low signal-to-noise, hidden pulses, and/or significant pulse overlap. The typical uncertainty in making this statement is < 0.4 s for pulses in Long GRBs (and < 0.2 s for 40% of the pulses) and perhaps < 0.1 s for pulses in Short GRBs. When considered along with the Epk decline found in GRB pulse evolution, this result implies that energy is injected at the beginning of each and every GRB pulse, and the subsequent spectral evolution, including the pulse peak intensity, represents radiated energy losses from this initial injection.
We present starburst models for far-infrared/sub-millimeter/millimeter (FIR/sub-mm/mm) line emission of molecular and atomic gas in an evolving starburst region, which is treated as an ensemble of non-interacting hot bubbles which drive spherical shells of swept-up gas into a surrounding uniform gas medium. These bubbles and shells are driven by stellar winds and supernovae within massive star clusters formed during an instantaneous starburst. The underlying stellar radiation from the evolving clusters affects the properties and structure of photodissociation regions (PDRs) in the shells, and hence the spectral energy distributions (SEDs) of the molecular and atomic line emission from these swept-up shells and the associated parent giant molecular clouds (GMCs) contains a signature of the stage of evolution of the starburst. The physical and chemical properties of the shells and their structure are computed using a a simple well known similarity solution for the shell expansion, a stellar population synthesis code, and a time-dependent PDR chemistry model. The SEDs for several molecular and atomic lines ($^{12}$CO and its isotope $^{13}$CO, HCN, HCO$^+$, C, O, and C$^+$) are computed using a non-local thermodynamic equilibrium (non-LTE) line radiative transfer model. By comparing our models with the available observed data of nearby infrared bright galaxies, especially M 82, we constrain the models and in the case of M 82, we provide estimates for the ages (5 - 6 Myr, 10 Myr) of recent starburst activity. We also derive a total H$_2$ gas mass of $\sim$ 2 - 3.4 $\times$ 10$^8$ M$_{\odot}$ for the observed regions of the central 1 kpc starburst disk of M 82.
We study the orbits, tidal heating and mass loss from satellites around close-in gas giant exoplanets. The focus is on large satellites which are potentially observable by their transit signature. We argue that even Earth-size satellites around hot Jupiters may be immune to destruction by orbital decay; detection of such a massive satellite would strongly constrain theories of tidal dissipation in gas giants, in a manner complementary to orbital circularization. The star's gravity induces significant periodic eccentricity in the satellite's orbit. The resulting tidal heating rates, per unit mass, are far in excess of Io's and dominate radioactive heating out to planet orbital periods of months for reasonable satellite tidal $Q$. Inside planet orbital periods of about a week, tidal heating can completely melt the satellite. Lastly, we compute an upper limit to the satellite mass loss rate due to thermal evaporation from the surface, valid if the satellite's atmosphere is thin and vapor pressure is negligible. Using this upper limit, we find that although rocky satellites around hot Jupiters with orbital periods less than a few days can be significantly evaporated in their lifetimes, detectable satellites suffer negligible mass loss at longer orbital periods.
We obtained VLA 21-cm observations of the galaxy ESO 481-G017 to determine the environment and trigger of remote star formation traced by a HII region found 43 kpc from the galaxy (in projection). ESO 481-G017 is an early type spiral galaxy with a HI mass of 1.1*10^9 Msun and a distance of 55 Mpc. The isolated HII region has a H-alpha luminosity of 10^38.1 erg s^-1 and minimal continuum emission suggesting that new stars have formed where little or no stars previously existed. The difference in velocity between the HI disk of ESO 481-G017 (3840-4000 km s^-1) and the isolated HII region (4701 +/- 80 km s^-1) indicates the origin of the HII region may be stars forming in a tidal feature or newly triggered star formation in a very low luminosity companion galaxy. The VLA observations shed light on the nature of this young object.
We present the result of a new VLA HI Imaging survey of Virgo galaxies, VIVA (the VLA Imaging survey of Virgo galaxies in Atomic gas). The survey includes high resolution HI data of 53 carefully selected late type galaxies (48 spirals and 5 irregular systems). The goal is to study environmental effects on HI gas properties of cluster galaxies to understand which physical mechanisms affect galaxy evolution in different density regions, and to establish how far out the impact of the cluster reaches. As a dynamically young cluster, Virgo contains examples of galaxies experiencing a variety of environmental effects. Its nearness allows us to study each galaxy in great detail. We have selected Virgo galaxies with a range of star formation properties in low to high density regions (at the projected distance from M87, d_87=0.3-3.3 Mpc). Contrary to pr evious studies, more than half of the galaxies in the sample (~60%) are fainter than 12 mag in B_T. Overall, the selected galaxies represent the late type Virgo galaxies (S0/a to Sd/Irr) down to m_p<~14.6 fairly well in morphological type, systemic velocity, subcluster membership, HI mass and deficiency. In this paper (VIVA I: the atlas and the HI properties), we present HI maps and properties, and describe the HI morphology and kinematics of individual galaxies in detail (abbreviated).
The observations carried out by the space missions CoRoT and Kepler provide a large set of asteroseismic data. Their analysis requires an efficient procedure first to determine if the star is reliably showing solar-like oscillations, second to measure the so-called large separation, third to estimate the asteroseismic information that can be retrieved from the Fourier spectrum. We develop in this paper a procedure, based on the autocorrelation of the seismic Fourier spectrum. We have searched for criteria able to predict the output that one can expect from the analysis by autocorrelation of a seismic time series. First, the autocorrelation is properly scaled for taking into account the contribution of white noise. Then, we use the null hypothesis H0 test to assess the reliability of the autocorrelation analysis. Calculations based on solar and CoRoT times series are performed in order to quantify the performance as a function of the amplitude of the autocorrelation signal. We propose an automated determination of the large separation, whose reliability is quantified by the H0 test. We apply this method to analyze a large set of red giants observed by CoRoT. We estimate the expected performance for photometric time series of the Kepler mission. Finally, we demonstrate that the method makes it possible to distinguish l=0 from l=1 modes. The envelope autocorrelation function has proven to be very powerful for the determination of the large separation in noisy asteroseismic data, since it enables us to quantify the precision of the performance of different measurements: mean large separation, variation of the large separation with frequency, small separation and degree identification.
Free-floating substellar candidates with estimated theoretical masses of as low as ~5 Jupiter masses have been found in the ~3 Myr old sigma Orionis open cluster. As the overlap with the planetary mass domain increases, the question of how these objects form becomes important. The determination of their number density and whether a mass cut-off limit exists is crucial to understanding their formation. We propose to search for objects of yet lower masses in the cluster and determine the shape of the mass function at low mass. Using new- and (re-analysed) published IZJHKs[3.6]-[8.0]-band data of an area of 840 arcmin2, we performed a search for LT-type cluster member candidates in the magnitude range J=19.5-21.5 mag, based on their expected magnitudes and colours. Besides recovering the T type object S Ori 70 and two other known objects, we find three new cluster member candidates, S Ori 72-74, with J=21 mag and within 12 arcmin of the cluster centre. They have theoretical masses of 4 (-2,+3) M_Jup and are among the least massive free-floating objects detected by direct imaging outside the Solar System. The photometry in archival Spitzer [3.6]-[5.8]-band images infers that S Ori 72 is an L/T transition candidate and S Ori 73 a T-type candidate, following the expected cluster sequence in the mid-infrared. Finally, the L-type candidate S Ori 74 with lower quality photometry is located at 11.8 arcsec (~4250 AU) of a stellar member of sigma Orionis and could be a companion. After contaminant correction in the area complete to J=21.1 mag, we estimate that there remain between zero and two cluster members in the mass interval 6-4 M_Jup. Our result suggests a possible turnover in the substellar mass spectrum below ~6 Jupiter masses, which should be investigated further by deeper photometric surveys.
The Nainital-Cape survey is a dedicated research programme to search and study pulsational variability in chemically peculiar stars in the Northern Hemisphere. The aim of the survey is to search such chemically peculiar stars which are pulsationally unstable. The observations of the sample stars were carried out in high-speed photometric mode using a three-channel fast photometer attached to the 1.04-m Sampurnanand telescope at ARIES. The new photometric observations confirmed that the pulsational period of star HD25515 is 2.78-hrs. The repeated time-series observations of HD113878 and HD118660 revealed that previously known frequencies are indeed present in the new data sets. We have estimated the distances, absolute magnitudes, effective temperatures and luminosities of these stars. Their positions in the H-R diagram indicate that HD25515 and HD118660 lie near the main-sequence while HD113878 is an evolved star. We also present a catalogue of 61 stars classified as null results, along with the corresponding 87 frequency spectra taken over the time scale 2002-2008. A statistical analysis of these null results shows, by comparison with past data, that the power of the noise in the light curves has slightly increased during the last few years.
We present a 0.8 -5 micron spectral library of 210 cool stars observed at a resolving power of R = lambda / Delta lambda ~ 2000 with the medium-resolution infrared spectrograph, SpeX, at the 3.0 m NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. The stars have well established MK spectral classifications and are mostly restricted to near-solar metallicities. The sample contains the F, G, K, and M spectral types with luminosity classes between I and V, but also includes some AGB, carbon, and S stars. In contrast to some other spectral libraries, the continuum shape of the spectra are measured and preserved in the data reduction process. The spectra are absolutely flux calibrated using Two Micron All Sky Survey (2MASS) photometry. Potential uses of the library include studying the physics of cool stars, classifying and studying embedded young clusters and optically obscured regions of the Galaxy, evolutionary population synthesis to study unresolved stellar populations in optically-obscured regions of galaxies, and synthetic photometry. The library is available in digital form from the IRTF website.
By using an average heavy nuclei model, the effects of the electron screening on electron capture (EC) in core-collapse supernovae are investigated. A one-dimension code based on the Ws15$M_{\sun}$ progenitor model is utilized to test the effects of electron screening during the collapsing process. The results show that, at high densities, the effects of EC on electron capture becomes significant. During the collapsing stage, the EC rate is decreased, the collapse timescale is prolonged and the leakage of the neutrino energy is increased. These effects lead to an appreciable decrease in the initial energy of the bounce shock wave. The effects of electron screeening in the other progenitor models are also discussed.
We study the weak gravitational lensing effects caused by a stochastic distribution of dark matter halos. We develop a simple approach to calculate the magnification probability distribution function which allows us to easily compute the magnitude bias and dispersion for an arbitrary data sample and a given universe model. We also consider the effects of large-scale cosmic inhomogeneities to the SNe magnitude-redshift relation, and conclude that such structures could bias the PDF enough to affect the extraction of cosmological parameters from the present-day limited size of SNe data samples. We also release turboGL, a simple and very fast (<= 1s) Mathematica code based on the method here presented.
Using the period and mass data of two hundred and seventy-nine extrasolar planets, we have constructed a coupled period-mass function through the non-parametric approach. This analytic expression of the coupled period-mass function has been obtained for the first time in this field. Moreover, due to a moderate period-mass correlation, the shapes of mass/period functions vary as a function of period/mass. These results of mass and period functions give way to two important implications: (1) the deficit of massive close-in planets is confirmed, and (2) the more massive planets have larger ranges of possible semi-major axes. These interesting statistical results will provide important clues into the theories of planetary formation.
We investigate a spherical collapse model with and without the spatial curvature. We obtain the exact solutions of dynamical quantities such as the ratio of the scale factor to its value at the turnaround epoch and the ratio of the overdensity radius to its value at the turnaround time with general cosmological parameters. The exact solutions of the overdensity at the turnaround epoch for the different models are also obtained. Thus, we are able to obtain the nonlinear overdensity at any epoch for the given model. We obtain that the nonlinear overdensity of the Einstein de Sitter Universe is 18 pi^2(fr{1}{2pi} + fr{3}{4})^2 ~ 147 instead of the well known value 18 pi^2 ~ 178. In the open Universe, perturbations are virialized earlier than in flat Universe and thus clusters are denser at the virial epoch. Also the critical density threshold from the linear theory at the virialized epoch is obtained as fr{3}{20} (9pi+6)^{fr{2}{3}} ~ 1.58 instead of fr{3}{20} (12pi)^{fr{2}{3}} ~ 1.69. This value is same for the close and the open Universes. Because we obtain the analytic forms of dynamical quantities, we are able to estimate the abundances of both virialized and non-virialized clusters at any epoch. Also the temperature and luminosity functions are able to be computed at any epoch. Unfortunately, the current concordance model prefers the almost flat Universe and the above results might be restricted by the academic interests only. However, mathematical structure of the physical evolution equations of the curved space is identical with that of the dark energy with the equation of state w = -fr{1}{3}. Thus, we are able to extend these analytic solutions to the general dark energy model and they will provide the useful tools for probing the properties of dark energy.
The appearance of eruptive space plasma processes, e.g., in eruptive flares as observed in the solar atmosphere, is usually assumed to be caused by magnetic reconnection. The process of magnetic reconnection is often connected with singular points of the magnetic field. We therefore analyse the system of stationary resistive/non-ideal magnetohydrodynamics (MHD) in the vicinity of singular points of flow and field to determine the boundary between reconnection solutions and non-reconnective solutions. We find conditions to enable the plasma to cross the magnetic separatrices also inside the current sheet, close to the current maximum. The results provide us with the topological and geometrical skeleton of the resistive MHD fields. We therefore have to perform a local analysis of almost all non-ideal MHD solutions with a generalized non-idealness. We use Taylor expansions of the magnetic field, the velocity field and all other physical quantities, including the non-idealness, and with the method of a comparison of the coefficients, the non-linear resistive MHD system is solved analytically. It turns out that not every non-ideal flow is a reconnective flow and that pure resistive/non-ideal MHD only allows for reconnection-like solutions, even if the non-idealness is localized to the region around the magnetic null point. It is necessary that the flow close to the magnetic X-point is also of X-point type to guarantee positive dissipation of energy and annihilation of magnetic flux. If the non-idealness has only a one-dimensional, sheet-like structure, only one separatrix line can be crossed by the plasma flow, similar to reconnective annihilation solutions.
We present the stellar and substellar mass function of the open cluster IC2391, plus its radial dependence, and use this to put constraints on the formation mechanism of brown dwarfs. Our multiband optical and infrared photometric survey with spectroscopic follow-up covers 11 square degrees, making it the largest survey of this cluster to date. We observe that there is no significant variation in the mass function below the substellar boundary at all three cluster radius intervals analyzed. This lack of variation is in favour of the ejection scenario but considering that IC2391 have an age ~3 times older than its crossing time, we might expect that most of the brown dwarfs with velocity dispersion greater than the escape velocity could have already escape the cluster. Although we observe a radial variation in the mass function over the range 0.15 to 0.5Msol, the radial profile of IC2391 does not give evidence of mass segregation. We also note a significant variation in the colour of the field star locus in colour-magnitude diagrams across the cluster and conclude that this is due to variable background extinction in the Galactic plane. From our preliminary spectroscopic follow-up to confirm brown dwarf status and cluster membership, we find that all spectra analyzed are M dwarfs (field or in the cluster), demonstrating the efficiency of our method to avoid contaminants (e.g. red giants) in our photometric selection. About a half of our photometric candidates are true physical members of the cluster and two are new spectroscopically-confirmed brown dwarf members of IC2391.
Mergers between a spiral and an elliptical (S+E mergers) are poorly studied so far despite the importance for galaxy evolution. NGC4441 is a nearby candidate for an advanced remnant of such a merger, showing typical tidal structures like an optical tail and two shells as well as two HI tails. The study of the molecular gas content gives clues on the impact of the recent merger event on the star formation. Simulations of S+E mergers predict contradictory scenarios concerning the strength and the extent of an induced starburst. Thus, observations of the amount and the distribution of the molecular gas, the raw material of star formation, are needed to understand the influence of the merger on the star formation history. 12CO and 13CO (1-0) and (2-1) observations were obtained using the Onsala Space Observatory 20m and IRAM 30m telescope as well as the Plateau de Bure interferometer. These data allow us to carry out a basic analysis of the molecular gas properties such as estimates of the molecular gas mass, its temperature and density and the star formation efficiency. The CO observations reveal an extended molecular gas reservoir out to ~4kpc, with a total molecular gas mass of ~5x10^8 M_sun. Furthermore, high resolution imaging shows a central molecular gas feature, most likely a rotating disc hosting most of the molecular gas ~4x10^8 M_sun. This nuclear disc shows a different sense of rotation than the large-scale HI structure, indicating a kinematically decoupled core. (abbreviated)
The very first stars in the Universe can be very massive, frequently reaching $10^3M_\odot$. If born in large numbers such massive stars can have strong impact on the subsequent star formation producing strong ionising radiation and contaminating the primordial gas with heavy elements. They would leave behind massive black holes that could act as seeds for growing supermassive black holes of active galactic nuclei. Given the anticipated fast rotation such stars would end their live as supermassive collapsars and drive powerful magnetically-dominated jets. In this letter we investigate the possibility of observing the bursts of high-energy emission similar to the Long Gamma Ray Bursts associated with normal collapsars. We show that during the collapse of supercollapsars, the Blandford-Znajek mechanism can extract up to $10^{56}$erg at a rate of few$\times10^{52}$erg/s. Due to the higher intrinsic time scale and higher redshift the observed burst duration increases by a factor of $\simeq 1000$ and can reach one day. Due to the high redshift the burst spectrum is expected to be soft, with the spectral energy distribution peaking at around 20-30keV. The expected total flux density is relatively low, $10^{-7}{erg} {cm}^{-2} {s}^{-1}$, but not prohibitive. If one supercollapsar is produced per every mini-halo of dark matter arising from the 3-$\sigma$ cosmological fluctuations then the whole sky frequency of such bursts could reach several tens per year.
The Crab Pulsar is a relatively young neutron star. The pulsar is the central star in the Crab Nebula, a remnant of the supernova SN 1054, which was observed on Earth in the year 1054. The Crab Pulsar has been extensively observed in the gamma-ray energy band by the Large Area Telescope (LAT), the main instrument onboard the Fermi Gamma-ray Space Telescope, during its first months of data taking. The LAT data have been used to reconstruct the fluxes and the energy spectra of the pulsed gamma-ray component and of the gamma-rays from the nebula. The results on the pulsed component are in good agreement with the previous measurement from EGRET, while the results on the nebula are consistent with the observations from Earth based telescopes.
We present high angular resolution PdBI images (beam of ~0.33'') of the J=27-26 line from several vibrational levels (v7=1 and v6=1) of HC3N toward Cepheus A HW2. These images reveal the two main heating sources in the cluster: one centered in the disk collimating the HW2 radio jet (the HW2 disk), and the other associated with a hot core 0.3'' northeast HW2 (the HC). This is the first time that vibrationally excited emission of HC3N is spatially resolved in a disk. The kinematics of this emission shows that the HW2 disk rotates following a Keplerian law. We derive the temperature profiles in the two objects from the excitation of HC3N along the HW2 disk and the HC. These profiles reveal that both objects are centrally heated and show temperature gradients. The inner and hotter regions have temperatures of 350+-30K and 270+-20K for the HW2 disk and the HC, respectively. In the cooler and outer regions, the temperature drops to 250+-30K in the HW2 disk, and to 220+-15K in the HC. The estimated luminosity of the heating source of the HW2 disk is ~2.2E4Lo, and 3000Lo for the HC. The most massive protostar in the HW2 region is the powering source of the HW2 radio jet. We discuss the formation of multiple systems in this cluster. The proximity of the HC to HW2 suggest that these sources likely form a binary system of B stars, explaining the observed precession of the HW2 radio jet.
We present hydrodynamical N-body simulations of clusters of galaxies with feedback taken from semi-analytic models of galaxy formation. The advantage of this technique is that the source of feedback in our simulations is a population of galaxies that closely resembles that found in the real universe. We demonstrate that, to achieve the high entropy levels found in clusters, active galactic nuclei must inject a large fraction of their energy into the intergalactic/intracluster media throughout the growth period of the central black hole. These simulations reinforce the argument of Bower et al. (2008), who arrived at the same conclusion on the basis of purely semi-analytic reasoning.
In this paper I will outline some of the aspects and problems of modern celestial mechanics and stellar dynamics, in the context of the quickly growing computing facilities. I will point the attention on the great advantages in using, for astrophysical simulations, the modern, fast and cheap Graphic Processing Units (GPUs) acting as true supercomputers. Finally, I present and discuss some characteristics and performances of a new double-parallel code exploiting the joint power of multicore CPUs and GPUs.
(abridged) As the favoured progenitors of long-duration gamma-ray bursts, massive stars may represent our best signposts of individual objects in the early Universe, but special conditions seem required to make these bursters, which might originate from the progenitor's rapid rotation and associated asymmetry. To obtain empirical constraints on the interplay between stellar rotation and wind asymmetry, we perform linear Halpha spectropolarimetry on a sample of 18 spectroscopically peculiar massive O stars, including OVz, Of?p, Oe, and Onfp stars, supplemented by an earlier sample of 20 O supergiants. Despite their rapid rotation (with vsin(i) up to 400 km/s) most O-type stars are found to be spherically symmetric, but with notable exceptions amongst specific object classes. We divide the peculiar O stars into four distinct categories: Groups III and IV include the Oe stars and Onfp stars, which are on the high-end tail of the O star rotation distribution and have in the past been claimed to be embedded in disks. Here we report the detection of a classical depolarization ``line effect'' in the Oe star HD 45314, but the overall incidence of line effects amongst Oe stars is significantly lower (1 out of 6) than amongst Be stars. The chance that the Oe and Be datasets are drawn from the same parent population is negligible (with 95% confidence). This implies there is as yet no evidence for a disk hypothesis in Oe stars, providing relevant constraints on the physical mechanism that is responsible for the Be phenomenon. Finally, we find that 3 out of 4 of the group IV Onfp stars show evidence for complex polarization effects, which are likely related to rapid rotation, and we speculate on the evolutionary links to B[e] stars.
A campaign is described, open to participation by interested AAVSO members, of follow-up observations for newly-discovered Cepheid variables in undersampled and obscured regions of the Galaxy. A primary objective being to use these supergiants to clarify the Galaxy's spiral nature. Preliminary multiband photometric observations are presented for three Cepheids discovered beyond the obscuring dust between the Cygnus & Aquila Rifts (40 \le l \le 50 degrees), a region reputedly tied to a segment of the Sagittarius-Carina arm which appears to cease unexpectedly. The data confirm the existence of exceptional extinction along the line of sight at upwards of A_V~6 magnitudes (d~2 kpc, l~47 degrees), however, the noted paucity of optical spiral tracers in the region does not arise solely from incompleteness owing to extinction. A hybrid spiral map of the Galaxy comprised of classical Cepheids, young open clusters & H II regions, and molecular clouds, presents a consistent picture of the Milky Way and confirms that the three Cepheids do not populate the main portion of the Sagittarius-Carina arm, which does not emanate locally from this region. The Sagitarrius-Carina arm, along with other distinct spiral features, are found to deviate from the canonical logarithmic spiral pattern. Revised parameters are also issued for the Cepheid BY Cas, and it is identified on the spiral map as lying mainly in the foreground to young associations in Cassiopeia. A Fourier analysis of BY Cas' light-curve implies overtone pulsation, and the Cepheid is probably unassociated with the open cluster NGC 663 since the distances, ages, and radial velocities do not match.
The absolute luminosity of the Fe Kalpha emission line from matter illuminated by X-rays in astrophysical sources is nontrivial to calculate except when the line-emitting medium is optically-thin to absorption and scattering. We characterize the Fe Kalpha line flux using a dimensionless efficiency, defined as the fraction of continuum photons above the Fe K shell absorption edge threshold energy that appear in the line. The optically-thin approximation begins to break down even for column densities as small as 2 x 10^22 cm^-2. We show how to obtain reliable estimates of the Fe Kalpha line efficiency in the case of cold, neutral matter, even for the Compton-thick regime. We find that, regardless of geometry and covering factor, the largest Fe Kalpha line efficiency is attained well before the medium becomes Compton-thick. For cosmic elemental abundances it is difficult to achieve an efficiency higher than a few percent under the most favorable conditions and lines of sight. For a given geometry, Compton-thick lines-of-sight may have Fe Kalpha line efficiencies that are orders of magnitude less than the maximum possible for that geometry. Configurations that allow unobscured views of a Compton-thick reflecting surface are capable of yielding the highest efficiencies. Our results can be used to estimate the predicted flux of the narrow Fe Kalpha line at ~6.4 keV from absorption models in AGN. In particular we show that contrary to a recent claim in the literature, absorption dominated models for the relativistic Fe Kalpha emission line in MCG -6-30-15 do not over-predict the narrow Fe Kalpha line for any column density or covering factor.
RR Lyrae stars play an important role as distance indicators and stellar population tracers. In this context the construction of accurate pulsation models is crucial to understand the observed properties and to constrain the intrinsic stellar parameters of these pulsators. The physical mechanism driving pulsation in RR Lyrae stars has been known since the middle of the 20th century and many efforts have been performed during the last few decades in the construction of more and more refined pulsation models. In particular, nonlinear pulsation models including a nonlocal time-dependent treatment of convection, such as the ones originally developed in Los Alamos in the seventies, allow us to reproduce all the relevant observables of radial pulsation and to establish accurate relations and methods to constrain the intrinsic stellar properties and the distance of these variables. The most recent results on RR Lyrae pulsation obtained through these kinds of models will be presented and a few still debated problems will be discussed.
We analyse four light curves obtained at high time resolution (~ 0.1 s) with the 11-m Southern African Large Telescope, at the ends of two normal outbursts and one superoutburst of the dwarf nova VW Hyi. All of these contain at least some Dwarf Nova Oscillations (DNOs), which, when at their highest amplitudes, are seen in unprecedented detail. In addition to the expected DNOs with periods > 20 s we find a previously unknown modulation at 13.39 s, but none at shorter periods. The various DNOs and their interaction with the longer period Quasi-periodic Oscillations are interpreted in terms of the model of magnetically controlled flow from an accretion disc proposed earlier in this series of papers. Our observations include rare DNOs very late in outburst; we find that the fundamental period does not increase beyond ~ 90 s, which is the same value that the independent ``longer period DNOs'' converge on.
We numerically study the interaction of inflation-produced magnetic fields with gravitational waves, both of which originate from quantum fluctuations during inflation. The resonance between the magnetic field perturbations and the gravitational waves has been suggested as a possible mechanism for magnetic field amplification. However, some analytical studies suggest that the effect of the inflationary gravitational waves is too small to provide significant amplification. Our numerical study shows more clearly how the interaction affects the magnetic fields and confirms the weakness of the influence of the gravitational waves. We present an investigation based on the magnetohydrodynamic approximation and take into account the differences of the Alfven speed.
The Intra-Cluster Medium is characterized by thermal emission, and by the presence of large scale magnetic fields. In some clusters of galaxies a diffuse non-thermal emission is also present, located at the cluster center and named radio halo. These sources indicate the existence of relativistic particles and magnetic fields in the cluster volume. In this paper we collect data on all known nearby cluster radio halos (z < 0.4), to discuss their statistical properties and to investigate their origin. We searched for published data on radio halos and reduced new and archive VLA data to increase the number of known radio halos. We present data on 31 radio halos, 1 new relic source, and 1 giant filament. We note the discovery of a small size diffuse radio emission in a cluster (A1213) with very low X-ray luminosity. Among statistical results we confirm the correlation between the average halo radio spectral index and the cluster temperature. We also discuss the high percentage of clusters where both a relic and a radio halo is present. The sample of radio halos discussed here represents the population of radio halos observable with present radio telescopes. The new telescope generation is necessary for a more detailed multifrequency study, and to investigate the possible existence of a population of radio halos with different properties.
The distributions of sunspot longitude at first appearance and at disappearance display an east-west asymmetry that results from a reduction in visibility as one moves from disk centre to the limb. To first order, this is explicable in terms of simple geometrical foreshortening. However, the centre-to-limb visibility variation is much larger than that predicted by foreshortening. Sunspot visibility is also known to be affected by the Wilson effect: the apparent dish shape of the sunspot photosphere caused by the temperature-dependent variation of the geometrical position of the tau=1 layer. In this article we investigate the role of the Wilson effect on the sunspot appearance distributions, deducing a mean depth for the umbral tau=1 layer of 500 to 1500 km. This is based on the comparison of observations of sunspot longitude distribution and Monte Carlo simulations of sunspot appearance using different models for spot growth rate, growth time and depth of Wilson depression.
The determination of the lower-end of the initial mass function (IMF) provides strong constraints on star formation theories. We report here on a search for isolated planetary-mass objects in the 3 Myr-old star-forming region IC 348. Deep, narrowband CH4off and CH4on images were obtained with CFHT/WIRCam over 0.11 sq.deg. in the central part of IC 348 to identify young T-dwarfs from their 1.6 um methane absorption bands. We report three faint T-dwarf candidates with CH4on-CH4off colours > 0.4 mag. Extinction was estimated for each candidate and lies in the range Av ~ 5-12 mag. Comparisons with T-dwarf spectral models, and colour/colour and colour/magnitude diagrams, reject two of the three candidates because of their extreme z' - J blueness. The one remaining object is not thought to be a foreground field dwarf because of a number density argument and also its strong extinction Av - 12 mag, or thought to be a background field T-dwarf which would be expected to be much fainter. Models and diagrams give this object a preliminary T6 spectral type. With a few Jupiter masses, the young T-dwarf candidate reported here is potentially amongst the youngest, lowest mass objects detected in a star-forming region so far. Its frequency is consistent with the extrapolation of current lognormal IMF estimates down to the planetary mass domain.
Irregular galaxies usually are smaller and less massive when compared to the spiral, S0 and elliptical counterparts. The radio observations show that the magnetic field in irregular galaxies is present and its value reaches the same level as in spiral galaxies. However the conditions in the medium of irregular galaxy are very unfavorable for amplification of the magnetic field due to slow rotation and low shearing rate. We investigate the cosmic ray driven dynamo in the interstellar medium of irregular galaxy. We study its efficiency under the conditions of slow rotation and a weak shearing. The star formation is also taken into account in our model and is parametrized by frequency of explosions and modulations of activity. The numerical model includes magnetohydrodynamical dynamo driven by cosmic rays injected into the interstellar medium through randomly exploding supernovae. Essential elements such as: vertical gravity of the disk, differential rotation in a local approximation - shearing box, and resistivity leading to magnetic reconnection are included in the model as well. We find that even slow galactic rotation with a low shearing rate gives an amplification of the magnetic field and that the fast rotation with a low value of the shear enhances the efficiency of the dynamo. Our simulations have shown that a high amount of magnetic energy leaves the simulation box becoming an efficient source of intergalactic magnetic fields.
A spectroscopic analysis of SDSS J160043.6+074802.9, a binary system containing a pulsating subdwarf-O (sdO) star with a late-type companion, yields Teff = 70 000 +/- 5000 K and log g = 5.25 +/- 0.30, together with a most likely type of K3V for the secondary star. We compare our results with atmospheric parameters derived by Fontaine et al. (2008) and in the context of existing evolution models for sdO stars. New and more extensive photometry is also presented which recovers most, but not all, frequencies found in an earlier paper. It therefore seems probable that some pulsation modes have variable amplitudes. A non-adiabatic pulsation analysis of uniform metallicity sdO models show those having log g > 5.3 to be more likely to be unstable and capable of driving pulsation in the observed frequency range.
We studied all blazars of known redshift detected by the Fermi satellite during its first three months survey. For the majority of them, pointed Swift observations ensures a good multiwavelength coverage, enabling us to to reliably construct their spectral energy distributions (SED). We model the SEDs using a one-zone leptonic model and study the distributions of the derived interesting physical parameters as a function of the observed gamma-ray luminosity. We confirm previous findings concerning the relation of the physical parameters with source luminosity which are at the origin of the blazar sequence. The SEDs allow to estimate the luminosity of the accretion disk for the majority of broad emitting line blazars, while for the line-less BL Lac objects in the sample upper limits can be derived. We find a positive correlation between the jet power and the luminosity of the accretion disk in broad line blazars. In these objects we argue that the jet must be proton-dominated, and that the total jet power is of the same order of (or slightly larger than) the disk luminosity. We discuss two alternative scenarios to explain this result.
Aims: The effect of frequency-dependent AGN core positions (``core-shifts'')
on radio Very Long Baseline Interferometry (VLBI) global astrometry
measurements is investigated.
Methods: The basic equations relating to VLBI astrometry are reviewed,
including the effects of source structure. A power-law representation of
core-shifts, based on both observations and theoretical considerations of jet
conditions, is incorporated.
Results: It is shown that, in the presence of core-shifts, phase and
group-delay astrometry measurements yield different positions. For a core
displacement from the jet base parametrized by Delta x (lambda) = k lambda^beta
group delays measure a ``reduced'' core-shift of (1-beta) Delta x (lambda). For
the astrophysically-significant case of beta = 1, group delays measure no shift
at all, giving the position of the jet base. At 8.4 GHz an estimated typical
offset between phase and group-delay positions of ~170 uas is smaller than the
current ~250 uas precision of group-delay positions of the sources used to
define the ICRF; however, this effect must be taken into account for future
measurements planned with improved accuracy when comparing with optical
positions of AGN to be obtained with the GAIA mission.
We consider astrophysical objects such as main-sequence stars, white-dwarfs and neutron stars in a noncommutative context. Noncommutativity is implemented via a deformed dispersion relation $E^{2}=p^{2}c^{2}(1+\lambda E)^{2}+m^{2}c^{4}$ from which we obtain noncommutative corrections to the pressure, particle number and energy densities for radiation and for a degenerate fermion gas. The main implications of noncommutativity for the considered astrophysical objects are examined and discussed.
We show that intermediate mass black holes conjectured to be the early precursors of supermassive black holes and surrounded by relic cold dark matter density spikes can act as particle accelerators with collisions, in principle, at arbitrarily high centre of mass energies in the case of Kerr black holes. While the ejecta from such interactions will be highly redshifted, we may anticipate the possibility of a unique probe of Planck-scale physics.
We study (p)reheating in modular (closed string) inflationary scenarios, with a special emphasis on Kahler moduli/Roulette models. It is usually assumed that reheating in such models occurs through perturbative decays. However, we find that there are very strong non-perturbative preheating decay channels related to the particular shape of the inflaton potential (which is highly nonlinear and has a very steep minimum). Preheating after modular inflation, proceeding through a combination of tachyonic instability and broad-band parametric resonance, is perhaps the most violent example of preheating after inflation known in the literature. Further, we consider the subsequent transfer of energy to the standard model sector in scenarios where the standard model particles are confined to a D7-brane wrapping the inflationary blow-up cycle of the compactification manifold or, more interestingly, a non-inflationary blow up cycle. We explicitly identify the decay channels of the inflaton in these two scenarios. We also consider the case where the inflationary cycle shrinks to the string scale at the end of inflation; here a field theoretical treatment of reheating is insufficient and one must turn instead to a stringy description. We estimate the decay rate of the inflaton and the reheat temperature for various scenarios.
Clifton's exact solution of f(R)=R^(1+\delta) gravity describing a dynamical spherical metric which is asymptotically Friedmann-Lemaitre-Robertson-Walker is studied. It is shown that it harbours a strong spacetime singularity and that this singularity is naked at late times.
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We discuss cosmological perturbation theory at third order, deriving the gauge transformation rules for metric and matter perturbations, and constructing third order gauge invariant quantities. We present the Einstein tensor components, the evolution equations for a perfect fluid, and the Klein-Gordon equation at third order, including scalar, vector and tensor perturbations. In doing so, we also give all second order tensor components and evolution equations in full, exhilarating generality.
In order to investigate the structure and dynamics of the recently discovered massive (M_* > 10^11 M_sun) compact z~2 galaxies, cosmological hydrodynamical/N-body simulations of a proto-cluster region have been undertaken. At z=2, the highest resolution simulation contains ~5800 resolved galaxies, of which 509, 27 and 5 have M_* > 10^10 M_sun, > 10^11 M_sun and > 4x10^11 M_sun, respectively. Effective radii and characteristic stellar densities have been determined for all galaxies. At z=2, for the definitely well resolved mass range of M_* > 10^11 Msun, the mass-size relation is consistent with observational findings for the most compact z~2 galaxies. The very high velocity dispersion recently measured for a compact z~2 galaxy (~510 km/s; van Dokkum et al 2009) can be matched at about the 1-sigma level, although a somewhat larger mass than the estimated M_* ~ 2 x 10^11 M_sun is indicated. For the above mass range, the galaxies have an average axial ratio <b/a> = 0.64 +/- 0.02 with a dispersion of 0.1, an average rotation to 1D velocity dispersion ratio <v/sigma> = 0.46 +/- 0.06 with a dispersion of 0.3, and a maximum value of v/sigma ~ 1.1. Rotation and velocity anisotropy both contribute in flattening the compact galaxies. Some of the observed compact galaxies appear flatter than any of the simulated galaxies. Finally, it is found that the massive compact galaxies are strongly baryon dominated in their inner parts, with typical dark matter mass fractions of order only 20% inside of r=2R_eff.
The solar chemical composition is an important ingredient in our understanding of the formation, structure and evolution of both the Sun and our solar system. Furthermore, it is an essential reference standard against which the elemental contents of other astronomical objects are compared. In this review we evaluate the current understanding of the solar photospheric composition. In particular, we present a re-determination of the abundances of nearly all available elements, using a realistic new 3-dimensional (3D), time-dependent hydrodynamical model of the solar atmosphere. We have carefully considered the atomic input data and selection of spectral lines, and accounted for departures from LTE whenever possible. The end result is a comprehensive and homogeneous compilation of the solar elemental abundances. Particularly noteworthy findings are significantly lower abundances of carbon, nitrogen, oxygen and neon compared with the widely-used values of a decade ago. The new solar chemical composition is supported by a high degree of internal consistency between available abundance indicators, and by agreement with values obtained in the solar neighborhood and from the most pristine meteorites. There is, however, a stark conflict with standard models of the solar interior according to helioseismology, a discrepancy that has yet to find a satisfactory resolution.
Pulsar timing arrays (PTAs) are designed to detect gravitational waves with periods from several months to several years, e.g. those produced by by wide supermassive black-hole binaries in the centers of distant galaxies. Here we show that PTAs are also sensitive to mergers of supermassive black holes. While these mergers occur on a timescale too short to be resolvable by a PTA, they generate a change of metric due to non-linear gravitational-wave memory which persists for the duration of the experiment and could be detected. We develop the theory of the single-source detection by PTAs, and derive the sensitivity of PTAs to the gravitational-wave memory jumps. We show that mergers of $10^8M_{\odot}$ black holes are $2-\sigma$-detectable (in a direction, polarization, and time-dependent way) out to co-moving distances of $\sim 1$ billion light years. Modern prediction for black-hole merger rates imply marginal to modest chance of an individual jump detection by currently developed PTAs. The sensitivity is expected to be somewhat higher for futuristic PTA experiments with SKA.
As of August 2008, over 30 multiple exoplanet systems are known, and 28% of
stars with planets show significant evidence of a second companion. I briefly
review these 30 systems individually, broadly grouping them into five
categories: 1) systems with 3 or more giant (Msini > 0.2 M_Jup) planets, 2)
systems with two giant planets in mean motion resonance (MMR), 3) systems with
two giant planets not in MMR but whose dynamical evolution is affected by
planet-planet interactions, 4) highly hierarchical systems, having two giant
planets with very large period ratios (> 30:1), and 5) systems of
``Super-Earths'', containing only planets with (Msini < 20 M_Earth).
It now appears that eccentricities are not markedly higher among planets in
known multiple planet systems, and that planets with Msini < 1 M_Jup have lower
eccentricities than more massive planets. The distribution of semimajor axes
for planets in multiplanet systems does not show the 3-day pile-up or the 1 AU
"jump" of the apparently-single planet distribution.
We briefly describe some recent observational results, mainly at X-ray wavelengths, on the winds of luminous active galactic nuclei (AGNs). These winds likely play a significant role in galaxy feedback. Topics covered include (1) Relations between X-ray and UV absorption in Broad Absorption Line (BAL) and mini-BAL quasars; (2) X-ray absorption in radio-loud BAL quasars; and (3) Evidence for relativistic iron K BALs in the X-ray spectra of a few bright quasars. We also mention some key outstanding problems and prospects for future advances; e.g., with the International X-ray Observatory (IXO).
We analyze samples of Spitzer Infrared Spectrograph (IRS) spectra of T Tauri stars in the Ophiuchus, Taurus, and Chamaeleon I star-forming regions, whose median ages lie in the <1 to 2 Myr range. The median mid-infrared spectra of objects in these three regions are similar in shape, suggesting, on average, similar disk structures. When normalized to the same stellar luminosity, the medians follow each other closely, implying comparable mid-infrared excess emission from the circumstellar disks. We use the spectral index between 13 and 31 micron and the equivalent width of the 10 micron silicate emission feature to identify objects whose disk configuration departs from that of a continuous, optically thick accretion disk. Transitional disks, whose steep 13-31 micron spectral slope and near-IR flux deficit reveal inner disk clearing, occur with about the same frequency of a few percent in all three regions. Objects with unusually large 10 micron equivalent widths are more common (20-30%); they could reveal the presence of disk gaps filled with optically thin dust. Based on their medians and fraction of evolved disks, T Tauri stars in Taurus and Chamaeleon I are very alike. Disk evolution sets in early, since already the youngest region, the Ophiuchus core (L1688), has more settled disks with larger grains. Our results indicate that protoplanetary disks show clear signs of dust evolution at an age of a few Myr, even as early as ~1 Myr, but age is not the only factor determining the degree of evolution during the first few million years of a disk's lifetime.
Using observations made with the XMM-Newton Observatory, we report the first X-ray detection of the high magnetic field radio pulsar PSR B1916+14. We show that the X-ray spectrum of the pulsar can be well fitted with an absorbed blackbody with temperature in the range of 0.08-0.23 keV, or a neutron star hydrogen atmosphere model with best-fit effective temperature of $\sim$0.10 keV, higher than expected from fast cooling models. The origin of the likely thermal emission is not well constrained by our short observation and is consistent with initial cooling or return-current heating. We found no pulsations in these data and set a 1$\sigma$ upper limit on the pulsed fraction in the 0.1--2 keV band of $\sim$0.7. Implications of these results for our understanding of the different observational properties of isolated neutron stars are discussed.
The Energetic X-ray Imaging Survey Telescope (EXIST) is a proposed next generation multi-wavelength survey mission. The primary instrument is a High Energy telescope (HET) that conducts the deepest survey for Gamma-ray Bursts (GRBs), obscured-accreting and dormant Supermassive Black Holes and Transients of all varieties for immediate followup studies by the two secondary instruments: a Soft X-ray Imager (SXI) and an Optical/Infrared Telescope (IRT). EXIST will explore the early Universe using high redshift GRBs as cosmic probes and survey black holes on all scales. The HET is a coded aperture telescope employing a large array of imaging CZT detectors (4.5 m^2, 0.6 mm pixel) and a hybrid Tungsten mask. We review the current HET concept which follows an intensive design revision by the HET imaging working group and the recent engineering studies in the Instrument and Mission Design Lab at the Goddard Space Flight Center. The HET will locate GRBs and transients quickly (<10-30 sec) and accurately (< 20") for rapid (< 1-3 min) onboard followup soft X-ray and optical/IR (0.3-2.2 micron) imaging and spectroscopy. The broad energy band (5-600 keV) and the wide field of view (~90 deg x 70 deg at 10% coding fraction) are optimal for capturing GRBs, obscured AGNs and rare transients. The continuous scan of the entire sky every 3 hours will establish a finely-sampled long-term history of many X-ray sources, opening up new possibilities for variability studies.
We present the UV, optical, X-ray, and radio properties of the Type IIb SN 2008ax discovered in NGC 4490. The observations in the UV are one of the earliest of a Type IIb supernova (SN). On approximately day four after the explosion, a dramatic upturn in the u and uvw1 (lambda_c = 2600 Angstroms) light curves occurred after an initial rapid decline which is attributed to adiabatic cooling after the initial shock breakout. This rapid decline and upturn is reminiscent of the Type IIb SN 1993J on day six after the explosion. Optical/near-IR spectra taken around the peak reveal prominent H-alpha, HeI, and CaII absorption lines. A fading X-ray source is also located at the position of SN 2008ax, implying an interaction of the SN shock with the surrounding circumstellar material and a mass-loss rate of the progenitor of M_dot = (9+/-3)x10^-6 solar masses per year. The unusual time evolution (14 days) of the 6 cm peak radio luminosity provides further evidence that the mass-loss rate is low. Combining the UV, optical, X-ray, and radio data with models of helium exploding stars implies the progenitor of SN 2008ax was an unmixed star in an interacting-binary. Modeling of the SN light curve suggests a kinetic energy (E_k) of 0.5x10^51 ergs, an ejecta mass (M_ej) of 2.9 solar masses, and a nickel mass (M_Ni) of 0.06 solar masses.
The study of warm molecular gas in the inner regions of protoplanetary disks is of key importance for the study of planet formation and especially for the transport of H2O and organic molecules to the surfaces of rocky planets/satellites. Recent Spitzer observations have shown that the mid-infrared spectra of protoplanetary disks are covered in emission lines due to water and other molecules. Here, we present a non-LTE 2D radiative transfer model of water lines in the 10-36 mum range that can be used to constrain the abundance structure of water vapor, given an observed spectrum, and show that an assumption of local thermodynamic equilibrium (LTE) does not accurately estimate the physical conditions of the water vapor emission zones. By applying the model to published Spitzer spectra we find that: 1) most water lines are subthermally excited, 2) the gas-to-dust ratio must be one to two orders of magnitude higher than the canonical interstellar medium ratio of 100-200, and 3) the gas temperature must be higher than the dust temperature, and 4) the water vapor abundance in the disk surface must be truncated beyond ~ 1 AU. A low efficiency of water formation below ~ 300 K may naturally result in a lower water abundance beyond a certain radius. However, we find that chemistry, may not be sufficient to produce an abundance drop of many orders of magnitude and speculate that the depletion may also be caused by vertical turbulent diffusion of water vapor from the superheated surface to regions below the snow line, where the water can freeze out and be transported to the midplane as part of the general dust settling. Such a vertical cold finger effect is likely to be efficient due to the lack of a replenishment mechanism of large, water-ice coated dust grains to the disk surface.
Most Globular Clusters are believed to host a single stellar populations. They can thus be considered a good place to study the Spite plateau and probe for possible evolutionary modifications of the Li content. We want to determine the Li content of subgiant (SG) and Main Sequence (MS) stars of the old, metal-poor globular cluster NGC 6397. This work was aimed not only at studying possible Li abundance variations but to investigate the cosmological Li discrepancy. Here, we present FLAMES/GIRAFFE observations of a sample of 84 SG and 79 MS stars in NGC 6397 selected in a narrow range of B-V colour and, therefore, effective temperatures. We determine both Teff and A(Li) using 3D hydrodynamical model atmospheres for all the MS and SG stars of the sample. We find a significant difference in the Li abundance between SG stars and MS stars, the SG stars having an A(Li) higher by almost 0.1 dex on average. We also find a decrease in the A(Li) with decreasing Teff, both in MS and SG stars, albeit with a significantly different slope for the two classes of stars. This suggests that the lithium abundance in these stars is, indeed, altered by some process, which is Teff-dependent. The Li abundance pattern observed in NGC 6397 is different from what is found among field stars, casting some doubt on the use of Globular Cluster stars as representative of Population II with respect to the Li abundance. None of the available theories of Li depletion appears to satisfactorily describe our observations.
We report on the first search for extra-terrestrial neutrino-induced cascades in IceCube. The analyzed data were collected in the year 2007 when 22 detector strings were installed and operated. We will discuss the analysis methods used to reconstruct cascades and to suppress backgrounds. Simulated neutrino signal events with a E-2 energy spectrum, which pass the background rejection criteria, are reconstructed with a resolution dlogE ~ 0.27 in the energy range from ~20 TeV to a few PeV. We present the range of the diffuse flux of extra-terrestrial neutrinos in the cascade channel in IceCube within which we expect to be able to put a limit.
We present a detailed analysis of the carbon and nitrogen abundances of two dwarf carbon-enhanced metal-poor (CEMP) stars: SDSS J1349-0229 and SDSS J0912+0216. We also report the oxygen abundance of SDSS J1349-0229. These stars are metal-poor, with [Fe/H] < -2.5, and were selected from our ongoing survey of extremely metal-poor dwarf candidates from the Sloan Digital SkySurvey (SDSS). The carbon, nitrogen and oxygen abundances rely on molecular lines which form in the outer layers of the stellar atmosphere. It is known that convection in metal-poor stars induces very low temperatures which are not predicted by `classical' 1D stellar atmospheres. To obtain the correct temperature structure, one needs full 3D hydrodynamical models. Using CO5BOLD 3D hydrodynamical model atmospheres and the Linfor3D line formation code, molecular lines of CH, NH, OH and C2 were computed, and 3D carbon, nitrogen and oxygen abundances were determined. The resulting carbon abundances were compared to abundances derived using atomic CI lines in 1D LTE and NLTE. There is not a good agreement between the carbon abundances determined from C2 bands and from the CH band, and molecular lines do not agree with the atomic CI lines. Although this may be partly due to uncertainties in the transition probabilities of the molecular bands it certainly has to do with the temperature structure of the outer layers of the adopted model atmosphere. We explore the influence of the 3D model properties on the molecular abundance determination. In particular, the choice of the number of opacity bins used in the model calculations and its subsequent effects on the temperature structure and molecular line formation is discussed. (Abridged)
We have performed hydrodynamical simulations of the long-time evolution of proto-neutron stars to study the nucleosynthesis using the resulting wind trajectories. Although the conditions found in the present wind models are not favourable for the production of heavy elements, a small enhancement of the entropy results in the production of r-process elements with A $\approx$ 195. This allows us to explore the sensitivity of their production to the hydrodynamical evolution (wind termination shock) and nuclear physics input used.
GRB 070610, which is also named Swift J195509.6+261406, is a peculiar Galactic transient with significant variability on short timescales in both X-ray and optical light curves. One possible explanation is that GRB 070610/Swift J195509.6 + 261406 is a soft gamma-ray repeater (SGR) in our Galaxy. Here we use the fireball model which is usually recognized as the standard model of gamma-ray burst (GRB) afterglows, and the energy injection hypothesis to interpret the X-ray and optical afterglow light curves of GRB 070610/Swift J195509.6 + 261406. It is found that the model is generally consistent with observations.
We review the available estimates of the masses of the compact object in Ultraluminous X-ray Sources (ULXs) and critically reconsider the stellar-mass versus intermediate-mass black hole interpretations. Black holes of several hundreds to thousands of $M_\odot$ are not required for the majority of ULXs, although they might be present in the handful of known hyper-luminous ($\sim 10^{41}$ erg s$^{-1}$) objects and/or some sources showing timing features in their power density spectra. At the same time, however, stellar mass BHs may be quite a reasonable explanation for ULXs below $\sim 10^{40}$ erg s$^{-1}$, but they need super-Eddington accretion and some suitable dependence of the beaming factor on the accretion rate in order to account for ULXs above this (isotropic) luminosity. We investigate in detail a 'third way' in which a proportion of ULXs contain $\approx 30-90 M_\odot$ black holes formed in a low metallicity environment and accreting in a slightly critical regime and find that it can consistently account for the properties of bright ULXs. Surveys of ULX locations looking for a statistically meaningful relationship between ULX position, average luminosity and local metallicity will provide a definitive test of our proposal.
The solution for the electromagnetic tornado in a vacuum gap of a pulsar that could serve as an explanation of the observed circular polarization of giant pulses from pulsars and might also explain the frequency strips observed in giant pulses spectrum is found.
We develop a model anisotropy best-fitting to the two-dimensional sky-map of multi-TeV galactic cosmic ray (GCR) intensity observed with the Tibet III air shower (AS) array. By incorporating a pair of intensity excesses in the hydrogen deflection plane (HDP) suggested by Gurnett et al., together with the uni-directional and bi-directional flows for reproducing the observed global feature, this model successfully reproduces the observed sky-map including the "skewed" feature of the excess intensity from the heliotail direction, whose physical origin has long remained unknown. These additional excesses are modeled by a pair of the northern and southern Gaussian distributions, each placed ~50 degree away from the heliotail direction. The amplitude of the southern excess is as large as ~0.2 %, more than twice the amplitude of the northern excess. This implies that the Tibet AS experiment discovered for the first time a clear evidence of the significant modulation of GCR intensity in the heliotail and the asymmetric heliosphere.
We perform a photometric analysis on deep, seeing-limited near-IR VLT images of the region NGC 346/N66 and a nearby control field of the SMC in order to locate the centres of active high- and intermediate-mass star formation through the identification of near-IR bright objects as candidate stellar sources under formation.We use archived imaging data obtained with the high-resolution camera ISAAC at VLT of NGC 346/N66 and we construct the near-IR colour-magnitude (CMD) and colour-colour diagrams (C-CD) of all detected sources. We investigate the nature of all stellar populations in the observed CMDs, and we identify all stellar sources that show significant near-IR excess emission in the observed C-CD. We select, thus, the best candidates for being young stellar sources. Based on their near-IR colours we select 263 candidate young stellar sources. This sample comprises a variety of objects such as intermediate-mass PMS and Herbig Ae/Be stars and possibly massive YSOs, providing original near-IR colours for them. The spatial distribution of the selected candidate sources shows that they are located along the dusty filamentary structures of N66 seen in mid- and far-IR dust emission and agrees very well with that of previously detected candidate YSOs and PMS stars. Our study provides an original accurate set of near-IR colours for candidate young stellar sources. While this adds significant information on the star formation process in NGC 346/N66, it is still necessary to reveal the exact nature of these objects, a process that requires the combination of multi-wavelength data, so that complete Spectral Energy Distributions can be constructed for individual sources. This would be a quite important follow-up study to that presented here.
We analyze the precursory anisotropy of a Forbush decrease observed with the multi-directional muon detector at Sao Martinho in Brazil on December 14, 2006. By subtracting contribution from the diurnal anisotropy precisely determined by the Global Muon Detector Network (GMDN), we succeed in extracting clear signatures of the precursor. The precursor first appeared ten hours prior to the onset of the Storm Sudden Commencement (SSC) as an increase of muon rate at the pitch angle of ~60 degrees around the IMF. This increase is consistent with the measurement of galactic cosmic rays reflected and accelerated by an interplanetary shock approaching toward the Earth with a radial speed of ~1160 km/sec. This intensity increase is observed for four hours and then followed by an intensity deficit known as a loss cone (LC) around ~0 degree pitch angle during the next four hours until the SSC onset. Weak signature of LC is also observed with Sao Martinho one day earlier on December 13, at the similar local time as December 14. This suggests that the LC appeared only 6.6 hours after the CME eruption on the sun, when the interplanetary shock was expected to be located 0.2 AU from the sun.
The sidereal anisotropy of galactic cosmic ray (GCR) intensity observed with the Tibet Air Shower (AS) experiment still awaits theoretical interpretation. The observed global feature of the anisotropy is well reproduced by a superposition of the bi-directional and uni-directional flows (BDF and UDF, respectively) of GCRs. If the orientation of the deduced BDF represents the orientation of the local interstellar magnetic field (LISMF), as indicated by best-fitting a model to the data, the UDF deviating from the BDF orientation implies a significant contribution from the streaming perpendicular to the LISMF. This perpendicular streaming is probably due to the drift anisotropy, because the contribution from the perpendicular diffusion is expected to be much smaller than the drift effect. The large amplitude deduced for the UDF indicates a large spatial gradient of the GCR density. We suggest that such a density gradient can be expected at the heliosphere sitting close to the boundary of the Local Interstellar Cloud (LIC), if the LIC is expanding. The spatial distribution of GCR density in the LIC reaches a stationary state because of the balance between the inward cross-field diffusion and the adiabatic cooling due to the expansion. We derive the steady-state distribution of GCR density in the LIC based on radial transport of GCRs in a spherical LIC expanding at a constant rate. By comparing the expected gradient with the observation by Tibet experiment, we estimate the perpendicular diffusion coefficient of multi-TeV GCRs in the local interstellar space.
Millisecond and binary pulsars are the most stable astronomical standards of frequency. They can be applied to solving a number of problems in astronomy and time-keeping metrology including the search for a stochastic gravitational wave background in the early universe, testing general relativity, and establishing a new time-scale. The full exploration of pulsar properties requires that proper unbiased estimates of spin and orbital parameters of the pulsar be obtained. These estimates depend essentially on the random noise components present in pulsar timing residuals. The instrumental white noise has predictable statistical properties and makes no harm for interpretation of timing observations, while the astrophysical/geophysical low-frequency noise corrupts them, thus, reducing the quality of tests of general relativity and decreasing the stability of the pulsar time scale.
The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) is a consortium of astronomers whose goal is the creation of a galactic scale gravitational wave observatory sensitive to gravitational waves in the nHz-microHz band. It is just one component of an international collaboration involving similar organizations of European and Australian astronomers who share the same goal. Gravitational waves, a prediction of Einstein's general theory of relativity, are a phenomenon of dynamical space-time generated by the bulk motion of matter, and the dynamics of space-time itself. They are detectable by the small disturbance they cause in the light travel time between some light source and an observer. NANOGrav exploits radio pulsars as both the light (radio) source and the clock against which the light travel time is measured. In an array of radio pulsars gravitational waves manifest themselves as correlated disturbances in the pulse arrival times. The timing precision of today's best measured pulsars is less than 100 ns. With improved instrumentation and signal-to-noise it is widely believed that the next decade could see a pulsar timing network of 100 pulsars each with better than 100 ns timing precision. Such a pulsar timing array (PTA), observed with a regular cadence of days to weeks, would be capable of observing supermassive black hole binaries following galactic mergers, relic radiation from early universe phenomena such as cosmic strings, cosmic superstrings, or inflation, and more generally providing a vantage on the universe whose revolutionary potential has not been seen in the 400 years since Galileo first turned a telescope to the heavens.
Hot accretion flows such as advection-dominated accretion flows are generally optically thin in the radial direction. Thus photons generated at some radii can cool or heat electrons at other radii via Compton scattering. Such global Compton scattering has previously been shown to be important for the dynamics of accretion flows. Here, we extend previous treatments of this problem by using accurate global general relativistic Monte Carlo simulations. We focus on an inner region of the accretion flow, for which we obtain a global self-consistent solution. As compared to the initial, not self-consistent solution, the final solution has both the cooling rate and the electron temperature significantly reduced at radii > 10 gravitational radii. On the other hand, the radiation spectrum of the self-consistent solution has the shape similar to that of the initial iteration, except for the high-energy cut-off being at an energy lower by a factor of ~ 2 and the bolometric luminosity decreased by a factor of ~ 2. We also compare the global Compton scattering model with local models in spherical and slab geometry. We find that the slab model approximates the global model significantly better than the spherical one. Still, neither local model gives a good approximation to the radial profile of the cooling rate, and the differences can be up to two orders of magnitude. The local slab model underestimates the cooling rate at outer regions whereas it overestimates that rate at inner regions.
We study the particle energy distribution in the cocoon surrounding Cygnus A, using radio images between 151 MHz and 15 GHz and a 200 ks Chandra ACIS-I image. We show that the excess low frequency emission in the the lobe further from Earth cannot be explained by absorption or excess adiabatic expansion of the lobe or a combination of both. We show that this excess emission is consistent with emission from a relic counterlobe and a relic counterjet that are being re-energized by compression from the current lobe. We detect hints of a relic hotspot at the end of the relic X-ray jet in the more distant lobe. We do not detect relic emission in the lobe nearer to Earth as expected from light travel-time effects assuming intrinsic symmetry. We determine that the duration of the previous jet activity phase was slightly less than that of the current jet-active phase. Further, we explain some features observed at 5 and 15 GHz as due to the presence of a relic jet.
Using simple stellar population synthesis, we model the bulge stellar contribution in the optical spectrum of a narrow-line Seyfert 1 galaxy RE J1034+396. We find that its bulge stellar velocity dispersion is $67.7\pm 8$ \kms. The supermassive black hole (SMBH) mass is about $(1-4)\times 10^6 \msun$ if it follows the well-known $\mbh-\sigma_*$ relation found in quiescent galaxies. We also derive the SMBH mass from the H$\beta$ second moment, which is consistent with that from its bulge stellar velocity dispersion. The SMBH mass of $(1-4)\times 10^6 \msun$ implies that the X-ray quasi-periodic oscillation (QPO) of RE J1034+396 can be scaled to a high-frequency QPO at 27-108 Hz found in Galactic black hole binaries with a 10 $\msun$ black hole. With the mass distribution in different age stellar populations, we find that the mean specific star formation rate (SSFR) over past 0.1 Gyr is $0.0163\pm 0.0011$ $\rm Gyr^{-1}$, the stellar mass in the logarithm is $10.155\pm 0.06$ in units of solar mass, and the current star formation rate is $0.23\pm 0.016 \msun \rm yr^{-1}$. RE J1034+396 does not follow the relation between the Eddington ratio and the SSFR suggested by Chen et al., although a larger scatter in their relation. We also suggest that about 7.0% of the total \ha luminosity and 50% of the total \oii luminosity come from the star formation process.
We present new information on galaxies in the vicinity of luminous radio galaxies and quasars at z=4,5,6. These fields were previously found to contain overdensities of Lyman Break Galaxies (LBGs) or spectroscopic Lyman alpha emitters. We use HST and Spitzer data to infer stellar masses, and contrast our results with large samples of LBGs in more average environments as probed by the Great Observatories Origins Deep Survey (GOODS). The following results were obtained. First, LBGs in both overdense regions and in the field at z=4-5 lie on a very similar sequence in a z'-[3.6] versus [3.6] color-magnitude diagram. This is interpreted as a sequence in stellar mass (log[M*/Msun] = 9-11) in which galaxies become increasingly red due to dust and age as their star formation rate (SFR) increases. Second, the two radio galaxies are among the most massive objects (log[M*/Msun]~11) known to exist at z~4-5, and are extremely rare based on the low number density of such objects as estimated from the ~25x larger area GOODS survey. We suggest that the presence of these massive galaxies and supermassive black holes has been boosted through rapid accretion of gas or merging inside overdense regions. Third, the total stellar mass found in the z=4 ``proto-cluster'' TN1338 accounts for <30% of the stellar mass on the cluster red sequence expected to have formed at z>4, based on a comparison with the massive X-ray cluster Cl1252 at z=1.2. Although future near-infrared observations should determine whether any massive galaxies are currently being missed, one possible explanation for this mass difference is that TN1338 evolves into a smaller cluster than Cl1252. This raises the interesting question of whether the most massive protocluster regions at z>4 remain yet to be discovered.
We present results of a series of magnetohydrodynamic (MHD) and hydro- dynamic (HD) 2.5D simulations of the morphology of outflows driven by nested wide-angle winds - i.e. winds which eminate from a central star as well as from an orbiting accretion disk. While our results are broadly relevent to nested wind systems we have tuned the parameters of the simulations to touch on issues in both Young Stellar Objects and Planetary Nebula studies. In particular our studies connect to open issues in the early evolution of Planetary Nebulae. We find that nested MHD winds exhibit marked morphological differences from the single MHD wind case along both dimensions of the flow. Nested HD winds on the other hand give rise mainly to geometric distortions of an outflow that is topologically similar to the flow arising from a single stellar HD wind. Our MHD results are insensitive to changes in ambient temperature between ionized and un-ionized circumstellar environments. The results are sensitive to the relative mass-loss rates, and to the relative speeds of the stellar and disk winds. We also present synthetic emission maps of both nested MHD and HD simulations. We find that nested MHD winds show knots of emission appearing on-axis that do not appear in the HD case.
We present a general picture of the ongoing formation and evolution of early-type galaxies via a specific evolutionary sequence starting in the blue cloud and ending in the low-mass end of the red sequence. This evolutionary sequence includes a Seyfert AGN phase in the green valley, but this phase occurs too late after the shutdown of star formation to be responsible for it. Thus, the bulk of black hole accretion in low-redshift early-type galaxies occurs in post-starburst objects, and not concurrent with star formation. On the other hand, a low-luminosity AGN phase switching on at an earlier stage when some star formation activity remains may be responsible for destroying the molecular gas reservoir fueling star formation.
MASSCLEAN is a new, sophisticated and robust stellar cluster image and photometry simulation package. This package is able to create color-magnitude diagrams and standard FITS images in any of the traditional optical and near-infrared bands based on cluster characteristics input by the user, including but not limited to distance, age, mass, radius and extinction. At the limit of very distant, unresolved clusters, we have checked the integrated colors created in MASSCLEAN against those from other simple stellar population (SSP) models with consistent results. Because the algorithm populates the cluster with a discrete number of tenable stars, it can be used as part of a Monte Carlo Method to derive the probabilistic range of characteristics (integrated colors, for example) consistent with a given cluster mass and age. We present the first ever mass dependent integrated colors as a function of age, derived from over 100,000 Monte Carlo runs, which can be used to improve the current age determination methods for stellar clusters.
The description is given for the speckle interferometer of the BTA 6-m telescope of the SAO RAS based on a new detector with an electron multiplication CCD. The main components of the instrument are microscope objectives, interference filters and atmospheric dispersion correction prisms. The PhotonMAX-512B CCD camera using a back-illuminated CCD97 allows up to 20 speckle images (with 512$\times$512 pix resolution) per second storage on the hard drive. Due to high quantum efficiency (93% in the maximum at 550 nm), and high transmission of its optical elements, the new camera can be used for diffraction-limited (0.02$''$) image reconstruction of $15^{m}$ stars under good seeing conditions. The main advantages of the new system over the previous generation BTA speckle interferometer are examined.
Spectral molecular line profile observations of star-forming molecular clouds sometimes show distinct red asymmetric double-peaked molecular line profiles with weaker blue peaks and stronger red peaks. For some star-forming molecular clouds, such molecular transitions with red asymmetric line profiles and blue asymmetric line profiles (i.e. blue asymmetric double-peaked molecular line profiles with weaker red peaks and stronger blue peaks) may coexist in spatially resolved spectral observations, while for others, such molecular transitions with red asymmetric line profiles may completely dominate in spatially resolved spectral observations. Blue asymmetric line profiles are usually interpreted as signals of central core collapses, while red asymmetric line profiles remain unexplained. In this paper, we advance a spherically symmetric self-similar hydrodynamic model framework for envelope expansions with core collapses (EECC) of a general polytropic molecular gas cloud under self-gravity. Based on such EECC hydrodynamic cloud models, we perform tracer molecular line profile calculations using the publicly available RATRAN code for star-forming clouds with spectroscopic signatures of red asymmetric line profiles. The presence of red asymmetric line profiles from molecular cloud cores indicates that EECC processes are most likely an essential hydrodynamic process of star formation. With spatial distributions, we explore various profiles of molecular lines for several tracer molecules in different settings of EECC dynamic models with and without shocks.
We have carried out a survey of the dense clumps associated with 14 embedded clusters in the C^18O (J=1-0) line emission with the Nobeyama 45m telescope in order to understand the formation and evolution of stellar clusters in dense clumps of molecular clouds. We have selected these clusters at distances from 0.3 to 2.1kpc and have mapped about 6' X 6' to 10' X 10'regions (corresponding to 3.8pc X 3.8pc at 2.1kpc) for all the clumps with 22" resolution (corresponding to Jeans length at 2.1kpc). We have obtained dense clumps with radii of 0.40-1.6pc, masses of 150-4600M_sun, and velocity widths in FWHM of 1.4-3.3kms^-1. Most of the clumps are found to be approximately in virial equilibrium, which implies that C^18O gas represents parental dense clumps for cluster formation. From the spatial relation between the distributions of clumps and clusters, we classified C^18O clumps into three types (Type A, B, and C). The C^18O clumps as classified into Type A have emission distributions with a single peak at the stellar clusters and higher brightness contrast than that of other target sources. Type B clumps have double or triple peaks which are associated with the cluster and moderately high brightness contrast structure. Type C clumps have also multiple peaks although they are not associated with the cluster and low brightness contrast structure. We suggest that our classification represents an evolutionary trend of cluster-forming dense clumps because dense gas in molecular clouds is expected to be converted into stellar constituents, or to be dispersed by stellar activities. Moreover, although there is a scatter, we found a tendency that the SFEs of the dense clumps increase from Type A to Type C, which also supports our scenario.
Star formation in relic HII regions of the first stars is investigated using magneto-hydrodynamical simulations with a nested grid method that covers 10 orders of magnitude in spatial scale and 20 orders of magnitude in density contrast. Due to larger fraction of H_2 and HD molecules, its prestellar thermal evolution is considerably different from that in the first star formation. Reflecting the difference, two hydrostatic cores appear in a nested manner: a protostar is enclosed by a transient hydrostatic core, which appears during the prestellar collapse. If the initial natal core rotates fast at a rate with rotational to gravitational energy ratio \beta_0 > 0.01-0.1, the transient hydrostatic core fragments to \sim 10 M_\odot sub-cores at density \sim 10^9 cm^-3. With smaller rotation energy, fragmentation occurs at higher density while a single protostar forms without fragmentation if rotation is extremely slow with \beta_0 < 10^-6 -10^-5. If magnetic field is present, these threshold values of \beta_0 is boosted owing to angular momentum transport by the magnetic breaking. Magnetic field also drives the protostellar outflows. With strong magnetic field, two distinct outflows are observed: The slower one emanates from the transient hydrostatic core, while the faster one from the protostar. These flows may affect the final stellar mass by ejecting some of masses in the initial core, and also may play some role in driving and maintenance of interstellar turbulence in young galaxies.
We present an analysis of regular timing observations of the high-magnetic-field Rotating Radio Transient (RRAT) J1819$-$1458 obtained using the 64-m Parkes and 76-m Lovell radio telescopes over the past five years. During this time, the RRAT has suffered two significant glitches with fractional frequency changes of $0.6\times10^{-6}$ and $0.1\times10^{-6}$. Glitches of this magnitude are a phenomenon displayed by both radio pulsars and magnetars. However, the behaviour of J1819$-$1458 following these glitches is quite different to that which follows glitches in other neutron stars, since the glitch activity resulted in a significant long-term net decrease in the slow-down rate. If such glitches occur every 30 years, the spin-down rate, and by inference the magnetic dipole moment, will drop to zero on a timescale of a few thousand years. There are also significant increases in the rate of pulse detection and in the radio pulse energy immediately following the glitches.
We present the observations of the afterglow of gamma-ray burst GRB 090102. We use optical data taken by the TAROT, REM, GROND, Palomar and NOT telescopes, and X-ray data taken by the XRT instrument on board the Swift spacecraft. This event features an unusual light curve. In X-rays, it presents a very monotonic decrease with no hint of temporal break from 0.005 to 6 days after the burst. In optical, the light curve presents a flattening after 1 ks. Before this break, the optical light curve is steeper than the X-ray one. In optical, no further break is observed up to 10 days after the burst. We tried to explain these observations in light of the standard fireball model, but we failed to do so. We then investigated several other models, like the cannonball model. We find that the explanation of the broad band data by any model requires a strong fine tuning when taking into account both optical and X-ray bands.
We have investigated some of the properties of dense sub-nuclear matter at the crustal region (both the outer crust and the inner crust region) of a magnetar. The relativistic version of Thomas-Fermi (TF) model is used in presence of strong quantizing magnetic field for the outer crust matter. The compressed matter in the outer crust, which is a crystal of metallic iron, is replaced by a regular array of spherically symmetric Wigner-Seitz (WS) cells. In the inner crust region, a mixture of iron and heavier neutron rich nuclei along with electrons and free neutrons has been considered. Conventional Harrison-Wheeler (HW) and Bethe-Baym-Pethick (BBP) equation of states are used for the nuclear mass formula. A lot of significant changes in the characteristic properties of dense crustal matter, both at the outer crust and the inner crust, have been observed.
It is well known that the diffuse gamma-rays are produced by the collisions between the galactic cosmic rays with the stellar matter (Hayakawa-Morrison Hypothesis). In this paper the author tries to estimate the contribution of star flares in the low energy component of the diffuse component of the galactic gamma-rays. It is expected that high energy part of the disuse component of the galactic gamma-rays reflects the power spectrum of galactic cosmic rays with gamma= -2.75 (in differential). In the galactic disk and bulge, there are more than 100 billion stars. They might often make flare and produce high energy cosmic rays by the flare in the energy range from 100 MeV to 1 TeV with the power index gamma= -3.75 (in differential). In this paper the author tries to estimate the contribution of the star flares to the diffuse component of gamma-rays.
A solar neutron detector has been launched on the Japan Exposure Module (JEM) of the International Space Station (ISS) in August 4, 2009. The detector works satsifactory. The detector comprises scintillation fiber and is designated as the Space Environment Data Acquisition (SEDA)-FIB. It tracks the recoil protons induced by neutrons and measurement of the proton energy using the range method. The energy resolution of the detector was measured using the proton beam at Riken. Herein, we report the energy resolution of the FIB detector by two different methods.
It is well known that the dark matter dominates the dynamics of galaxies and clusters of galaxies. Its constituents remain a mystery despite an assiduous search for them over the past three decades. Recent results from the satellite-based PAMELA experiment detect an excess in the positron fraction at energies between 10-100 GeV in the secondary cosmic ray spectrum. Other experiments namely ATIC, HESS and FERMI show an excess in the total electron (\ps + \el) spectrum for energies greater 100 GeV. These excesses in the positron fraction as well as the electron spectrum could arise in local astrophysical processes like pulsars, or can be attributed to the annihilation of the dark matter particles. The second possibility gives clues to the possible candidates for the dark matter in galaxies and other astrophysical systems. In this article, we give a report of these exciting developments.
Small angular scale (high l) studies of cosmic microwave background anisotropies require accurate knowledge of the statistical properties of extragalactic sources at cm-mm wavelengths. We have used a 30 GHz dual-beam receiver (OCRA-p) on the Torun 32-m telescope to measure the flux densities of 121 sources in VSA fields selected at 15 GHz with the Ryle Telescope. We have detected 57 sources above a limiting flux density of 5mJy, of which 31 sources have a flux density greater than 10mJy, which is our effective completeness limit. From these measurements we derive a surface density of sources above 10mJy at 30 GHz of 2.0+/-0.4 per square degree. This is consistent with the surface density obtained by Mason et al. (2009) who observed a large sample of sources selected at a much lower frequency (1.4 GHz). We have also investigated the dependence of the spectral index distribution on flux density by comparing our results with those for sources above 1 Jy selected from the WMAP 22 GHz catalogue. We conclude that the proportion of steep spectrum sources increases with decreasing flux density, qualitatively consistent with the predictions of deZotti et al. (2005). We find no evidence for an unexpected population of sources whose spectra rise towards high frequencies, which would affect our ability to interpret current high resolution CMB observations at 30 GHz and above.
This study employed grain dynamic models to examine the density distribution of debris discs, and discussed the effects of the collisional time-intervals of asteroidal bodies, the maximum grain sizes, and the chemical compositions of the dust grains of the models, in order to find out whether a steady out-moving flow with an 1/R profile could be formed. The results showed that a model with new grains every 100 years, a smaller maximum grain size, and a composition C400 has the best fit to the 1/R profile because: (1) the grains have larger values of beta on average,therefore, they can be blown out easily; (2) the new grains are generated frequently enough to replace those have been blown out. With the above two conditions, some other models can have a steady out-moving flow with an approximate 1/R profile. However, those models in which new grains are generated every 1000 years have density distributions far from the profile of a continuous out-moving flow. Moreover, the analysis on the signatures of planets in debris discs showed that there are no indications when a planet is in a continuous out-moving flow, however, the signatures are obvious in a debris disc with long-lived grains.
Many brightest cluster galaxies (BCGs) at the centers of X-ray selected clusters exhibit clear evidence for recent star formation. However, studies of BCGs in optically-selected clusters show that star formation is not enhanced when compared to control samples of non-BCGs of similar stellar mass. Here we analyze a sample of 113 BCGs in low redshift (z<0.1), optically-selected clusters, a matched control sample of non-BCGs, and a smaller sample of BCGs in X-ray selected clusters. We convolve the SDSS images of the BCGs to match the resolution of the GALEX data and we measure UV-optical colours in their inner and outer regions. We find that optically-selected BCGs exhibit smaller scatter in optical colours and redder inner NUV-r colours than the control galaxies, indicating that they are a homogenous population with very little ongoing star formation. The BCGs in the X-ray selected cluster sample span a similar range in optical colours, but have bluer NUV-r colours. Among X-ray selected BCGs, those located in clusters with central cooling times of less than 1 Gyr are significantly bluer than those located in clusters where the central gas cooling times are long. Our main conclusion is that the location of a galaxy at the centre of its halo is not sufficient to determine whether or not it is currently forming stars. One must also have information about the thermodynamic state of the gas in the core of the halo.
We have developed the method that allows us to estimate the magnetic field strength at the horizon of a supermassive black hole (SMBH) through the observed polarization of optical emission of the accreting disk surrounding SMBH. The known asymptotic formulae for the Stokes parameters of outgoing radiation are azimuthal averaged, which corresponds to an observation of the disk as a whole. We consider two models of the embedding 3D-magnetic field, the regular field, and the regular field with an additional chaotic (turbulent) component. It is shown that the second model is preferable for estimating the magnetic field in NGC 4258. For estimations we used the standard accretion disk model assuming that the same power-law dependence of the magnetic field follows from the range of the optical emission down to the horizon. The observed optical polarization from NGC 4258 allowed us to find the values 10^3 - 10^4 Gauss at the horizon, depending on the particular choice of the model parameters. We also discuss the wavelength dependencies of the light polarization, and possibly applying them for a more realistic choice of accretion disk parameters.
The purpose of this paper is to characterize the statistical properties of solar granulation in the photosphere and low chromosphere up to 650 km. We use velocity and intensity variations obtained at different atmospheric heights from observations in BaII 4554 A. The observations were done during good seeing conditions at the VTT at the Observatorio del Teide on Tenerife. The line core forms rather high in the atmosphere and allows granulation properties to be studied at heights that have been not accessed before in similar studies. In addition, we analyze the synthetic profiles of the BaII 4554 A line by the same method computed taking NLTE effects into account in the 3D hydrodynamical model atmosphere. We suggest a 16-column model of solar granulation depending on the direction of motion and on the intensity contrast measured in the continuum and in the uppermost layer. We calculate the heights of intensity contrast sign reversal and velocity sign reversal. We show that both parameters depend strongly on the granulation velocity and intensity at the bottom photosphere. The larger the two parameters, the higher the reversal takes place in the atmosphere. On average, this happens at about 200-300 km. We suggest that this number also depends on the line depth of the spectral line used in observations. Despite the intensity and velocity reversal, about 40% of the column structure of granulation is preserved up to heights around 650 km.
Rapidly oscillating Ap stars exhibit an astrophysically interesting combination of strong, dipolar-like magnetic fields and high-overtone p-mode pulsations similar to the Sun. Recent time-resolved spectroscopy of these stars unravelled a complex picture of propagating magneto-acoustic pulsation waves, with amplitude and phase strongly changing as a function of atmospheric height. To interpret these observations and gain a new insight into the atmospheric dynamics of roAp stars we have carried out 2-D time-dependent, non-linear magneto-hydrodynamical simulations of waves for a realistic atmospheric stratification of a cool Ap star. We explore a grid of simulations in a wide parameter space, treating oscillations of the velocity, magnetic field and thermodynamic quantities in a self-consistent manner. Our simulations foster a new understanding of the influence of the atmosphere and the magnetic field on the propagation and reflection properties of magneto-acoustic waves, formation of node surfaces, and relative variation of different quantities. Our simulations reproduce all main features of the observed pulsational behavior of roAp stars. We show, for the first time, that the overall depth dependence of the pulsations in roAp atmospheres is strongly influenced by the density inversion at the photospheric base.
Recent 1.2-mm continuum observations have shown the giant HII region NGC3576 to be embedded in the centre of an extended filamentary dust-cloud. The bulk of the filament away from the HII region contains a number of clumps seen only at (sub-)millimetre wavelengths and which may host massive protostellar objects at a very early stage of evolution. We have used the Australia Telescope Compact Array (ATCA) to image the cloud for the NH3(1,1), (2,2) and (4,4) transitions, 22 GHz water masers, and 23 GHz continuum emission. We also utilised the 22-m Mopra antenna to map the region for the molecular lines 13CO (1-0), C18O (1-0), HCO+ (1-0), H13CO+ (1-0), CS (1-0) and N2H+ (1-0).The HII region is observed to be expanding into the molecular cloud, sweeping up a clumpy shell of gas, while the central star cluster is dispersing the molecular gas to the east. Temperatures are highest adjacent to the central HII region, indicating that the embedded cluster of young stars there is heating the gas. Six new water masers were detected in the arms of the filament, all associated with NH3 emission peaks, confirming that star-formation has begun within these cores. Core masses range from 5 to 516 solar masses and most appear to be gravitationally bound. Complementary results by Andr\'e et al. (2008) imply that seven cores will go on to form massive stars between 15 and 50 solar masses. The large scale velocity structure of the filament is smooth, but at least one clump shows the signature of inward gas motions via asymmetries in the NH3 (1,1) line profiles. The same clump exhibits an enhanced abundance of N2H+, which coupled with an absence of CO indicates depletion onto the dust grain surface. The HII region at the heart of NGC3576 is potentially triggering the formation of massive stars in the bulk of the associated cloud.
We use XMM-Newton data to carry out a detailed study of the Si, Fe and Ni abundances in the cool cores of a representative sample of 26 local clusters. We have performed a careful evaluation of the systematic uncertainties related to the instruments, the plasma codes and the spectral modeling finding that the major source of uncertainty is in the plasma codes. Our Si, Fe, Ni, Si/Fe and Ni/Fe distributions feature only moderate spreads (from 20% to 30%) around their mean values strongly suggesting similar enrichment processes at work in all our cluster cores. Our sample averaged Si/Fe ratio is comparable to those measured in samples of groups and high luminosity ellipticals implying that the enrichment process in ellipticals, dominant galaxies in groups and BCGs in clusters is quite similar. Although our Si/Fe and Ni/Fe abundance ratios are fairly well constrained, the large uncertainties in the supernovae yields prevent us from making a firm assessment of the relative contribution of type Ia and core-collapsed supernovae to the enrichment process. All that can really be said with some certainty is that both contribute to the enrichment of cluster cores.
Over the last decades, numerous wide (>1000 AU) binaries have been discovered in the Galactic field and halo. The origin of these wide binaries cannot be explained by star formation or by dynamical interactions in the Galactic field. We explain their existence by wide binary formation during the dissolution phase of young star clusters. In this scenario, two single stars that leave the dissolving cluster at the same time, in the same direction, and with similar velocities, form a new, very wide binary. Using N-body simulations we study how frequently this occurs, and how the orbital parameters of such binaries depend on the properties of the cluster from which they originate. The resulting wide binary fraction for individual star clusters is 1-30%, depending on the initial conditions. As most stars form as part of a binary or multiple system, we predict that a large fraction of these wide binaries are in fact wide triple and quadruple systems.
We investigate the oscillating effective equation of state (EoS) of the universe around the phantom divide in the framework of $F(R)$ gravity. We illustrate the behavior of $F(R)$ with realizing multiple crossings of the phantom divide.
We present the star formation history (SFH) and its variations with
galactocentric distance for the Local Group dwarf galaxy of Phoenix.
Color-magnitude diagram was obtained from WFPC2@HST reaching the oldest main
sequence turnoffs. The IAC-star and IAC-pop codes and the MinnIAC suite have
been used to obtain the star formation rate as a function of time, metallicity,
and radius. We find that Phoenix has had ongoing but gradually decreasing star
formation over nearly a Hubble time. The highest level of star formation
occurred from the formation of the galaxy till 10.5 Gyr ago, when 50% of the
total star formation had already taken place. From that moment, star formation
continues at a significant level until 6 Gyr ago, and at a very low level till
the present time. The chemical enrichment law shows a trend of slowly
increasing metallicity as a function of time till 8--6 Gyr ago, when Z starts
to increase steeply to the current value.
Young stars are found in the inner region of the galaxy only, but
intermediate-age and old stars can be found at all galactocentric distances.
This study shows that star formation started at all galactocentric distances in
Phoenix at an early epoch. Our results are compatible with a scenario in which
the star formation region envelope slowly shrinks as time goes on, possibly as
a natural result of pressure support reduction as gas supply diminishes. As a
consequence, star formation stopped first in outer regions and the scale-length
of the stellar mass density distribution decreased with time. No traces of a
true, old halo are apparent in Phoenix either in its stellar age distribution
or in the stellar mass density distribution, at least out to 0.5 kpc (about 2.5
scale-lengths) from the center.
The production of high-energy neutrinos is possible in several types of astrophysical sources. The detection of such neutrinos would yield information complementary to that provided by photons and charged particles. This paper presents results from a point source analysis using data taken during 2007 with the five-line partial configuration of the ANTARES neutrino telescope. This search was performed using a binned and an unbinned method. Results are presented both for a list of candidate sources and an all-sky scan. No evidence for any statistically significant neutrino excess has been found. The corresponding upper limits on the cosmic neutrino flux have been determined. Assuming an $E^{-2}$ spectrum for the sources and for the energy range 3 -- 400 TeV, the upper limits at 90% C.L. are $E^{2}d\phi_{\nu_{\mu}}/dE \sim 3-10\times10^{-10}$ TeV cm$^{-2}$ s$^{-1}$, obtained for declinations between $-90^{\circ}$ and $15^{\circ}$.
A sample of edge-on spiral galaxies aimed at a study of the main structural
and photometric parameters of edge-on galaxies both of early and late types is
presented. The data were taken from the 2MASS in the J, H and K_s filters. The
sample consists of 175 galaxies in the K_s-filter, 169 galaxies in the H-filter
and 165 galaxies in the J-filter. We present bulge and disc decompositions of
each galaxy image. All galaxies have been modelled with a Sersic bulge and
exponential disc with the BUDDA v2.1 package.
The main conclusions of our general statistical analysis of the sample are:
(1) The distribution of the apparent bulge axis ratio q_b for the subsample
with n < 2 can be attributed to triaxial, nearly prolate bulges that are seen
from different projections, while n > 2 bulges seem to be oblate spheroids with
moderate flattening.
(2) For the sample galaxies, the effective radius of the bulge r_{e,b}, the
disc scalelength h and the disc scaleheight z_0 are well correlated. However,
there is a clear trend for the ratio r_{e,b}/h to increase with n.
(3) There is a hint that the fundamental planes of discs, which links only
disc parameters and the maximum rotational velocity of gas, are different for
galaxies with different bulges.
(4) The investigation of the Photometric Plane of sample bulges shows that
the plane is not flat and has a prominent curvature towards small values of n.
For bulges this fact was not noticed earlier.
(5) The clear relation between the flattening of stellar discs h/z_0 and the
relative mass of a spherical component, including a dark halo, is confirmed not
for bulgeless galaxies but for galaxies with massive bulges. (Abridged)
We study two dimensional turbulent magnetic reconnection in a compressible fluid in the gas pressure dominated limit. We use open boundary conditions and start from a Harris current sheet configuration with a uniform total pressure. A small perturbation to the vector potential initiates laminar reconnection at the Sweet-Parker rate, which is allowed to evolve for several dynamical times. Subsequently sub-Alfvenic turbulence is produced through random forcing at small wave numbers. The magnetic field topology near the current sheet is strongly affected by the turbulence. However, we find that the resulting reconnection speed depends on the resistivity. In contrast to previous results in three dimensions, we find no evidence for fast reconnection. The reconnection speed exhibits large variations but the time averages increase smoothly with the strength of the turbulence.
Dwarf galaxies are the numerically dominating population in the dense regions of the universe. Although they seem to be simple systems at first view, the stellar populations of dwarf elliptical galaxies (dEs) might be fairly complex. Nucleated dEs are of particular interest, since a number of objects exhibit different stellar populations in their nuclei and host galaxy. We present stellar population parameters obtained from integrated optical spectra using a Lick index analysis of seven nucleated dwarf elliptical galaxies and their nuclei. After subtracting the scaled galaxy spectra from the nucleus spectra, we compared them with one another and explore their stellar populations. As a preliminary result, we find that the luminosity weighted ages of the nuclei slightly lower than those of galaxies, however, we do not see any significant difference in metallicity of the host galaxies and their nuclei.
We present near infrared spectroscopy of the recurrent nova RS Oph obtained on several occasions after its latest outburst in 2006 February. The 1-5 mircon spectra are dominated by the red giant, but the H I, He I, and coronal lines present during the eruption are present in all our observations. From the fits of the computed infrared spectral energy distributions to the observed fluxes we find T_eff=4200+/-200,K for the red giant. The first overtone CO bands at 2.3 micron, formed in the atmosphere of the red giant, are variable. The spectra clearly exhibit an infrared excess due to dust emission longward of 5 micron; we estimate an effective temperature for the emitting dust shell of 500K, and find that the dust emission is also variable, being beyond the limit of detection in 2007. Most likely, the secondary star in RS Oph is intrinsically variable.
We used deep wide field photometric observations to derive the fraction of binary systems in a sample of five high-latitude Galactic open clusters. By analysing the color distribution of Main Sequence stars we derived the minimum fraction of binary systems required to reproduce the observed color-magnitude diagram morphologies. We found that all the analysed clusters contain a minimum binary fraction larger than 11% within the core radius. The estimated global fractions of binary systems range from 35% to 70% depending on the cluster. The comparison with homogeneous estimates performed in globular clusters indicates that open clusters hold a significantly higher fraction of binary systems, as predicted by theoretical models and N-body simulations. A dependence of the relative fraction of binary systems on the cluster mass has been detected, suggesting that the binary disruption within the cluster core is the dominant process that drives the fraction of binaries in stellar systems.
The relative abundance of deuterium (D) in the solar atmosphere is not known. D is not only destroyed in stars, it is also synthesized in the atmospheres of active stars (Prodanovic & Fields 2003). In several cases, production of D in the sun has been detected when solar flares occur, using both energetic particle measurements (Anglin 1975) and by detection of 2.223 MeV gamma rays emitted by D (Terekhov et al. 1996; Shih et al. 2009). We describe a project to measure the abundance of deuterium in the solar wind, and to monitor its evolution during a several-year period. The instrument consists of two grids, a tritium target, and semiconductor particle detectors. The grids, which are hemispherical and concentric, accelerate the incident solar wind ions using a potential difference on the order of ~80 to 100 kV and concentrate the ions on the tritium target. A fraction of the solar wind deuterons thus accelerated interact with the target to produce 3.6 MeV alpha particles, some of which are recorded by adjacent semiconductor detectors. A similar instrument was successfully tested in space in 1975 in order to observe positive auroral ions in a hydrogen aurora.
We compute 3D gasdynamical models of jet outflows from the central AGN, that carry mass as well as energy to the hot gas in galaxy clusters and groups. These flows have many attractive attributes for solving the cooling flow problem: why the hot gas temperature and density profiles resemble cooling flows but show no spectral evidence of cooling to low temperatures. Subrelativistic jets, described by a few parameters, are assumed to be activated when gas flows toward or cools near a central SMBH. Using approximate models for a rich cluster (A1795), a poor cluster (2A 0336+096) and a group (NGC 5044), we show that mass-carrying jets with intermediate mechanical efficiencies ($\sim10^{-3}$) can reduce for many Gyr the global cooling rate to or below the low values implied by X-spectra, while maintaining $T$ and $\rho$ profiles similar to those observed, at least in clusters. Groups are much more sensitive to AGN heating and present extreme time variability in both profiles. Finally, the intermittency of the feedback generates multiple generations of X-ray cavities similar to those observed in Perseus cluster and elsewhere. Thus we also study the formation of buoyant bubbles and weak shocks in the ICM, along with the injection of metals by SNIa and stellar winds.
We combine a series of high-resolution simulations with semi-analytic galaxy formation models to follow the evolution of a system resembling the Milky Way and its satellites. The semi-analytic model is based on that developed for the Millennium Simulation, and successfully reproduces the properties of galaxies on large scales, as well as those of the Milky Way. In this model, we are able to reproduce the luminosity function of the satellites around the Milky Way by preventing cooling in haloes with Vvir < 16.7 km/s (i.e. the atomic hydrogen cooling limit) and including the impact of the reionization of the Universe. The physical properties of our model satellites (e.g. mean metallicities, ages, half-light radii and mass-to-light ratios) are in good agreement with the latest observational measurements. We do not find a strong dependence upon the particular implementation of supernova feedback, but a scheme which is more efficient in galaxies embedded in smaller haloes, i.e. shallower potential wells, gives better agreement with the properties of the ultra-faint satellites. Our model predicts that the brightest satellites are associated with the most massive subhaloes, are accreted later (z $\lta$ 1), and have extended star formation histories, with only 1 per cent of their stars made by the end of the reionization. On the other hand, the faintest satellites were accreted early, are dominated by stars with age > 10 Gyr, and a few of them formed most of their stars before the reionization was complete. Objects with luminosities comparable to those of the classical MW satellites are associated with dark matter subhaloes with a peak circular velocity $\gta$ 10 km/s, in agreement with the latest constraints.
The identification of very high energy photons is of great importance for the understanding of the origin of extreme energy cosmic rays (EECR). Several can be the sources of high energy photons at Earth. A guaranteed component is the flux of high energy photons expected as a consequence of the interaction of cosmic rays with the cosmic photon background. Another contribution may be expected as by-product at the acceleration sites of protons and nuclei, although such flux should be strongly suppressed for distant sources. On the other hand, top-down scenarios involving the decay of super heavy relic particles or topological defects, even if not currently favored, have as a characteristic signature an increasingly dominant flux of photons at the highest energies. In this work we study the statistical separation between hadron and photon showers at energies where both, LPM effect and magnetospheric interactions are important for the development of the cascades. We consider a detector with the same orbital characteristics as JEM-EUSO, but disregard trigger and reconstruction efficiencies, in order to define the maximum ideal discrimination power attainable.
In the following paper we present an internal shocks model, iShocks, for simulating a variety of relativistic jet scenarios; these scenarios can range from a single ejection event to an almost continuous jet, and are highly user configurable. Although the primary focus in the following paper is black hole X-ray binary jets, the model is scale and source independent and could be used for supermassive black holes in active galactic nuclei or other flows such as jets from neutron stars. Discrete packets of plasma (or `shells') are used to simulate the jet volume. A two-shell collision gives rise to an internal shock, which acts as an electron re-energization mechanism. Using a pseudo-random distribution of the shell properties, the results show how for the first time it is possible to reproduce a flat/inverted spectrum (associated with compact radio jets) in a conical jet whilst taking the adiabatic energy losses into account. Previous models have shown that electron re-acceleration is essential in order to obtain a flat spectrum from an adiabatic conical jet: multiple internal shocks prove to be efficient in providing this re-energization. We also show how the high frequency turnover/break in the spectrum is correlated with the jet power, $\nu_b \propto L_{\textrm W}^{\sim 0.6}$, and the flat-spectrum synchrotron flux is correlated with the total jet power, $F_{\nu}\propto L_{\textrm W}^{\sim 1.4}$. Both the correlations are in agreement with previous analytical predictions.
Neutrinos are a very promising messenger at tens of EeV and above. They can be produced by several channels, namely as by products of hadronic interactions at the sources, as the main products of the decay of super massive particles and, in a guaranteed way, as the result of the propagation of UHECR through the bath of microwave relic photons. A new era of very large exposure space observatories, of which the JEM-EUSO mission is a prime example, is on the horizon and, with it, it is even larger the possibility of astrophysical neutrino detection at the highest energies. In the present work we use a combination of the PYTHIA interaction code with the CONEX shower simulation package in order to produce fast one-dimensional simulations of neutrino initiated showers in air. We make a detail study of the structure of the corresponding longitudinal profiles, but focus our physical analysis mainly on the development of showers at mid and high altitudes, where they can be an interesting target for space fluorescence observatories.
The simulations of extensive air showers as well as the detectors involved in their detection play a fundamental role in the study of the high energy cosmic rays. At the highest energies the detailed simulation of air showers is very costly in processing time and disk space due to the large number of secondary particles generated in interactions with the atmosphere, e.g. $\sim 10^{11}$ for $10^{20}$ eV proton shower. Therefore, in order to increase the statistics, it is quite common to recycle single showers many times to simulate the detector response. In this work we present a detailed study of the artificial effects introduced by the multiple use of single air showers for the detector simulations. In particular, we study the effects introduced by the repetitions in the kernel density estimators which are frequently used in composition studies.
By looking for possible violation of the generalized second law, we might be able to find regions in the space of theories and states that do not allow holographic dual descriptions. We revisit three proposals for violation of the generalized second law in the simplest Higgs phase of gravity called ghost condensate. Two of them, (i) analogue of Penrose process and (ii) semiclassical heat flow, are based on Lorentz breaking effects, by which particles of different species can have different limits of speed. We show that processes in both (i) and (ii) are always slower than accretion of ghost condensate and cannot decrease the total entropy before the accretion increases the entropy. The other proposal is to use (iii) negative energy carried by excitations of ghost condensate. We prove an averaged null energy condition, which we conjecture prevents the proposal (iii) from violating the generalized second law in a coarse-grained sense.
The equation in the title describes the number of bright images of a point source under lensing by an elliptic object with isothermal density. We prove that this equation has at most 6 solutions. Any number of solutions from 1 to 6 can actually occur.
Gravitational waves from compact objects provide information about their structure, probing deep into strong-gravity regions. Here we illustrate how the presence or absence of an event horizon can produce qualitative differences in the gravitational waves emitted by ultra-compact objects. In order to set up a straw-man ultra-compact object with no event horizon, but which is otherwise almost identical to a black hole, we consider the nonrotating thin-shell gravastar model first proposed by Mazur and Mottola, which has a Schwarzschild exterior, a de Sitter interior and an infinitely thin shell with finite surface energy and tension separating the two regions. As viewed from the external space-time, the shell can be located arbitrarily close to the Schwarzschild radius, so a gravastar might seem indistinguishable from a black hole when tests are only performed on its external metric. We study the linearized dynamics of the gravastar, and in particular the junction conditions connecting internal and external gravitational perturbations. As a first application of the formalism we compute polar and axial oscillation modes of a thin-shell gravastar. We show that the quasinormal mode spectrum is completely different from that of a black hole, even in the limit when its surface redshift becomes infinite. Polar QNMs depend on the equation of state of matter on the shell and can be used to distinguish between different gravastar models. Our calculations suggest that low-compactness gravastars could be unstable when the sound speed on the shell is superluminal.
Gravitational waveforms and production could be considerably affected by gravitomagnetic corrections considered in relativistic theory of orbits. Beside the standard periastron effect of General Relativity, new nutation effects come out when c^{-3} corrections are taken into account. Such corrections emerge as soon as matter-current densities and vector gravitational potentials cannot be discarded into dynamics. We study the gravitational waves emitted through the capture, in the gravitational field of massive binary systems (e.g. a very massive black hole on which a stellar object is inspiralling) via the quadrupole approximation, considering precession and nutation effects. We present a numerical study to obtain the gravitational wave luminosity, the total energy output and the gravitational radiation amplitude. From a crude estimate of the expected number of events towards peculiar targets (e.g. globular clusters) and in particular, the rate of events per year for dense stellar clusters at the Galactic Center (SgrA*), we conclude that this type of capture could give signatures to be revealed by interferometric GW antennas, in particular by the forthcoming laser interferometer space antenna LISA.
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Motivated by recent observations that suggest a low density of old stars around the Milky Way supermassive black hole, evolutionary models for the nuclear star cluster are considered that postulate a parsec-scale core as initial conditions. Gravitational encounters cause the core to shrink; a core of initial radius 1-1.5 pc evolves to a size of 0.5 pc after 10 Gyr, roughly the size of the observed core. The absence of a Bahcall-Wolf cusp is naturally explained. In these models, the time for a 10-Solar-mass black hole to spiral in to the Galactic center from an initial distance of 5 pc can be much greater than 10 Gyr. Assuming that the stellar black holes had the same phase-space distribution initially as the stars, their density after 5-10 Gyr is predicted to rise very steeply going into the stellar core, but to remain substantially below the densities inferred from steady-state models that include a steep density cusp in the stars. The implications of these models are discussed for the rates of gravitational wave inspiral events and of other physical processes that depend on a high density of stars or stellar mass black holes near Sagittarius A*.
In recent work, we have shown that it is possible to link quantitatively many
aspects of damped Lyman alpha (DLA) absorbers in the spectra of quasars to high
resolution simulations of galaxy formation. Using runs from the same series of
hydrodynamic numerical studies, we consider the expected properties of Lyman
alpha absorbers seen in the spectra of high redshift (z>2) gamma ray burst
afterglows (GRB-DLAs). If GRBs are associated with the death of massive stars,
their afterglows provide insights into otherwise unprobed regions of
protogalactic objects, but detailed physical interpretations are currently
embryonic.
We find that median impact parameters (measured from the potential minimum)
are approximately 1 kpc for GRBs compared with 4 kpc for QSO-DLAs. However, an
equally important difference is that GRB-DLAs are predominantly associated with
halos of mass 10^10<M_vir/M_sol<10^12, an order of magnitude larger than the
hosts of QSO-DLAs. Accordingly, there are differences in the stellar properties
of hosts.
Our simulations accurately predict the form of the GRB-DLA HI column density
distribution, producing quantitative agreement for N_HI>10^19 cm^-2, but they
somewhat underpredict the incidence of low column densities N_HI<10^19 cm^-2.
Line-of-sight neutral gas metallicities predicted by our simulations (10^-2 <
Z/Z_sol < 1) are consistent with the modest observational constraints. Because
of large internal dispersions in gas metallicities, this agreement is not
significantly compromised by imposing a cut-off on the metallicity of stars
able to launch GRBs (Z_star<Z_sol/3), confounding claims that the observed
metallicity of GRB-DLAs poses a challenge to current GRB models. (Abridged.)
We analyze the clustering of red and blue galaxies from four samples spanning a redshift range of 0.4<z<2.0 to test the various scenarios by which galaxies evolve onto the red sequence. The data are taken from the UKIDSS Ultra Deep Survey, DEEP2, and COMBO-17. The use of clustering allows us to determine what fraction of the red sequence is made up of central galaxies and satellite galaxies. At all redshifts, including z=0, the data are consistent with ~60% of satellite galaxies being red or quenched, implying that ~1/3 of the red sequence is comprised of satellite galaxies. More than three-fourths of red satellite galaxies were moved to the red sequence after they were accreted onto a larger halo. The constant fraction of satellite galaxies that are red yields a quenching time for satellite galaxies that depends on redshift in the same way as halo dynamical times; t_Q ~ (1+z)^{-1.5}. In three of the four samples, the data favor a model in which red central galaxies are a random sample of all central galaxies; there is no preferred halo mass scale at which galaxies make the transition from star-forming to red and dead. The large errors on the fourth sample inhibit any conclusions. Theoretical models in which star formation is quenched above a critical halo mass are excluded by these data. A scenario in which mergers create red central galaxies imparts a weaker correlation between halo mass and central galaxy color, but even the merger scenario creates tension with red galaxy clustering at redshifts above 0.5. These results suggest that the mechanism by which central galaxies become red evolves from z=0.5 to z=0.
Robust fast methods to classify variable light curves in large sky surveys are becoming increasingly important. While it is relatively straightforward to identify common periodic stars and particular transient events (supernovae, novae, microlensing), there is no equivalent for non-periodic continuously varying sources (quasars, aperiodic stellar variability). In this paper we present a fast method for modeling and classifying such sources. We demonstrate the method using ~ 86,000 variable sources from the OGLE-II survey of the LMC and ~ 2,700 mid-IR selected quasar candidates from the OGLE-III survey of the LMC and SMC. We discuss the location of common variability classes in the parameter space of the model. In particular we show that quasars occupy a distinct region of variability space, providing a simple quantitative approach to the variability selection of quasars.
We study the dynamic efficiency of conversion of kinetic-to-thermal/magnetic energy of internal shocks in relativistic magnetized outflows. We model internal shocks as being caused by collisions of shells of plasma with the same energy flux and a non-zero relative velocity. The contact surface, where the interaction between the shells takes place, can break up either into two oppositely moving shocks (in the frame where the contact surface is at rest), or into a reverse shock and a forward rarefaction. We find that for moderately magnetized shocks (magnetization $\sigma\simeq 0.1$), the dynamic efficiency in a single two-shell interaction can be as large as 40%. Thus, the dynamic efficiency of moderately magnetized shocks is larger than in the corresponding unmagnetized two-shell interaction. If the slower shell propagates with a sufficiently large velocity, the efficiency is only weakly dependent on its Lorentz factor. Consequently, the dynamic efficiency of shell interactions in the magnetized flow of blazars and gamma-ray bursts is effectively the same. These results are quantitatively rather independent on the equation of state of the plasma. The radiative efficiency of the process is expected to be a fraction $f_r<1$ of the estimated dynamic one, the exact value of $f_r$ depending on the particularities of the emission processes which radiate away the thermal or magnetic energy of the shocked states.
We present structural parameters for the seven intermediate-age and old star clusters NGC121, Lindsay 1, Kron 3, NGC339, NGC416, Lindsay 38, and NGC419 in the Small Magellanic Cloud. We fit King profiles and Elson, Fall, and Freeman profiles to both surface-brightness and star count data taken with the Advanced Camera for Surveys aboard the Hubble Space Telescope. Clusters older than 1 Gyr show a spread in cluster core radii that increases with age, while the youngest clusters have relatively compact cores. No evidence for post core collapse clusters was found. We find no correlation between core radius and distance from the SMC center, although consistent with other studies of dwarf galaxies, some relatively old and massive clusters have low densities. The oldest SMC star cluster, the only globular NGC121, is the most elliptical object of the studied clusters. No correlation is seen between ellipticity and distance from the SMC center. The structures of these massive intermediate-age (1-8 Gyr) SMC star clusters thus appear to primarily result from internal evolutionary processes.
Large-scale dynamos have recently been obtained in local numerical simulations with turbulent convection in a number of setups and substantial effort has subsequently gone into distinguishing the physical processes responsible for the generation of the large-scale fields. Although local models can be used to demonstrate the existence of large-scale dynamos in different settings, results of such models usually bear little resemblance to stellar dynamos operating in spherical geometry. As a way of bridging the gap between idealised local Cartesian models and more realistic but far more demanding full-sphere calculations, we present preliminary results obtained from simulations in rotating spherical wedge domains. By covering smaller latitudinal and longitudinal extents, these models allow higher spatial resolution. For sufficiently rapid rotation we find equatorial acceleration and an oscillatory large-scale dynamo.
We present a new deep optical study of a luminosity limited sample of nearby elliptical galaxies, attempting to observe the effects of gravitational interactions on the ISM of these objects. This study is motivated by recent observations of M86, a nearby elliptical galaxy that shows possible evidence for gas heating through a recent gravitational interaction. The complete sample includes luminous ellipticals in clusters, groups and the field. For each of the galaxies we objectively derive a tidal parameter which measures the deviation of the stellar body from a smooth, relaxed model and find that 73% of them show tidal disturbance signatures in their stellar bodies. This is the first time that such an analysis is done on a statistically complete sample and it confirms that elliptical galaxies continue to grow and evolve through gravitational interactions even in the local Universe. Our study of ellipticals in a wide range of interaction stages, along with available ISM data will attempt to shed light on this possibly alternative mechanism for maintaining the observed ISM temperatures of elliptical galaxies.
We present arcsecond-resolution Submillimeter Array (SMA) polarimetric observations of the 880 um continuum emission from the protoplanetary disks around two nearby stars, HD 163296 and TW Hydrae. Although previous observations and theoretical work have suggested that a 2-3% polarization fraction should be common for the millimeter continuum emission from such disks, we detect no polarized continuum emission above a 3-sigma upper limit of 7 mJy in each arcsecond-scale beam, or <1% in integrated continuum emission. We compare the SMA upper limits with the predictions from the exploratory Cho & Lazarian (2007) model of polarized emission from T Tauri disks threaded by toroidal magnetic fields, and rule out their fiducial model at the ~10-sigma level. We explore some potential causes for this discrepancy, focusing on model parameters that describe the shape, magnetic field alignment, and size distribution of grains in the disk. We also investigate related effects like the magnetic field strength and geometry, scattering off of large grains, and the efficiency of grain alignment, including recent advances in grain alignment theory, which are not considered in the fiducial model. We discuss the impact each parameter would have on the data and determine that the suppression of polarized emission plausibly arises from rounding of large grains, reduced efficiency of grain alignment with the magnetic field, and/or some degree of magnetic field tangling (perhaps due to turbulence). A poloidal magnetic field geometry could also reduce the polarization signal, particularly for a face-on viewing geometry like the TW Hya disk. The data provided here offer the most stringent limits to date on the polarized millimeter-wavelength emission from disks around young stars.
We present the results of observations of the very low surface brightness (VLSB) dwarf galaxy SDSS J092609.45+334304.1 with extreme parameters which indicate its unevolved status. Namely, its value of O/H, derived as an average of that in two adjacent HII regions at the NE edge of the disc, corresponds to the parameter 12+log(O/H)=7.12+-0.02, which is amongst two lowest known. The total HI flux measurement obtained with the Nancay Radio Telescope and the photometric results imply that the galaxy ratio M(HI)/L_B ~3.0, is among the top known in the Local Volume. The galaxy is situated in the region of a nearby underdense region known as the Lynx-Cancer void, where other, unevolved galaxies, including DDO 68, HS 0832+3542 and SAO 0822+3545, are known to be present. The total mass of this almost edge-on VLSB galaxy is ~8.3 times larger than its baryonic mass, implying the dynamical dominance of Dark Matter (DM) halo. The (u-g), (g-r) colours of outer parts of this galaxy are consistent with the ages of its main stellar population of 1--3 Gyr. Thanks to the galaxy isolation, the small effect of current or recent star formation (SF), its proximity and rather large HI flux (~2.5 Jy km/s), this VLSB dwarf is a good laboratory for the detailed study of DM halo properties through HI kinematics and the star formation processes in very metal-poor low surface density environment. This finding, along with the discovery of other unusual dwarf galaxies in this void, provides evidence for the relation between galaxy evolution and its very low-density environment for the baryonic mass range of 10^{8} to 10^{9} Mo. This relation seems to be consistent with that expected in the LambdaCDM models of galaxy and structure formation.
We investigate two potential mechanisms that will produce X-ray and gamma-ray flashes from Type Ia supernovae (SN-Ia). The mechanisms are the breakout of the thermonuclear burning front as it reaches the surface of the white dwarf and the interaction of the rapidly expanding envelope with an accretion disk. Based on the delayed-detonation scenario and detailed radiation-hydro calculation which include nuclear networks, we find that both mechanisms produce ~1 second flashes of high energy radiation with peak luminosities of 10^48 to 10^50 erg/sec with fast rises and exponential declines. The X- and gamma-ray visibility of a SN-Ia will depend strongly on self absorption within the progenitor system, specifically on the properties of the accretion disk and its orientation towards the observer. Such X-ray and gamma-ray flashes could be detected as triggered events by Gamma-Ray Burst (GRB) detectors on satellites, with events in current GRB catalogs. We have searched through the GRB catalogs (for the BATSE, HETE, and Swift experiments) for GRBs that occur at the extrapolated time of explosion and in the correct direction for known Type Ia supernovae with radial velocity of less than 3,000 km/s. For BATSE about 12.9+-3.6 nearby SNe Ia should have been detected, but only 0.8+-0.7 non-coincidental matches have been found. With the HETE and Swift satellites, we expect to see 5.6+-1.3 SN-Ia flashes from known nearby SNe Ia but, yet, no SN-Ia flashes were detected. These place observational limits that the bolometric peak luminosity of SN-Ia Flashes must be less ~10^46 erg/s. We attribute the difference between theory and observational limits to the absorption of the X- and gamma-rays by the accretion disk of large scale height or common envelope that would be smothering the white dwarf.
We present the results of Suzaku observation of the radio halo cluster Abell 2319. The metal abundance in the central cool region is found to be higher than the surrounding region, which was not resolved in the former studies. We confirm that the line-of-sight velocities of the intracluster medium in the observed region are consistent with those of the member galaxies of entire A2319 and A2319A subgroup for the first time, though any velocity difference within the region is not detected. On the other hand, we do not find any signs of gas motion relevant to A2319B subgroup. Hard X-ray emission from the cluster is clearly detected, but its spectrum is likely thermal. Assuming a simple single temperature model for the thermal component, we find that the upper limit of the non-thermal inverse Compton component becomes $2.6 \times 10^{-11}$ erg s$^{-1}$ cm$^{-2}$ in the 10-40 keV band, which means that the lower limit of the magnetic field is 0.19 $\mu$G with the radio spectral index 0.92. Although the results slightly depend on the detailed spectral modeling, it is robust that the upper limit of the power-law component flux and lower limit of the magnetic field strength become $\sim 3 \times 10^{-11}$ erg s$^{-1}$ cm$^{-2}$ and $\sim 0.2 \mu$G, respectively. Considering the lack of a significant amount of very hot ($\sim 20$ keV) gas and the strong bulk flow motion, it is more likely that the relativistic non-thermal electrons responsible for the radio halo are accelerated through the intracluster turbulence rather than the shocks.
The discovery of a short-lived gamma-ray burst at a surprisingly early epoch in the history of the Universe shows how much is still unknown about the evolution of the parent systems of such bursts.
We present infrared imaging from IRIS2 on the Anglo-Australian Telescope that shows the barred spiral galaxy IC 4933 has not just an inner ring encircling the bar, but also a star-forming nuclear ring 1.5 kpc in diameter. Imaging in the u' band with GMOS on Gemini South confirms that this ring is not purely an artifact due to dust. Optical and near-infrared colours alone however cannot break the degeneracy between age, extinction, and burst duration that would allow the star formation history of the ring to be unraveled. Integral field spectroscopy with the GNIRS spectrograph on Gemini South shows the equivalent width of the Pa-beta line to peak in the north and south quadrants of the ring, indicative of a bipolar azimuthal age gradient around the ring. The youngest star-forming regions do not appear to correspond to where we expect to find the contact points between the offset dust lanes and the nuclear ring unless the nuclear ring is oval in shape, causing the contact points to lead the bar by more than 90 degrees.
Through Spitzer Space Telescope's observations, Su et al. (2005) show that the Vega debris disc is dominated by grains which are small enough to be blown out by radiation pressure. This implies the lifetime of Vega debris disc's grains is relatively short, about 1000 years, and a continuous dust production is necessary to maintain the observed debris disc. However, Krivov et al. (2006)'s theoretical calculations show that the Vega debris disc is dominated by 10 micro-meter grains, which would be in bound orbits and thus long-lived, provided that the disc is in a steady state. In order to solve the above contradiction, through dynamical simulations, we determine the grains' orbital evolutions and density profiles and seek a model of size distribution which can reproduce the observed surface brightness. Our results show that a self-consistent dynamical model with a 1/R disc density profile can be constructed when the grains have a power-law size distribution. Moreover, both types of models, dominated by short-lived and long-lived grains, are consistent with the observational data.
The merger of a super-massive binary black hole (SBBH) is one of the most extreme events in the universe with a huge amount of energy released by gravitational radiation. Although the characteristic gravitational wave (GW) frequency around the merger event is far higher than the nHz regime optimal for pulsar timing arrays (PTAs), nonlinear GW memory might be a critical smoking gun of the merger event detectable with PTAs. In this paper, basic aspects of this interesting observation are discussed for SBBHs, and the detection numbers of their memory and inspiral GWs are estimated for ongoing and planned PTAs. We find that the expected detection number would be smaller than unity for the two-types of signals even with the Square Kilometer Array. We also provide various scaling relations that would be useful to study detection probabilities of GWs from individual SBBHs with PTAs.
We present results from a study of the kinematic structure of star-forming galaxies at redshift z~3 selected in the VVDS, using integral-field spectroscopy of rest-frame optical nebular emission lines, in combination with rest-frame UV spectroscopy, ground-based optical/near-IR and Spitzer photometry. We also constrain the underlying stellar populations to address the evolutionary status of these galaxies. We infer the kinematic properties of four galaxies: VVDS-20298666, VVDS-020297772, VVDS-20463884 and VVDS-20335183 with redshifts z = 3.2917, 3.2878, 3.2776, and 3.7062, respectively. While VVDS-20463884 presents an irregular velocity field with a peak in the local velocity dispersion of the galaxy shifted from the centre of the galaxy, VVDS-20298666 has a well-resolved gradient in velocity over a distance of ~4.5 kpc with a peak-to-peak amplitude of v = 91 km/s . We discovered that the nearby galaxy, VVDS-020297772 (which shows traces of AGN activity), is in fact a companion at a similar redshift with a projected separated of 12 kpc. In contrast, the velocity field of VVDS-020335183 seems more consistent with a merger on a rotating disk. However, all of the objects have a high local velocity dispersion (sigma ~ 60-70 km/s), which gives v/sigma < 1. It is unlikely that these galaxies are dynamically cold rotating disk of ionized gas.
For a dense stellar matter, which is electrically neutral and in beta equilibrium, the electron chemical potential, mu_e, will depend nontrivially on baryonic matter density. It is generally expected that as density increases, the electron chemical potential will increase and new degrees of freedom will emerge as mu_e becomes comparable to their energy scales. Assuming the electrical neutrality and beta equilibrium for the stellar matter, we have studied how the density dependence of lepton chemical potentials varies for different models of nuclear interactions that are constrained by experiments up to nuclear matter density, n_0, but extrapolate differently(unconstrained) beyond n_0 and calculated the relative abundances of nucleons(neutron and proton) and leptons(electron and muon) and their density dependencies. We find that the density dependence of the electron chemical potential is strongly dependent on the structure of the nuclear symmetry energy relevant to softness/stfness of the nuclear matter EOS that measures the energy relevant to the neutron-proton asymmetry. As a consequence, the relative abundances of neutrons, protons, electrons, and muons as well as the kaon condensation are strongly dependent on the nuclear symmetry energy. An intriguing result in our finding is that contrary to the accepted lore, kaon condensation in neutron star matter, which is considered to be the first phase transitions beyond n_0 and plays a crucial role in certain scenarios of compact-star formation, is not directly tied to the softness or stiffness of the EOS beyond n_0. This point is illustrated with a "super-soft" EOS that is fit to the pi^-/pi^+ ratio data of GSI which excludes kaon condensation at any density.
We present a wide field census of resolved stellar populations in the northern half of M81, conducted with Suprime-Cam on the 8-m Subaru telescope and covering an area ~ 0.3 square degrees. The resulting color-magnitude diagram reaches over one magnitude below the red giant branch (RGB) tip, allowing a detailed comparison between the young and old stellar spatial distributions. The surface density of stars with ages <~ 100 Myr is correlated with that of neutral hydrogen in a manner similar to the disk-averaged Kennicutt-Schmidt relation. We trace this correlation down to gas densities of ~ 2 x 10^20 cm^{-2}, lower than typically probed with H-alpha flux. Both diffuse light and resolved RGB star counts show compelling evidence for a faint, extended structural component beyond the bright optical disk, with a much flatter surface brightness profile. The star counts allow us to probe this component to significantly fainter levels than is possible with the diffuse light alone. From the colors of its RGB stars, we estimate this component has a peak global metallicity [M/H] ~ -1.1 +/- 0.3 at deprojected radii 32 - 44 kpc assuming an age of 10 Gyr and distance of 3.6 Mpc. The spatial distribution of its RGB stars follows a power-law surface density profile, I(r) ~ r^{-gamma}, with gamma ~ 2. [Abridged]
One class of protoplanetary disc models, the X-wind model, predicts strongly subkeplerian orbital gas velocities, a configuration which can be sustained by magnetic tension. We investigate disc-planet interactions in these subkeplerian discs, focusing on orbital migration for low-mass planets and gap formation for high-mass planets. We use linear calculations and non-linear hydrodynamical simulations to measure the torque and look at gap formation. In both cases, the subkeplerian nature of the disc is treated as a fixed external constraint. We show that, depending on the degree to which the disc is subkeplerian, the torque on low-mass planets varies between the usual Type I torque and the one-sided outer Lindblad torque, which is also negative but an order of magnitude larger. In strongly subkeplerian discs, corotation effects can be ignored, making migration fast and inward. Gap formation near the planet's orbit is more difficult in such discs, since there are no resonances close to the planet accommodating angular momentum transport. In stead, the location of the gap is shifted inwards with respect to the planet, leaving the planet on the outside of a surface density depression. Depending on the degree to which a protoplanetary disc is subkeplerian, disc-planet interactions can be very different from the usual Keplerian picture, making these discs in general more hazardous for young planets.
We report on deep imaging of a remote M31 globular cluster, MGC1, obtained with Gemini/GMOS. Our colour-magnitude diagram for this object extends ~5 magnitudes below the tip of the red giant branch and exhibits features consistent with an ancient metal-poor stellar population, including a long, well-populated horizontal branch. The red giant branch locus suggests MGC1 has a metal abundance [M/H] ~ -2.3. We measure the distance to MGC1 and find that it lies ~160 kpc in front of M31 with a distance modulus of 23.95 +/- 0.06. Combined with its large projected separation of 117 kpc from M31 this implies a deprojected radius of Rgc = 200 +/- 20 kpc, rendering it the most isolated known globular cluster in the Local Group by some considerable margin. We construct a radial brightness profile for MGC1 and show that it is both centrally compact and rather luminous, with Mv = -9.2. Remarkably, the cluster profile shows no evidence for a tidal limit and we are able to trace it to a radius of at least 450 pc, and possibly as far as ~900 pc. The profile exhibits a power-law fall-off with exponent -2.5, breaking to -3.5 in its outermost parts. This core-halo structure is broadly consistent with expectations derived from numerical models, and suggests that MGC1 has spent many gigayears in isolation.
Following our long-term work on development of model-independent data analysis methods for reconstructing the one-dimensional velocity distribution function of halo WIMPs as well as for determining their mass and couplings on nucleons by using data from direct Dark Matter detection experiments directly, we combined the simulation programs to a compact system: AMIDAS (A Model-Independent Data Analysis System). For users' convenience an online system has also been established at the same time. AMIDAS has the ability to do full Monte Carlo simulations, faster theoretical estimations, as well as to analyze (real) data sets recorded in direct detection experiments without modifying the source code. In this article, I give an overview of functions of the AMIDAS code based on the use of its website.
The Fringe Sensor Unit (FSU) is the central element of the Phase Referenced Imaging and Micro-arcsecond Astrometry (PRIMA) dual-feed facility and provides fringe sensing for all observation modes, comprising off-axis fringe tracking, phase referenced imaging, and high-accuracy narrow-angle astrometry. It is installed at the Very Large Telescope Interferometer (VLTI) and successfully servoed the fringe tracking loop during the initial commissioning phase. Unique among interferometric beam combiners, the FSU uses spatial phase modulation in bulk optics to retrieve real-time estimates of fringe phase after spatial filtering. A R=20 spectrometer across the K-band makes the retrieval of the group delay signal possible. The FSU was integrated and aligned at the VLTI in summer 2008. It yields phase and group delay measurements at sampling rates up to 2 kHz, which are used to drive the fringe tracking control loop. During the first commissioning runs, the FSU was used to track the fringes of stars with K-band magnitudes as faint as m_K=9.0, using two VLTI Auxiliary Telescopes (AT) and baselines of up to 96 m. Fringe tracking using two Very Large Telescope (VLT) Unit Telescopes (UT) was demonstrated. During initial commissioning and combining stellar light with two ATs, the FSU showed its ability to improve the VLTI sensitivity in K-band by more than one magnitude towards fainter objects, which is of fundamental importance to achieve the scientific objectives of PRIMA.
Dark energy observations may be explained within general relativity using an inhomogeneous Hubble-scale depression in the matter density and accompanying curvature, which evolves naturally out of an Einstein-de Sitter model. We present a simple parameterization of a void which can reproduce concordance model distances to arbitrary accuracy, but can parameterize away from this to give a smooth density profile everywhere. We show how the Hubble constant isn't just a nuisance parameter in inhomogeneous models because it affects the shape of the distance-redshift relation. Independent Hubble-rate data from age estimates can in principle serve to break the degeneracy between concordance and void models, but the data is not yet able to achieve this. Using the latest Constitution supernova dataset we show that robust limits can be placed on the size of a void which is roughly independent of its shape. However, the sharpness of the profile at the origin cannot be well constrained due to supernova being dominated by peculiar velocities in the local Universe. We illustrate our results using some recently proposed diagnostics for the Friedmann models.
We present the relations between Halpha equivalent width, optical brightness
and X-ray flux of Be/X-ray binary system SAX J2103.5+4545, by analyzing the
optical photometric and spectroscopic observations together with the X-ray
observations.
In the photometric observations PSF photometry were applied using MIDAS and
its DAOPHOT package. The reduction and analysis of spectra were done by using
MIDAS and its suitable packages. The X-ray outburst of the system occurred just
after the optical outburst. The nearly symmetric Halpha emission line profiles
observed during the beginning of optical outburst turn into asymmetric profiles
with increased EW values during the dissipation of Be disc. Halpha lines
changed from emission to absorption during the observation period. The observed
double peaked HeI emission lines might come from the accretion disc of neutron
star which is temporarily formed at the time of X-ray outburst.
We focus on the influence of the Coriolis acceleration on the stochastic excitation of oscillation modes in convective regions of rotating stars. Our aim is to estimate the asymmetry between excitation rates of prograde and retrograde modes. We extend the formalism derived for obtaining stellar $p$- and $g$-mode amplitudes (Samadi & Goupil 2001, Belkacem et al. 2008) to include the effect of the Coriolis acceleration. We then study the special case of uniform rotation for slowly rotating stars by performing a perturbative analysis. This allows us to consider the cases of the Sun and the CoRoT target HD 49933. We find that, in the subsonic regime, the influence of rotation as a direct contribution to mode driving is negligible in front of the Reynolds stress contribution. In slow rotators, the indirect effect of the modification of the eigenfunctions on mode excitation is investigated by performing a perturbative analysis of the excitation rates. It turns out that the excitation of solar $p$ modes is affected by rotation with excitation rates asymmetries between prograde and retrograde modes of the order of several percents. Solar low-order $g$ modes are also affected by uniform rotation and their excitation rates asymmetries are found to reach up to 10 %. The CoRoT target HD 49933 is rotating faster than the Sun ($\Omega / \Omega_\odot \approx 8$) and we show that the resulting excitation rates asymmetry is about 10 % for the excitation rates of $p$ modes. We have then demonstrated that $p$ and $g$ mode excitation rates are modified by uniform rotation through the Coriolis acceleration. Study of the effect of differential rotation is dedicated to a forthcoming paper.
I review various phenomena associated with mass-accreting white dwarfs (WDs)
in the view of supersoft X-ray sources. When the mass-accretion rate is low
(\dot M_{acc} < a few \times 10^{-7} M_\sun yr^{-1}), hydrogen nuclear burning
is unstable and nova outbursts occur. A nova is a transient supersoft X-ray
source (SSS) in its later phase which timescale depends strongly on the WD
mass. The X-ray turn on/off time is a good indicator of the WD mass. At an
intermediate mass-accretion rate an accreting WD becomes a persistent SSS with
steady hydrogen burning. For a higher mass-accretion rate, the WD undergoes
"accretion wind evolution" in which the WD accretes matter from the equatorial
plane and loses mass by optically thick winds from the other directions. Two
SSS, namely RX J 0513-69 and V Sge, are corresponding objects to this accretion
wind evolution. We can specify mass increasing WDs from light-curve analysis
based on the optically thick wind theory using multiwavelength observational
data including optical, IR, and supersoft X-rays.
Mass estimates of individual objects give important information for the
binary evolution scenario of type Ia supernovae.
We present extensive, high-density Swift observations of V2491 Cyg (Nova Cyg
2008 No. 2). Observing the X-ray emission from only one day after the nova
discovery, the source is followed through the initial brightening, the
Super-Soft Source phase and back to the pre-outburst flux level. The evolution
of the spectrum throughout the outburst is demonstrated. The UV and X-ray
light-curves follow very different paths, although changes occur in them around
the same times, indicating a link between the bands. Flickering in the
late-time X-ray data indicates the resumption of accretion.
We show that if the white dwarf is magnetic, it would be among the most
magnetic known; the lack of a periodic signal in our later data argues against
a magnetic white dwarf, however. We also discuss the possibility that V2491 Cyg
is a recurrent nova, providing recurrence timescale estimates.
Cosmogenic neutrinos are expected from ultrahigh energy cosmic rays undergoing the GZK process and anticipated to be observed by detecting air showers from the decays of tau leptons. We use CORSIKA simulated shower structure to calculate the coherent geosynchrotron radio emissions of the tau decay showers above $10^{17}$eV. We present the pattern and spectrum of radio waves and discuss their detections by radio antennae.
We present a unified model of optical and ultraviolet (UV) light curves for one of the fastest classical novae, V838 Herculis (Nova Herculis 1991), and estimate its white dwarf (WD) mass. Based on an optically thick wind theory of nova outbursts, we model the optical light curves with free-free emission and the UV 1455 \AA light curves with blackbody emission. Our models of 1.35 \pm 0.02 M_\sun WD reproduce simultaneously the optical and UV 1455 \AA observations. The mass lost by the wind is \Delta M_{wind} \sim 2 \times 10^{-6} M_\sun. We provide new determinations of the reddening, E(B-V) = 0.53 \pm 0.05, and of the distance, 2.7 \pm 0.5 kpc.
We consider the conditional galaxy density around each galaxy, and study its fluctuations in the newest samples of the Sloan Digital Sky Survey Data Release 7. Over a large range of scales, both the average conditional density and its variance show a nontrivial scaling behavior, which resembles to criticality. The density depends, for 10 < r < 80 Mpc/h, only weakly (logarithmically) on the system size. Correspondingly, we find that the density fluctuations follow the Gumbel distribution of extreme value statistics. This distribution is clearly distinguishable from a Gaussian distribution, which would arise for a homogeneous spatial galaxy configuration. We also point out similarities between the galaxy distribution and critical systems of statistical physics.
The Moon is a unique platform for fundamental astrophysical measurements of gravitation, the Sun, and the Universe. Lacking a permanent ionosphere and, on the farside, shielded from terrestrial radio emissions, a radio telescope on the Moon will be an unparalleled heliospheric and astrophysical observatory. Crucial stages in particle acceleration near the Sun can be imaged and tracked. The evolution of the Universe before and during the formation of the first stars will be traced, yielding high precision cosmological constraints. Lunar Laser Ranging of the Earth-Moon distance provides extremely high precision constraints on General Relativity and alternative models of gravity, and also reveals details about the interior structure of the Moon. With the aim of providing additional perspective on the Moon as a scientific platform, this white paper describes key research projects in these areas of astrophysics from the Moon that are being undertaken by the NLSI-funded LUNAR consortium. The NASA Lunar Science Institute (NLSI) recently funded 7 mostly university-based teams to study science of, on, and from the Moon. The LUNAR consortium was selected by the NLSI for astrophysical research and education that focuses on the key, unique instruments that most effectively take scientific advantage of sites on the lunar surface - low frequency heliophysics and cosmology, and lunar laser ranging. We are submitting this white paper to the Planetary Sciences Decadal Survey to provide additional perspective on the value of Moon for conducting cutting-edge research in astrophysics and gravitational physics by describing our key projects for LUNAR. This program of astrophysics from the Moon complements as well as takes advantage of expected scientific infrastructure on the Moon during the next few decades.
The role of the environment on galaxy evolution is still not fully understood. In order to quantify and set limits on the role of nurture one must identify and study a sample of isolated galaxies. The AMIGA project "Analysis of the Interstellar Medium of Isolated GAlaxies" is doing a multi-wavelength study of a large sample of isolated galaxies in order to examine their interstellar medium and star formation activity. We processed data for 950 galaxies from the Catalogue of Isolated Galaxies (CIG, Karachentseva 1973) and evaluated their isolation using an automated star-galaxy classification procedure (down to M_B ~17.5) on large digitised POSS-I fields surrounding each isolated galaxy (within a projected radius of at least 0.5 Mpc). We defined, compared and discussed various criteria to quantify the degree of isolation for these galaxies: e.g. Karachentseva's revised criterion, local surface density computations, estimation of the external tidal force affecting each isolated galaxy. We found galaxies violating Karachentseva's original criterion, and we defined various subsamples of galaxies according to their degree of isolation. Additionally, we sought for the redshifts of the primary and companion galaxies to access the radial dimension. We also applied our pipeline to triplets, compact groups and clusters and interpret the isolated galaxy population in light of these control samples.
(Abridged) We have computed a grid of 900 numeric dynamic model atmospheres (DMAs) using a well-tested computer code. This grid of models covers most of the expected combinations of stellar parameters, which are the stellar temperature, the stellar luminosity, the stellar mass, the abundance of condensible carbon, and the velocity amplitude of the pulsation. The resultant mass-loss rates and wind speeds are clearly affected by the choice of stellar temperature, mass, luminosity and the abundance of available carbon. In certain parts of the parameter space there is also an inevitable mass-loss threshold, below which a dust-driven wind is not possible. Contrary to some previous studies, we find a strong dependence on the abundance of free carbon, which turns out to be a critical parameter. Furthermore, we have found that the dust grains that form in the atmosphere may grow too large for the commonly-used small-particle approximation of the dust opacity to be strictly valid. This may have some bearing on the wind properties, although further study of this problem is needed before quantitative conclusions can be drawn. The wind properties show relatively simple dependences on stellar parameters above the mass-loss threshold, while the threshold itself is of a more complicated nature. Hence, we chose not to derive any simplistic mass-loss formula, but rather provide a mass-loss prescription in the form of an easy-to-use FORTRAN routine. Since this mass-loss routine is based on data coming from an essentially self-consistent model of mass loss, it may therefore serve as a better mass-loss prescription for stellar evolution calculations than empirical formulae. Furthermore, we conclude that there are still some issues that need to be investigated, such as the r\^ole of grain-sizes.
We present calculations of magnetic potential associated with the perturbation of Saturn's magnetic field by a rotating, equatorially-situated disc of plasma. Such structures are central to the dynamics of the rapidly rotating magnetospheres of Saturn and Jupiter. They are `fed' internally by sources of plasma from moons such as Enceladus (Saturn) and Io (Jupiter). We use a scaled form of Euler potentials for the Jovian magnetodisc field (Caudal, 1986). In this formalism, the magnetic field is assumed to be azimuthally symmetric about the planet's axis of rotation, and plasma temperature is constant along a field line. We perturb the dipole potential by using simplified distributions of plasma pressure and angular velocity for both planets, based on observations by Cassini (Saturn) and Voyager (Jupiter). Our results quantify the degree of radial `stretching' exerted on the dipolar field lines through the plasma's rotational motion and pressure. A simplified version of the field model, the `homogeneous disc', can be used to easily estimate the distance of transition in the outer magnetosphere between pressure-dominated and centrifugally-dominated disc structure. We comment on the degree of equatorial confinement as represented by the scale height associated with disc ions of varying mass and temperature. For Saturn, we identify the principal forces which contribute to the magnetodisc current and make comparisons between the field structure predicted by the model and magnetic field measurements from Cassini. For Jupiter, we reproduce Caudal's original calculation in order to validate our model implementation. We also show that compared to Saturn, where plasma pressure gradient is, on average, weaker than centrifugal force, the outer plasmadisc of Jupiter is clearly a pressure-dominated structure.
There are significant advantages in combining Hinode observations with
ground-based instruments that can observe additional spectral diagnostics at
higher data rates and with greater flexibility. However, ground-based
observations, because of the random effects of weather and seeing as well as
the complexities data analysis due to changing instrumental configurations,
have traditionally been less efficient than satellite observations in producing
useful datasets. Future large ground-based telescopes will need to find new
ways to optimize both their operational efficiency and scientific output.
We have begun experimenting with service-mode or queue-mode observations at
the Dunn Solar Telescope using the Interferometric Bidimensional Spectrometer
(IBIS) as part of joint Hinode campaigns. We describe our experiences and the
advantages of such an observing mode for solar physics.
We present new adaptive optics echelle spectra of the near-infrared [Fe II] lines in the redshifted and blueshifted jets from the T Tauri star RW Aur. The spectra have an unprecedented combination of high spatial and spectral resolution that makes it possible to trace the dynamics of the flow to a projected distance of only 10 AU from the source. As noted by previous studies, the redshifted flow is much slower than its fainter blueshifted counterpart. Our observations clearly show that both the radial velocities and the emission line widths are larger closer to the source on both sides of the jet. The line widths are 20% - 30% of the jet velocity on both sides of the flow, significantly larger than would be produced by a divergent constant velocity flow. The observed line widths could arise from a layered velocity structure in the jet or from magnetic waves. A bright knot in the redshifted jet has no concomitant increase in line width, implying that it is not heated by a bow shock. Alternate heating mechanisms include planar shocks, ambipolar diffusion and magnetic reconnection.
The diffuse Galactic gamma-ray data from the region of the
Galactic Center has been collected by the LAT instrument on the
Fermi Gamma-Ray Space Telescope. In this paper we argue that it may be able
to provide an unambiguous evidence of originating, in addition to known
astrophysical sources, from dark matter annihilations in the halo,
independently of the mass and other properties of the dark matter particle. We
also show that the recently released high precision data from mid-latitudes is
already providing an upper bound, albeit still a weak one, on the cuspiness of
the dark matter density profile as a function of the mass of the dark matter
assumed to be a stable neutralino of minimal supersymmetry.
With its rapid-response capability and multiwavelength complement of instruments, the Swift satellite has transformed our physical understanding of gamma-ray bursts (GRBs). Providing high-quality observations of hundreds of bursts, and facilitating a wide range of follow-up observations within seconds of each event, Swift has revealed an unforeseen richness in observed burst properties, shed light on the nature of short-duration bursts, and helped realize the promise of GRBs as probes of the processes and environments of star formation out to the earliest cosmic epochs. These advances have opened new perspectives on the nature and properties of burst central engines, interactions with the burst environment from microparsec to gigaparsec scales, and the possibilities for non-photonic signatures. Our understanding of these extreme cosmic sources has thus advanced substantially; yet more than 40 years after their discovery, GRBs continue to present major challenges on both observational and theoretical fronts.
The propagation trajectories of the highest energy cosmic rays can be deflected by Galactic magnetic field (GMF) and expected correlation between their arrival directions and the positions of their sources can be disturbed. In order to explore whether the possible correlation is disturbed or not, we simulate the arrival distribution of the highest energy protons (HEPs) with energies above $6 \times 10^{19}$ eV taking 4 different GMF models into account and investigate the cross-correlation between the protons and sources assumed in the simulation. We show that the error of cross-correlation function adopted in this study is sufficiently small by accumulating 200 events. We also find that the correlation is not disturbed largely in many cases after 200 events accumulation and positive signals of the correlation are significantly expected at angular scale of 3-5$^{\circ}$. Only in the cases of the northern sky with axisymmetric spiral structures of GMF, the cross-correlation functions are consistent with no correlation or have very low significance even if the correlation is positive at small angular scale. Conversely, no observation of the correlation within this scale implies no field reversal of GMF outside the solar system. Finally, we draw the possible source directions of recently published data of the Pierre Auger Observatory (PAO) assuming the composition to be purely protons and irons, and demonstrate that the pure-iron assumption may break the correlation of the PAO data with the large-scale structure of local Universe.
In 2006 I published an article in GeminiFocus that examined funding for
astronomical adaptive optics related technology and instrumentation in the
United States from 1995 through mid-2006. That article concluded that based on
projections then current, AO implementation on public and private telescopes in
the U.S. will soon seriously lag that on the ESO VLT as measured by funds
available. It called for a significant infusion of public funds for AO
development and implementation so that when combined with private funds, the
U.S. astronomical community as a whole would be able to take full advantage of
AO systems on both public and private telescopes. In 2006 I estimated that the
total amount of public (NSF) funds that would be available in 2009 for AO
related non-science activities would be about $6M.
This article updates the analysis done in my previous article. I show that
for 2009 the funds for AO related non-science activities are about $7M in spite
of the termination of the AODP program. Federal stimulus funds via the ARRA may
be responsible for a not insignificant part of this $7M. I make the probably
optimistic prediction that in 2010 there will be $6M in new NSF funds avaialbe
for AO related non-science work.
Thus there has been no significant real increase of public funding for AO
development and implementation. If private funding in the US and the level of
ESO AO funding is close to the values predicted in my previous article, then
ESO on one observatory, is outspending all US AO efforts spread over about a
dozen observatories by a factor of three.
Analyses of very accurate CoRoT space photometry, past Johnson V photoelectric photometry and high-resolution \'echelle spectra led to the determination of improved and consistent fundamental stellar properties of both components of AU Mon. We derived new, accurate ephemerides for both the orbital motion (with a period of 11.113d) and the long-term, overall brightness variation (with a period of 416.9d) of this strongly interacting Be + G semi-detached binary. It is shown that this long-term variation must be due to attenuation of the total light by some variable circumbinary material. We derived the binary mass ratio $M_{\rm G}/M_{\rm B}$ = 0.17\p0.03 based on the assumption that the G-type secondary fills its Roche lobe and rotates synchronously. Using this value of the mass ratio as well as the radial velocities of the G-star, we obtained a consistent light curve model and improved estimates of the stellar masses, radii, luminosities and effective temperatures. We demonstrate that the observed lines of the B-type primary may not be of photospheric origin. We also discover rapid and periodic light changes visible in the high-quality residual CoRoT light curves. AU Mon is put into perspective by a comparison with known binaries exhibiting long-term cyclic light changes.
It is well known that interstellar travel is bounded by the finite speed of light, but on very large scales any rocketeer would also need to consider the influence of cosmological expansion on their journey. This paper examines accelerated journeys within the framework of Friedmann- Lemaitre-Robertson-Walker universes, illustrating how the duration of a fixed acceleration sharply divides exploration over interstellar and intergalactic distances. Furthermore, we show how the universal expansion increases the difficulty of intergalactic navigation, with small uncertainties in cosmological parameters resulting in significantly large deviations. This paper also shows that, contrary to simplistic ideas, the motion of any rocketeer is indistinguishable from Newtonian gravity if the acceleration is kept small.
We compute analytic expressions for the non-linearity parameters characterizing the bi- and tri-spectrum of primordial curvature perturbations generated during an inflationary epoch of the early universe driven by an arbitrary number fields. We assume neither slow roll nor a separable potential; instead, to compute Non-Gaussianities, we assume a separable Hubble parameter. We apply the formalism to an exact solvable toy-model and show under which conditions observably large non-Gaussianities are produced.
The role of the interaction in crossing the phantom divide line in the Chaplygin gas model is discussed. We obtain some general properties of the interactions that allow the model to arrive at the phantom divide line. We show that these properties put some conditions on the ratio of dark matter to dark energy density.
Liquid Xenon (LXe) is an excellent material for experiments designed to detect dark matter in the form of Weakly Interacting Massive Particles (WIMPs). A low energy detection threshold is essential for a sensitive WIMP search. The understanding of the relative scintillation efficiency (Leff) and ionization yield of low energy nuclear recoils in LXe is limited for energies below 10 keV. In this paper, we present new measurements that extend the energy down to 4 keV, finding that Leff decreases with decreasing energy. We also measure the quenching of scintillation efficiency due to the electric field in LXe, finding no significant field dependence.
We discuss a very general theory of gravity, of which Lagrangian is an arbitrary function of the curvature invariants, on the brane. In general, the formulation of the junction conditions (except for Euler characteristics such as Gauss-Bonnet term) leads to the powers of the delta function and requires regularization. We suggest the way to avoid such a problem by imposing the metric and its first derivative to be regular at the brane, the second derivative to have a kink, the third derivative of the metric to have a step function discontinuity, and no sooner as the fourth derivative of the metric to give the delta function contribution to the field equations. Alternatively, we discuss the reduction of the fourth-order gravity to the second order theory by introducing extra scalar and tensor fields: the scalaron and the tensoron. In order to obtain junction conditions we apply two methods: the application of the Gauss-Codazzi formalism and the application of the generalized Gibbons-Hawking boundary terms which are appended to the appropriate actions. In the most general case we derive junction conditions without assuming the continuity of the scalaron and the tensoron on the brane. The derived junction conditions can serve studying the cosmological implications of the higher-order brane gravity models.
Presented in this paper is a technique that we propose for extracting the physical parameters of a rotating stellar core collapse from the observation of the associated gravitational wave signal from the collapse and core bounce. Data from interferometric gravitational wave detectors can be used to provide information on the mass of the progenitor model, precollapse rotation and the nuclear equation of state. We use waveform libraries provided by the latest numerical simulations of rotating stellar core collapse models in general relativity, and from them create an orthogonal set of eigenvectors using principal component analysis. Bayesian inference techniques are then used to reconstruct the associated gravitational wave signal that is assumed to be detected by an interferometric detector. Posterior probability distribution functions are derived for the amplitudes of the principal component analysis eigenvectors, and the pulse arrival time. We show how the reconstructed signal and the principal component analysis eigenvector amplitude estimates may provide information on the physical parameters associated with the core collapse event.
The theoretical description of compact structures that share some key features with mass varying particles allows for a simple analysis of equilibrium and stability for massive stellar bodies. We investigate static, spherically symmetric solutions of Einstein equations for a system composed by non-baryonic matter (neutrinos or dark matter) which forms stable structures through attractive forces mediated by a background scalar-field (dark energy). Assuming that the dark matter, or massive neutrinos, consist of a gas of weakly interacting particles, the coupling with the scalar field is translated into an effective dependence of the mass of the compounding particle on the radial coordinate of the curved spacetime. The stability analysis reveals that these static solutions become dynamically unstable for different Buchdahl limits of the ratio between the total mass-energy and the stellar radius, $M/R$. We also find regular solutions that for an external observer resemble Schwarzschild black-holes. Our analysis leaves unanswered the question whether such solutions, which are both regular and stable, do exist.
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We have obtained deep, high spatial resolution images of the central region of Abell 1795 at Halpha and [NII] (6583A) with the Maryland Magellan Tunable Filter (MMTF), and in the far-ultraviolet (FUV) with the Advanced Camera for Surveys Solar Blind Channel on the Hubble Space Telescope (HST). The superb image quality of the MMTF data has made it possible to resolve the known SE filament into a pair of thin, intertwined filaments extending for ~50 kpc, with a width < 1 kpc. The presence of these thin, tangled strands is suggestive of a cooling wake where runaway cooling is taking place, perhaps aided by an enhanced magnetic field in this region. The HST data further resolve these strands into chains of FUV-bright stellar clusters, indicating that these filaments are indeed sites of on-going star formation, but at a rate ~2 orders of magnitude smaller than the mass-deposition rates predicted from the X-ray data. The elevated [NII]/Halpha ratio and large spatial variations of the FUV/Halpha flux ratio across the filaments indicate that O-star photoionization is not solely responsible for the ionization. The data favor collisional heating by cosmic rays either produced in-situ by magnetohydrodynamical processes or conducted in from the surrounding intracluster medium.
We present a thorough literature study of the most-massive star, m_max, in several young star clusters in order to assess whether or not star clusters are populated from the stellar initial mass function (IMF) by random sampling over the mass range 0.01 < m < 150 M_sol without being constrained by the cluster mass, M_ecl. The data reveal a partition of the sample into lowest mass objects (M_ecl < 10^2 M_sol), moderate mass clusters (10^2 M_sol < M_ecl < 10^3 M_sol) and rich clusters above 10^3 M_sol. Additionally, there is a plateau of a constant maximal star mass (m_max ~ 25 M_sol) for clusters with masses between 10^3 M_sol and 4 10^3 M_sol. Statistical tests of this data set reveal that the hypothesis of random sampling from the IMF between 0.01 and 150 M_sol is highly unlikely for star clusters more massive than 10^2 M_sol with a probability of p ~ 2 10^-7 for the objects with M_ecl between 10^2 M_sol and 10^3 M_sol and p ~ 3 10^-9 for the more massive star clusters. Also, the spread of m_max values at a given M_ecl is smaller than expected from random sampling. We suggest that the basic physical process able to explain this dependence of stellar inventory of a star cluster on its mass may be the interplay between stellar feedback and the binding energy of the cluster-forming molecular cloud core. Given these results, it would follow that an integrated galactic initial mass function (IGIMF) sampled from such clusters would automatically be steeper in comparison to the IMF within individual star clusters.
We present a study of the properties of the star-cluster systems around pseudo-bulges of late-type spiral galaxies using a sample of 11 galaxies with distances from 17 to 37 Mpc. Star clusters are identified from multiband HST ACS and WFPC2 imaging data by combining detections in 3 bands (F435W and F814W with ACS and F606W with WFPC2). The photometric data are then compared to population synthesis models to infer the masses and ages of the star clusters. Photometric errors and completeness are estimated by means of artificial source Monte Carlo simulations. Dust extinction is estimated by considering F160W NICMOS observations of the central regions of the galaxies, augmenting our wavelength coverage. In all galaxies we identify star clusters with a wide range of ages, from young (age < 8 Myr) blue clusters, with typical mass of 10^3 Msun to older (age > 100-250 Myr), more massive, red clusters. Some of the latter might likely evolve into objects similar to the Milky Way's globular clusters. We compute the specific frequencies for the older clusters with respect to the galaxy and bulge luminosities. Specific frequencies relative to the galaxy light appear consistent with the globular cluster specific frequencies of early-type spirals. We compare the specific frequencies relative to the bulge light with the globular cluster specific frequencies of dwarf galaxies, which have a surface-brightness profile that is similar to that of the pseudo-bulges in our sample. The specific frequencies we derive for our sample galaxies are higher than those of the dwarf galaxies, supporting an evolutionary scenario in which some of the dwarf galaxies might be the remnants of harassed late-type spiral galaxies which hosted a pseudo-bulge.
Spectra of the He I 10830 Angstrom line were obtained with NIRSPEC on the Keck 2 telescope for metal-deficient field giant stars. This line is ubiquitous in stars with T_eff greater than 4500K and M_V fainter than -1.5. Fast outflows are detected from the majority of stars and about 40 percent of the outflows have sufficient speed to allow escape of material from the star as well as from a globular cluster. Outflow speeds and line strengths do not depend on metallicity suggesting the driving mechanism for these winds derives from magnetic and/or hydrodynamic processes. Gas outflows are present in every luminous giant, but are not detected in all stars of lower luminosity indicating possible variability. Mass loss rates ranging from 3X10(-10) to 6X10(-8) solar mass/yr estimated from the Sobolev approximation represent values with evolutionary significance for red giant branch (RGB) and red horizontal branch (RHB) stars. We estimate that 0.2 M_sun will be lost on the RGB, and the torque of this wind can account for observations of slowly rotating RHB stars in the field. About 0.1-0.2 M_sun will be lost on the RHB itself. This first empirical determination of mass loss on the RHB may contribute to the appearance of extended horizontal branches in globular clusters. The spectra appear to resolve the problem of missing intracluster material in globular clusters. Opportunities exist for 'wind smothering' of dwarf stars by winds from the evolved population, possibly leading to surface pollution in regions of high stellar density.
In the absence of constraints from the binary companion or supernova remnant, the standard method for estimating pulsar ages is to infer an age from the rate of spin-down. While the generic spin-down age may give realistic estimates for normal pulsars, it can fail for pulsars with very short periods. Details of the spin-up process during the low mass X-ray binary phase pose additional constraints on the period (P) and spin-down rates (Pdot) that may consequently affect the age estimate. Here, we propose a new recipe to estimate millisecond pulsar (MSP) ages that parametrically incorporates constraints arising from binary evolution and limiting physics. We show that the standard method can be improved by this approach to achieve age estimates closer to the true age whilst the standard spin-down age may over- or under-estimate the age of the pulsar by more than a factor of ~10 in the millisecond regime. We use this approach to analyze the population on a broader scale. For instance, in order to understand the dominant energy loss mechanism after the onset of radio emission, we test for a range of plausible braking indices. We find that a braking index of n=3 is consistent with the observed MSP population. We demonstrate the existence and quantify the potential contributions of two main sources of age corruption: the previously known "age bias" due to secular acceleration and "age contamination" driven by sub-Eddington progenitor accretion rates. We explicitly show that descendants of LMXBs that have accreted at very low rates will exhibit ages that appear older than the age of the Galaxy. We further elaborate on this technique, the implications and potential solutions it offers regarding MSP evolution, the underlying age distribution and the post-accretion energy loss mechanism.
The aim of this work is a detailed analysis of transit light curves from TrES-1 and TrES-2, obtained over a period of three to four years, in order to search for variabilities in observed mid-transit times and to set limits for the presence of additional third bodies. Using the IAC 80cm telescope, we observed transits of TrES-1 and TrES-2 over several years. Based on these new data and previously published work, we studied the observed light curves and searched for variations in the difference between observed and calculated (based on a fixed ephemeris) transit times. To model possible transit timing variations, we used polynomials of different orders, simulated O-C diagrams corresponding to a perturbing third mass and sinusoidal fits. For each model we calculated the $\chi^2$ residuals and the False Alarm Probability (FAP). For TrES-1 we can exclude planetary companions ($>$ 1 M$_{\oplus}$) in the 3:2 and 2:1 MMRs with high FAPs based on our transit observations from ground. Additionally, the presence of a light time effect caused by e. g. a 0.09 M$_\odot$ mass star at a distance of 7.8 AU is possible. As for TrES-2, we found a better ephemeris of T$_c = 2 453 957.63512(28) + 2.4706101(18) \times$Epoch and a good fit for a sine function with a period of 0.2 days, compatible with a moon around TrES-2 and an amplitude of 57 s, but it was not a uniquely low $\chi^2$ value that would indicate a clear signal. In both cases, TrES-1 and TrES-2, we were able to put upper limits on the presence of additional perturbers masses. We also conclude that any sinusoidal variations that might be indicative of exomoons need to be confirmed with higher statistical significance by further observations, noting that TrES-2 is in the field-of-view of the Kepler Space Telescope.
The quest to discover the most distant galaxies has developed rapidly in the last decade. We are now exploring redshifts of 6 and beyond, when the Universe was less than a billion years old, an epoch when the previously-neutral intergalactic medium was reionized. The continuing discovery of galaxies at progressively higher and higher redshifts has been driven by the availability of large telescopes on the ground and in space, improvements in detector technology, and new search strategies. Over the past 4 years, the Lyman break technique has been shown to be effective in isolating z~6 star-forming i'-drop galaxies through spectroscopic confirmation with large ground-based telescopes (Keck, Gemini and the ESO VLTs). Narrow-band imaging, notably with the wide field of the Subaru telescope, has also produced samples of Lyman-alpha emitters at these redshifts. A The discovery of this i'-drop galaxy population has been used to infer the global star formation rate density at this epoch (z~6), and we are now beginning to constrain the contribution to reionization of the UV flux from these galaxies. Infrared data from the Spitzer Space Telescope has been used to determine the spectral energy distributions (SEDs) from the rest-frame UV to the optical, and constrain the previous star formation histories, masses and ages. The indications are that much of the stellar mass of these galaxies might have formed in vigorous bursts at z>6. The next big advances would be to test the population synthesis modelling of these z~6 galaxies through spectroscopy of the rest-frame optical (rather than crude broad-band SEDs), and also to push the observational horizon for galaxies further to directly explore star formation during the reionization epoch. JWST is likely to have a profound impact on realising these goals.
We present a preliminary survey of 58 radio sources within the isoplanatic patches (r < 25") of bright (11<R<12) stars suitable for use as natural guide stars with high-order adaptive optics (AO). An optical and near-infrared imaging survey was conducted utilizing tip-tilt corrections in the optical and AO in the near-infrared. Spectral Energy Distributions (SEDs) were fit to the multi-band data for the purpose of obtaining photometric redshifts using the Hyperz code. Several of these photometric redshifts were confirmed with spectroscopy, a result that gives more confidence to the redshift distribution for the whole sample. Additional long-wavelength data from Spitzer, SCUBA, SHARC2, and VLA supplement the optical and near-infrared data. We find the sample generally follows and extends the magnitude-redshift relation found for more powerful local radio galaxies. The survey has identified several reasonably bright (H=19-20) objects at significant redshifts (z>1) that are now within the capabilities of the current generation of AO-fed integral-field spectrographs. These objects constitute a unique sample that can be used for detailed ground-based AO studies of galactic structure, evolution, and AGN formation at high redshift.
We use a model for the evolution of galaxies in the far-IR based on the LambdaCDM cosmology to make detailed predictions for upcoming cosmological surveys with the Herschel Space Observatory. We use the combined GALFORM semi-analytical galaxy formation model and GRASIL spectrophotometric code to compute galaxy SEDs including the reprocessing of radiation by dust. The model, which is the same as that in Baugh et al. (2005), assumes two different IMFs: a normal solar neighbourhood IMF for quiescent star formation in disks, and a very top-heavy IMF in starbursts triggered by galaxy mergers. We have shown previously that the top-heavy IMF appears necessary to explain the number counts and redshifts of faint sub-mm galaxies. In this paper, we present predictions for galaxy luminosity functions, number counts and redshift distributions in the Herschel imaging bands. We find that source confusion will be a serious problem in the deepest planned surveys. We also show predictions for physical properties such as star formation rates and stellar, gas and halo masses, together with fluxes at other wavelengths (from the far-UV to the radio) relevant for multi-wavelength follow-up observations. We investigate what fraction of the total IR emission from dust and of the high-mass star formation over the history of the Universe should be resolved by planned surveys with Herschel, and find a fraction ~30-50%, depending on confusion. Finally, we show that galaxies in Herschel surveys should be significantly clustered.
We examine the effects of density stratification on magnetohydrodynamic turbulence driven by the magnetorotational instability in local simulations that adopt the shearing box approximation. Our primary result is that, even in the absence of explicit dissipation, the addition of vertical gravity leads to convergence in the turbulent energy densities and stresses as the resolution increases, contrary to results for zero net flux, unstratified boxes. The ratio of total stress to midplane pressure has a mean of ~0.01, although there can be significant fluctuations on long (>~50 orbit) timescales. We find that the time averaged stresses are largely insensitive to both the radial or vertical aspect ratio of our simulation domain. For simulations with explicit dissipation, we find that stratification extends the range of Reynolds and magnetic Prandtl numbers for which turbulence is sustained. Confirming the results of previous studies, we find oscillations in the large scale toroidal field with periods of ~10 orbits and describe the dynamo process that underlies these cycles.
High-energy photons from cosmological emitters suffer attenuation due to pair production interactions with the extragalactic background light (EBL). The collective emission of any high-energy emitting cosmological population will exhibit an absorption feature at the highest energies. We calculate this absorption feature in the collective emission of blazars for various models of the blazar gamma-ray luminosity function (GLF) and the EBL. We find that models of the blazar GLF that predict higher relative contributions of high-redshift blazars to the blazar collective spectrum result in emission that is more susceptible to attenuation by the EBL, and hence result in more prominent absorption features, allowing for better differentiation amongst EBL models. We thus demonstrate that observations of such an absorption feature will contain information regarding both the blazar GLF and the EBL, and we discuss tests for EBL models and the blazar GLF that will become possible with upcoming Fermi observations.
We post-process galaxy star formation histories in cosmological hydrodynamics simulations to test quenching mechanisms associated with AGN. By comparing simulation results to color-magnitude diagrams and luminosity functions of SDSS galaxies, we examine whether "quasar mode" or "radio mode" AGN feedback can yield a realistic red sequence. Both cases yield red sequences distinct from the blue cloud, decent matches to the luminosity function, and galaxies that are too blue by about 0.1 magnitudes in g-r. Our merger-based prescription for quasar mode feedback, however, yields a red sequence build-up inconsistent with observations: the luminosity function lacks a characteristic knee, and the brightest galaxies include a small number of young stars.
Galaxy clusters are valuable cosmological probes. However, cluster mass estimates rely on observable quantities that are affected by complicated baryonic physics in the intracluster medium (ICM), including feedback from active galactic nuclei (AGN). Cosmological simulations have started to include AGN feedback using subgrid models. In order to make robust predictions, the systematics of different implementations and parametrizations need to be understood. We have developed an AGN subgrid model in FLASH that supports a few different black hole accretion models and feedback models. We use this model to study the effect of AGN on X-ray cluster observables and its dependence on model variations.
We present a preliminary study of using the template of an ensemble of evolving shells we developed for M 82 (Yao 2009). we apply the model to represent various stages of starburst evolution in a well known sample of nearby luminous infrared galaxies (LIRGs). In this way, we attempt to interpret the relationship between the degree of molecular excitation and ratio of far-infrared (FIR) to CO luminosity to possibly reflect different stages of the evolution of star-forming activity within their nuclear regions.
In almost 30 years of operation, the Very Large Array (VLA) has proved to be a remarkably flexible and productive radio telescope. However, the basic capabilities of the VLA have changed little since it was designed. A major expansion utilizing modern technology is currently underway to improve the capabilities of the VLA by at least an order of magnitude in both sensitivity and in frequency coverage. The primary elements of the Expanded Very Large Array (EVLA) project include new or upgraded receivers for continuous frequency coverage from 1 to 50 GHz, new local oscillator, intermediate frequency, and wide bandwidth data transmission systems to carry signals with 16 GHz total bandwidth from each antenna, and a new digital correlator with the capability to process this bandwidth with an unprecedented number of frequency channels for an imaging array. Also included are a new monitor and control system and new software that will provide telescope ease of use. Scheduled for completion in 2012, the EVLA will provide the world research community with a flexible, powerful, general-purpose telescope to address current and future astronomical issues.
We have implemented a simple model to identify the likely sites of the first stars and galaxies in the high-resolution simulations of the formation of galactic dark matter halos of the Aquarius Project. The first star in a galaxy like the Milky Way formed around redshift z=35; by z=10, the young galaxy contained up to 30000 dark matter haloes capable of forming stars by molecular hydrogen cooling. These minihaloes were strongly clustered and feedback may have severely limited the actual number of Population III stars that formed. By the present day, the remnants of the first stars would be strongly concentrated to the centre of the main halo. If a second generation of long-lived stars formed near the first (Population II.5), we would expect to find half of them within 30kpc/h of the galactic centre and a significant fraction in satellites where they may be singled out by their anomalous metallicity patterns. The first halo in which gas could cool by atomic hydrogen line radiation formed at z=25; by z=10, the number of such first galaxies had increased to ~300. Feedback might have decreased the number of first galaxies at the time they undergo significant star formation, but not the number that survive to the present because near neighbours merge. Half of all the "first galaxies" that form before z=10 merge with the main halo before z~3 and most lose a significant fraction of their mass. However, today there should still be more than 20 remnants orbiting within the central ~30kpc/h of the Milky Way. These satellites have circular velocities of a few kilometers per second or more, comparable to those of known Milky Way dwarfs. They are a promising hunting ground for the remnants of the earliest epoch of star formation.
Using the period and mass data of two hundred and seventy-nine extrasolar planets, we have constructed a coupled period-mass function through the non-parametric approach. This analytic expression of the coupled period-mass function has been obtained for the first time in this field. Moreover, due to a moderate period-mass correlation, the shapes of mass/period functions vary as a function of period/mass. These results of mass and period functions give way to two important implications: (1) the deficit of massive close-in planets is confirmed, and (2) the more massive planets have larger ranges of possible semi-major axes. These interesting statistical results will provide important clues into the theories of planetary formation.
We report on Chandra X-ray Observatory (CXO) High-Energy Transmission Grating (HETG) spectra of the dipping Low Mass X-ray Binary (LMXB) 1A 1744-361 during its July 2008 outburst. We find that its persistent emission is well modeled by a blackbody (kT ~ 1.0 keV) plus power-law ($\Gamma$ ~ 1.7) with an absorption edge at 7.6 keV. In the residuals of the combined spectrum we find a significant absorption line at 6.961+/-0.002 keV, consistent with the Fe XXVI (hydrogen-like Fe) 2 - 1 transition. We place an upper limit on the velocity of a redshifted flow of v < 221 km/s. We find an equivalent width for the line of 27^+2_-3 eV, from which we determine a column density of 7+/-1x10^17 cm^-2 via a curve-of-growth analysis. Using XSTAR simulations, we place a lower limit on the ionization parameter of > 10^3.6 erg cm/s. The properties of this line are consistent with those observed in other dipping LMXBs. Using Rossi X-ray Timing Explorer (RXTE) data accumulated during this latest outburst we present an updated color-color diagram which clearly shows that 1A 1744-361 is an "atoll" source. Finally, using additional dips found in the RXTE and CXO data we provide an updated orbital period estimate of 52+/-5 minutes.
We evaluate the ability of cosmic microwave background (CMB) polarization spectra to constrain cosmological models by analyzing a multi-experiment CMB dataset including the final analysis of the QUaD experiment. We provide the best limits on parameters from combined polarization data and find that QUaD spectra combined with additional CMB datasets using the optimal pivot scale of k_p=0.013 Mpc-1 prefer standard LCDM parameters of {omch2, ombh2, H_0, A_s, n_s, tau}={0.113, 0.0224, 70.6, 2.29 times 10^-9, 0.960, 0.086}, with the confidence regions of ombh2, omch2 and H_0 tightened due to the presence of QUaD data. QUaD alone constrains cold dark matter and baryon densities and the acoustic scale very well. The temperature and polarization sub-sets each provide good limits on cosmological parameters which are consistent with values obtained from a combination of existing CMB data. We incorporate small-scale CMB data to provide the tightest constraint on tensor modes from CMB data alone.
Sulfur abundances are derived for a sample of ten B main-sequence star members of the Orion association. The analysis is based on LTE plane-parallel model atmospheres and non-LTE line formation theory by means of a self-consistent spectrum synthesis analysis of lines from two ionization states of sulfur, SII and SIII. The observations are high-resolution spectra obtained with the ARCES spectrograph at the Apache Point Observatory. The abundance distribution obtained for the Orion targets is homogeneous within the expected errors in the analysis: A(S)=7.15+/-0.05. This average abundance result is in agreement with the recommended solar value (both from modelling of the photospheres in 1-D and 3-D, and meteorites) and indicates that little, if any, chemical evolution of sulfur has taken place in the last ~4.5 billion years. The sulfur abundances of the young stars in Orion are found to agree well with results for the Orion nebulae, and place strong constraints on the amount of sulfur depletion onto grains as being very modest or nonexistent. The sulfur abundances for Orion are consistent with other measurements at a similar galactocentric radius: combined with previous results for other OB-type stars produce a relatively shallow sulfur abundance gradient with a slope of -0.037+/-0.012 dex/kpc.
A sample of RR Lyrae (RRL) variables from the Southern Edgeworth-Kuiper Belt Object survey in regions overlapping the expected position of debris from the interaction of the Sagittarius (Sgr) dwarf galaxy with the Milky Way (RA ~ 20 and 21.5 h; distance 16-21 kpc) has been followed up spectroscopically and photometrically. The 21 photometrically confirmed type ab RRLs have mean [Fe/H] = -1.79 +/- 0.08, consistent with the abundances found for RRLs in a different portion of the Sgr tidal debris stream. The distribution of velocities in the Galactic standard of rest frame (V_GSR) of the 26 RRLs in the region is not consistent with a smooth halo population. Upon comparison with the Sgr disruption models of Law et al (2005), a prominent group of five stars having highly negative radial velocities (V_GSR ~ -175 kms-1) is consistent with predictions for old trailing debris when the Galactic halo is modeled as oblate. The observations also require that the recent trailing debris stream has a broader spread perpendicular to the Sgr plane than predicted by the models. We have also investigated the possible association of the Virgo Stellar Stream (VSS) with Sgr debris by comparing radial velocities for RRLs in the region with the same models, finding similarities in the velocity-position trends. As suggested by our earlier work, the stars in the VSS region with large negative V_GSR values are likely to be old leading Sgr debris, but we find that while old trailing Sgr debris may well make a contribution at positive V_GSR values, it is unlikely to fully account for the VSS feature. Overall we find that further modeling is needed, as trailing arm data generally favors oblate models while leading arm data favors prolate models, with no single potential fitting all the observed data.
Global linear stability analysis of a self-similar solution describing the interaction of a relativistic shell with an ambient medium is performed. The solution is shown to be unstable to convective Rayleigh-Taylor modes having angular scales smaller than the causality scale. Longer wavelength modes are stable and decay with time. For modes of sufficiently large spherical harmonic degree $l$ the dimensionless growth rate scales as $\sqrt{l/\Gamma}$, where $\Gamma$ is the Lorentz factor of the shell. The instability commences at the contact interface separating the shocked eject a and shocked ambient gas and propagates to the shocks. The reverse shock front responds promptly to the in stability and exhibits rapidly growing distortions at early times. Propagation to the forward shock is slower, and it is anticipated that the region near the contact will become fully turbulent before the instability is communicated to the forward shock. The non-universality of the Blandford-McKee blast wave solution suggests that turbulence generated by the instability in the shocked ambient medium may decay slowly with time and may be the origin of magnetic fields over a long portion of the blast wave evolution. It is also speculated that the instability may affect the emission from the shocked ejecta in the early post-prompt phase of GRBs.
The Tarantula Survey is an ambitious ESO Large Programme that has obtained multi-epoch spectroscopy of over 1,000 massive stars in the 30 Doradus region of the Large Magellanic Cloud. Here we introduce the scientific motivations of the survey and give an overview of the observational sample. Ultimately, quantitative analysis of every star, paying particular attention to the effects of rotational mixing and binarity, will be used to address fundamental questions in both stellar and cluster evolution.
We test the modified Newtonian dynamics (MOND) theory with the velocity-dispersion profiles of Galactic globular clusters populating the outermost region of the Milky Way halo, where the Galactic acceleration is lower than the characteristic MOND acceleration a_0. For this purpose, we constructed self-consistent, spherical models of stellar systems in MOND, which are the analogues of the Newtonian King models. The models are spatially limited, reproduce well the surface-brightness profiles of globular clusters, and have velocity-dispersion profiles that differ remarkably in shape from the corresponding Newtonian models. We present dynamical models of six globular clusters, which can be used to efficiently test MOND with the available observing facilities. A comparison with recent spectroscopic data obtained for NGC2419 suggests that the kinematics of this cluster might be hard to explain in MOND.
Aims: The launch of Swift has allowed many more novae to be observed regularly over the X-ray band. Such X-ray observations of novae can reveal ejecta shocks and the nuclear burning white dwarf, allowing estimates to be made of the ejecta velocity. Methods: We analyse XMM-Newton and Swift X-ray and UV observations of the nova V598 Pup, which was initially discovered in the XMM-Newton slew survey. These data were obtained between 147 and 255 days after the nova outburst, and are compared with the earlier, brighter slew detection. Results: The X-ray spectrum consists of a super-soft source, with the soft emission becoming hotter and much fainter between days ~147 and ~172 after the outburst, and a more slowly declining optically thin component, formed by shocks with kT ~ 200-800 eV (corresponding to velocities of 400-800 km s^-1). The main super-soft phase had a duration of less than 130 days. The Reflection Grating Spectrometer data show evidence of emission lines consistent with optically thin emission of kT ~100 eV and place a limit on the density of the surrounding medium of log(n_e/cm^-3) < 10.4 at the 90 % level. The UV emission is variable over short timescales and fades by at least one magnitude (at lambda ~ 2246-2600 angstrom) between days 169 and 255.
We re-examine the method of deprojection of proper motions, which has been used for finding the velocity ellipsoid of stars in the nearby Galaxy. This method is only legitimate if the lines of sight to the individual stars are uncorrelated with the stars' velocities. Very simple models are used to show that spurious results similar to ones recently reported are obtained when velocity dispersion decreases with galactocentric radius in the expected way. A scheme that compensates for this bias is proposed.
In 2007-2008 we carried out a new multiwavelength campaign of the Whole Earth Blazar Telescope (WEBT) on BL Lacertae, involving three pointings by the XMM-Newton satellite, to study its emission properties. The source was monitored in the optical-to-radio bands by 37 telescopes. The brightness level was relatively low. Some episodes of very fast variability were detected in the optical bands. The X-ray spectra are well fitted by a power law with photon index of about 2 and photoelectric absorption exceeding the Galactic value. However, when taking into account the presence of a molecular cloud on the line of sight, the data are best fitted by a double power law, implying a concave X-ray spectrum. The spectral energy distributions (SEDs) built with simultaneous radio-to-X-ray data at the epochs of the XMM-Newton observations suggest that the peak of the synchrotron emission lies in the near-IR band, and show a prominent UV excess, besides a slight soft-X-ray excess. A comparison with the SEDs corresponding to previous observations with X-ray satellites shows that the X-ray spectrum is extremely variable. We ascribe the UV excess to thermal emission from the accretion disc, and the other broad-band spectral features to the presence of two synchrotron components, with their related SSC emission. We fit the thermal emission with a black body law and the non-thermal components by means of a helical jet model. The fit indicates a disc temperature greater than 20000 K and a luminosity greater than 6 x 10^44 erg/s.
We investigate a class of dark energy models in which the equation of state undergoes a rapid transition and for which the Hubble SNIa diagram is known to be poorly discriminant. Interestingly enough, we find that transitions at high redshift can lead to distortion in the correlation function of dark matter at lower redshift. We therefore use a combination of the SN1a Hubble diagram, CMB data and power spectrum from the SDSS luminous red galaxies to constrain the redshift of a possible transition. We find that the fundamental cosmological parameters are well constrained independently of the presence of a transition. Acceptable transitions from an equation of state close to $w = 0$ to a value close to -1 are strongly rejected at redshifts much higher than those for which LSS data are available: the transition redshift can be rejected up to a value as high as 10. We conclude that no preference for a transition appears from present-day data.
Context: The Red MSX Source (RMS) survey has identified a large sample of candidate massive young stellar objects (MYSOs) and ultra compact (UC) HII regions from a sample of ~2000 MSX and 2MASS colour selected sources. Aims: To search for H2O masers towards a large sample of young high mass stars and to investigate the statistical correlation of H2O masers with the earliest stages of massive star formation. Methods: We have used the Mopra Radio telescope to make position-switched observations towards ~500 UCHII regions and MYSOs candidates identified from the RMS survey and located between 190\degr < l < 30\degr. These observations have a 4$\sigma$ sensitivity of ~1 Jy and a velocity resolution of ~0.4 km/s.} Results: We have detected 163 H2O masers, approximately 75% of which were previously unknown. Comparing the maser velocities with the velocities of the RMS sources, determined from 13CO observations, we have identified 135 RMS-H2O maser associations, which corresponds to a detection rate of ~27%. Taking into account the differences in sensitivity and source selection we find our detection rate is in general agreement with previously reported surveys. Conclusions: We find similar detection rates for UCHII regions and MYSOs candidates, suggesting that the conditions needed for maser activity are equally likely in these two stages of the star formation process. Looking at the detection rate as a function of distance from the Galactic centre we find it significantly enhanced within the solar circle, peaking at ~37% between 6-7 kpc, which is consistent with previous surveys of UC HII regions, possibly indicating the presence of a high proportion of more luminous YSOs and HII regions.
Polycyclic aromatic hydrocarbon (PAH) emission has been found in both starbursts and modestly starforming galaxies, but the relation between starforming activity and PAH luminosity is still a matter of debate. The different correlation degrees could be caused by the variety of optical and far-infrared sample selection criteria. In order to obtain a census of the typical properties of PAH emitting galaxies, we here study moderately distant galaxies which have been selected by their PAH emission. Combining the ISOCAM Parallel Survey at 6.7 micron with 2MASS we have colour-selected a sample of 120 candidates for strong PAH emission. We obtained optical and mid-infrared spectra of 75 and 19 sources, respectively, and analysed IRAS-ADDSCANs and available Spitzer 3.6-160 micron photometry. The Spitzer spectra exhibit clear PAH features and corroborate that our photometric selection criteria trace the PAH emission of galaxies fairly well. The optical spectra show emission lines, at median redshift z~0.1, as well as Hdelta and CaII absorption, indicating ongoing starformation as well as post-starbursts. The mid- and far-infrared spectral energy distributions (SEDs) provide evidence for a broad range of far-infrared (FIR) luminosities, but in general the dust is colder than for starburst galaxies like M82. For most galaxies the monocromatic luminosity peaks at about equal height at optical, 6.7 micron (PAH) and FIR wavelengths. In about 15% of the sources the FIR luminosity exceeds the optical and PAH energy output by about a factor 5-10 despite the cool dust temperature; in these galaxies a large dust mass of 10^8 - 10^9 M_sun is inferred.
We developed a new technique for registration of the far solar corona from ground-based observations at distances comparable to those obtained from space coronagraphs. It makes possible visualization of fine details of studied objects invisible by naked eye. Here we demonstrate that streamers of the electron corona sometimes punch the dust corona and that the shape of the dust corona may vary with time. We obtained several experimental evidences that the far coronal streamers (observed directly only from the space or stratosphere) emit only in discrete regions of the visible spectrum like resonance fluorescence of molecules and ions in comets. We found that interaction of the coronal streamers with the dust corona can produce molecules and radicals, which are known to cause the resonance fluorescence in comets.
In this paper we make an effort to understand the interaction of turbulence generated by the magnetorotational instability (MRI) with turbulence from other sources, such as supernova explosions (SNe) in galactic disks. First we perform a linear stability analysis (LSA) of non-ideal MRI to derive the limiting value of Ohmic diffusion that is needed to inhibit the growth of the instability for different types of rotation laws. With the help of a simple analytical expression derived under first-order smoothing approximation (FOSA), an estimate of the limiting turbulence level and hence the turbulent diffusion needed to damp the MRI is derived. Secondly, we perform numerical simulations in local cubes of isothermal nonstratified gas with external forcing of varying strength to see whether the linear result holds for more complex systems. Purely hydrodynamic calculations with forcing, rotation and shear are made for reference purposes, and as expected, non-zero Reynolds stresses are found. In the magnetohydrodynamic calculations, therefore, the total stresses generated are a superposition of the forcing and MRI contributions. To separate these contributions, we perform reference runs with MRI-stable shear profiles (angular velocity increasing outwards), which suggest that the MRI-generated stresses indeed become strongly suppressed as function of the forcing. The Maxwell to Reynolds stress ratio is observed to decrease an order of magnitude as the turbulence level due to external forcing exceeds the predicted limiting value, which we interpret as a sign of MRI suppression. Finally, we apply these results to estimate the limiting radius inside of which the SN activity can suppress the MRI, arriving at a value of 13.6kpc
The irradiation of protoplanetary discs by central stars is the main heating mechanism for discs, resulting in their flared geometric structure. In a series of papers, we investigate the deep links between 2D self-consistent disc structure and planetary migration in irradiated discs, focusing particularly on those around M stars. In this first paper, we analyse the thermal structure of discs that are irradiated by an M star by solving the radiative transfer equation by means of a Monte Carlo code. Our simulations of irradiated hydrostatic discs are realistic and self-consistent in that they include dust settling with multiple grain sizes (N=15), the gravitational force of an embedded planet on the disc, and the presence of a dead zone (a region with very low levels of turbulence) within it. We show that dust settling drives the temperature of the mid-plane from an $r^{-3/5}$ distribution (well mixed dust models) toward an $r^{-3/4}$. The dead zone, meanwhile, leaves a dusty wall at its outer edge because dust settling in this region is enhanced compared to the active turbulent disc at larger disc radii. The disc heating produced by this irradiated wall provides a positive gradient region of the temperature in the dead zone in front of the wall. This is crucially important for slowing planetary migration because Lindblad torques are inversely proportional to the disc temperature. Furthermore, we show that low turbulence of the dead zone is self-consistently induced by dust settling, resulting in the Kelvin-Helmholtz instability (KHI). We show that the strength of turbulence arising from the KHI in the dead zone is $\alpha=10^{-5}$.
We study F(R) modified gravity models that are capable of driving the accelerating epoch of the Universe at the present space-time curvature whilst not destroying the standard Big Bang and Inflationary cosmology. Recent studies have shown that a weak curvature singularity can arise generically in F(R) dynamical evolution. Beyond it, the global predictability is lost that signals an internal incompleteness of these models. In this work we show that this problem is cured by adding a quadratic correction with a sufficiently small coefficient to the F(R) function at large curvatures, as the singularity disappears. It appears that by modifying gravity at low curvatures to induce late time acceleration, we are forced to also modify gravity at high curvature. Such carefully constructed models can yield both an early time inflationary epoch and a late time de Sitter phase. The reheating epoch in these `unified' models of primordial and present dark energy is completely different from that of the old R + R^{2}/6M^{2} inflationary model, mainly due to the fact that values of the effective gravitational constant at low and intermediate curvatures are different for positive and negative R.
We propose a new scenario for the evolution of a binary of primordial black holes (PBHs). We consider the dynamical friction by ambient dark matter and gravitational interaction between a binary and a circumbinary disk, assuming PBHs do not constitute the bulk of dark matter. After the turnaround, a PBH binary loses the energy and angular momentum by the two processes, which are very effective for a typical configuration. Finally the binary coalesces due to the emission of gravitational waves in a time scale much shorter than the age of the universe. We estimate the density parameter of the resultant gravitational wave background. Astrophysical implication concerning supermassive black holes is also discussed.
We present three new methods for determining the age of groups of pre-main-sequence stars. The first, creating empirical isochrones allows us to create a robust age ordering, but not to derive actual ages. The second, using the width of the gap in colour-magnitude space between the pre-main-sequence and main-sequence (the radiative convective gap) has promise as a distance and extinction independent measure of age, but is as yet uncalibrated. Finally we discuss tau-squared fitting of the main sequence as the stars approach the terminus of the main sequence. This method suggests that there is a factor two difference between these "nuclear" ages, and more conventional pre-main-sequence contraction ages.
One of the prominent science goal of the ELTs will be to study the physics and mass assembly of galaxies at very high redshifts. Here, we present the galaxy evolution science case for EAGLE, which is a NIR multi-integral field spectrograph for the E-ELT currently under phase A study. We summarize results of simulations conducted to derive high-level requirements. In particular, we show how we have derived the specifications for the ensquared energy that the AO system needs to provide to reach the scientific goals of the instrument. Finally, we present future strategies to conduct galaxy surveys with EAGLE.
Valuable insights into galaxy evolution can be gleaned from studies of resolved stellar populations in the local Universe. Deep photometric surveys have provided tracers of the star-formation histories in galaxies from 0.8-16 Mpc, but without robust chemical abundances and stellar kinematics from spectroscopy, their sub-structures and assembly histories remain hidden from us. In this context, we introduce the EAGLE design study for a multi--integral-field-unit, near-infrared spectrograph for the European Extremely Large Telescope (E-ELT). EAGLE will exploit the unprecedented light-gathering power of the E-ELT to deliver AO-corrected spectroscopy across a large (38.5 sq. arcmin) field, truly revolutionising our view of stellar populations in the Local Volume.
Main-sequence stars earlier than spectral type ~F6 or so are expected to rotate rapidly due to their radiative exteriors. This rapid rotation leads to an oblate stellar figure. It also induces the photosphere to be hotter (by up to several thousand Kelvin) at the pole than at the equator as a result of a process called gravity darkening that was first predicted by von Zeipel (1924). Transits of extrasolar planets across such a non-uniform, oblate disk yield unusual and distinctive lightcurves that can be used to determine the relative alignment of the stellar rotation pole and the planet orbit normal. This spin-orbit alignment can be used to constrain models of planet formation and evolution. Orderly planet formation and migration within a disk that is coplanar with the stellar equator will result in spin-orbit alignment. More violent planet-planet scattering events should yield spin-orbit misaligned planets. Rossiter-McLaughlin measurements of transits of lower-mass stars show that some planets are spin-orbit aligned, and some are not. Since Rossiter-McLaughlin measurements are difficult around rapid rotators, lightcurve photometry may be the best way to determine the spin-orbit alignment of planets around massive stars. The Kepler mission will monitor ~10^4 of these stars within its sample. The lightcurves of any detected planets will allow us to probe the planet formation process around high-mass stars for the first time.
Using imaging that shows four magnitudes of main sequence stars, we have discovered that the Galactic globular cluster NGC 1851 is surrounded by a halo that is visible from the tidal radius of 700 arcsec (41 pc) to more than 4500 arcsec (>250 pc). This halo is symmetric and falls in density as a power law of $r^{-1.24}$. It contains approximately 0.1% of the dynamical mass of NGC 1851. There is no evidence for tidal tails. Current models of globular cluster evolution do not explain this feature, although simulations of tidal influences on dwarf spheroidal galaxies qualitatively mimic these results. Given the state of published models it is not possible to decide between creation of this halo from isolated cluster evaporation, or from tidal or disk shocking, or from destruction of a dwarf galaxy in which this object may have once been embedded.
In this paper we select large spectral averages of data from the Cassini Composite Infrared Spectrometer (CIRS) obtained in limb-viewing mode at low latitudes (30S--30N), greatly increasing the path length and hence signal-to-noise ratio for optically thin trace species such as propane. By modeling and subtracting the emissions of other gas species, we demonstrate that at least six infrared bands of propane are detected by CIRS, including two not previously identified in Titan spectra. Using a new line list for the range 1300-1400cm -1, along with an existing GEISA list, we retrieve propane abundances from two bands at 748 and 1376 cm-1. At 748 cm-1 we retrieve 4.2 +/- 0.5 x 10(-7) (1-sigma error) at 2 mbar, in good agreement with previous studies, although lack of hotbands in the present spectral atlas remains a problem. We also determine 5.7 +/- 0.8 x 10(-7) at 2 mbar from the 1376 cm-1 band - a value that is probably affected by systematic errors including continuum gradients due to haze and also an imperfect model of the n6 band of ethane. This study clearly shows for the first time the ubiquity of propane's emission bands across the thermal infrared spectrum of Titan, and points to an urgent need for further laboratory spectroscopy work, both to provide the line positions and intensities needed to model these bands, and also to further characterize haze spectral opacity. The present lack of accurate modeling capability for propane is an impediment not only for the measurement of propane itself, but also for the search for the emissions of new molecules in many spectral regions.
We consider simple models based on core collapse or pair-formation supernovae to account for the light curve of the transient SCP06F6. A radioactive decay diffusion model provides estimates of the mass of the required radioactive nickel and the ejecta as functions of the unknown redshift. An opacity change such as by dust formation or a recombination front may account for the rapid decline from maximum. We particularly investigate two specific redshifts: $z=0.143$, for which Gaensicke et al. (2008) have proposed that the unidentified broad absorption features in the spectrum of SCP06F6 are C$_{2}$ Swan bands, and $z=0.57$ based on a crude agreement with the Ca H&K and UV iron-peak absorption features that are characteristic of supernovae of various types. The ejected masses and kinetic energies are smaller for a more tightly constrained model invoking envelope recombination. We also discuss the possibilities of circumstellar matter (CSM) shell diffusion and shock interaction models. In general, optically-thick CSM diffusion models can fit the data with the underlying energy coming from an energetic buried supernova. Models in which the CSM is of lower density so that the shock energy is both rapidly thermalized and radiated tend not to be self-consistent. We suggest that a model of SCP06F6 worth futher exploration is one in which the redshift is $\sim$ 0.57, the spectral features are Ca and iron peak elements, and the light curve is powered by the diffusive release of a substantial amount of energy from nickel decay or from an energetic supernova buried in the ejecta of an LBV-like event.
We present a numerical scheme for modelling unresolved turbulence in cosmological adaptive mesh refinement codes. As a first application, we study the evolution of turbulence in the intra-cluster medium and in the core of a galaxy cluster. Simulations with and without subgrid scale model are compared in detail. Since the flow in the ICM is subsonic, the global turbulent energy contribution at the unresolved length scales is smaller than 1% of the internal energy. We find that the production of turbulence is closely correlated with merger events occurring in the cluster environment, and its dissipation locally affects the cluster energy budget. Because of this additional source of dissipation, the core temperature is larger and the density is smaller in the presence of subgrid scale turbulence than in the standard adiabatic run, resulting in a higher entropy core value.
Traversable wormhole are primarily useful as "gedanken-experiments" and as a theoretician's probe of the foundations of general relativity. In this work, we construct exact solutions of static and pseudo-spherically symmetric wormholes by adding exotic matter to a vacuum solution referred to as a degenerate solution of class A. The usual 2-d spheres are replaced by pseudo-spheres, which are still surfaces of revolution around an axis, but now consist of a negative and constant curvature. The physical properties and characteristics of these intriguing solutions are explored, and through the mathematics of embedding it is shown that particular constraints are placed on the shape function, that differ significantly from the Morris-Thorne wormhole. In particular, it is shown that the energy density is always negative and the radial pressure is positive, at the throat, contrary to the Morris-Thorne counterpart. Specific solutions are also presented by considering interesting equations of state, and by imposing restricted choices for the shape function or the redshift function. These new wormhole geometries are denoted as the alter ego of the Morris-Thorne wormhole.
Because the interaction is well-known, Coulomb excitation is one of the best tools for the investigation of nuclear properties. In the last 3 decades new reaction theories for Coulomb excitation have been developed such as: (a) relativistic Coulomb excitation, (b) Coulomb excitation at intermediate energies, and (c) multistep coupling in the continuum. These developments are timely with the advent of rare isotope facilities. Of special interest is the Coulomb excitation and dissociation of weakly-bound systems. I review the Coulomb excitation theory, from low to relativistic collision energies. Several applications of the theory to situations of interest in nuclear physics and nuclear astrophysics are discussed.
Most numerical models of binary stars - in particular neutron stars in compact binaries - assume the companions to be either corotational or irrotational. Either one of these assumptions leads to a significant simplification in the hydrodynamic equations of stationary equilibrium. In this paper we develop a new formalism for the construction of binary stars with circulation intermediate between corotational and irrotational. Generalizing the equations for irrotational flow we cast the Euler equation, which is an algebraic equation in the case of corotational or irrotational fluid flow, as an elliptic equation for a new auxiliary quantity. We also suggest a parameterized decomposition of the fluid flow that allows for a variation of the stellar circulation.
We parameterize the initial flux composition of high energy astrophysical neutrinos as (Phi_e^0 : Phi_mu^0 : Phi_tau^0) = (1 : n : 0), where n characterizes the source. All usually assumed neutrino sources appear as limits of this simple parametrization. We investigate how precise neutrino telescopes can pin down the value of n. We furthermore show that there is a neutrino mixing scenario in which the ratio of muon neutrinos to the other neutrinos takes a constant value regardless of the initial flux composition. This occurs when the muon neutrino survival probability takes its minimal allowed value. The phenomenological consequences of this very predictive neutrino mixing scenario are given.
A promising extension of general relativity is Chern-Simons (CS) modified gravity, in which the Einstein-Hilbert action is modified by adding a parity-violating CS term, which couples to gravity via a scalar field. In this work, we consider the interesting, yet relatively unexplored, dynamical formulation of CS modified gravity, where the CS coupling field is treated as a dynamical field, endowed with its own stress-energy tensor and evolution equation. We consider the possibility of observationally testing dynamical CS modified gravity by using the accretion disk properties around slowly-rotating black holes. The energy flux, temperature distribution, the emission spectrum as well as the energy conversion efficiency are obtained, and compared to the standard general relativistic Kerr solution. It is shown that the Kerr black hole provide a more efficient engine for the transformation of the energy of the accreting mass into radiation than their slowly-rotating counterparts in CS modified gravity. Specific signatures appear in the electromagnetic spectrum, thus leading to the possibility of directly testing CS modified gravity by using astrophysical observations of the emission spectra from accretion disks.
We study the impact of a strongly first-order electro-weak phase transition on the thermal relic abundance of particle species that could constitute the dark matter and that decoupled before the phase transition occurred. We define a dilution factor induced by generic first-order phase transitions, and we explore the parameter space of the minimal supersymmetric extension to the Standard Model to determine which phase transition temperatures and dilution factors are relevant for the lightest neutralino as a dark matter candidate. We then focus on a specific toy-model setup that could give rise to a strongly first-order electro-weak phase transition, and proceed to a detailed calculation of dilution factors and transition temperatures, comparing our findings to actual neutralino dark matter models. Typical models that would produce an excessive thermal relic density and that can be salvaged postulating a strongly first-order electro-weak phase transition include massive (multi-TeV) wino or higgsino-like neutralinos, as well as bino-like neutralinos in a wider mass range, with masses as low as 400 GeV. If LHC data indicate an inferred thermal neutralino relic abundance larger than the cold dark matter density, the mismatch could thus potentially be explained by electro-weak scale physics that will also be thoroughly explored with collider experiments in the near future.
A dilaton could be the dominant messenger between Standard Model fields and dark matter. The measured dark matter relic abundance relates the dark matter mass and spin to the conformal breaking scale. The dark matter-nucleon spin-independent cross section is predicted in terms of the dilaton mass. We compute the current constraints on the dilaton from LEP and Tevatron experiments, and the gamma-ray signal from dark matter annihilation to dilatons that could be observed by Fermi-LAT.
In this paper we study the magnetic susceptibility and other thermodynamic properties of the polarized nuclear matter at finite temperature using the lowest order constrained variational (LOCV) method employing the $AV_{18}$ potential. Our results show a monotonic behavior for the magnetic susceptibility which indicates that the spontaneous transition to the ferromagnetic phase does not occur for this system.
Solar and KamLAND data are in slight tension when interpreted in the standard two-flavor oscillations framework and this may be alleviated allowing for a non-zero value of the mixing angle theta_13. Here we show that, likewise, non-standard flavor-changing interactions (FCI), possibly intervening in the propagation of solar neutrinos, are equally able to alleviate this tension and therefore constitute a potential source of confusion in the determination of theta_13. By performing a full three-flavor analysis of solar and KamLAND data in presence of FCI we provide a quantitative description of the degeneracy existing between theta_13 and the vectorial coupling eps_e\tau^dV characterizing the non-standard transitions between nu_e and nu_tau in the forward scattering process with d-type quarks. We find that couplings with magnitude eps_e\tau^dV ~ 10%, compatible with the existing bounds, can mimic the non-zero values of theta_13 indicated by the latest analyses.
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The baryon acoustic oscillation (BAO) method for constraining the expansion history is adversely affected by non-linear structure formation, which washes out the correlation function peak created at decoupling. To increase the constraining power of low z BAO experiments, it has been proposed that one use the observed distribution of galaxies to "reconstruct'' the acoustic peak. Recently Padmanabhan, White and Cohn provided an analytic formalism for understanding how reconstruction works within the context of Lagrangian perturbation theory. We extend that formalism to include the case of biased tracers of the mass and, because the quantitative validity of LPT is questionable, we investigate reconstruction in N-body simulations. We find that LPT does a good job of explaining the trends seen in simulations for both the mass and for biased tracers and comment upon the implications this has for reconstruction.
We utilize the newly-acquired, ultra-deep WFC3/IR observations over the HUDF to search for star-forming galaxies at z~8-8.5, only 600 million years from recombination, using a Y_{105}-dropout selection. The new 4.7 arcmin**2 WFC3/IR observations reach to ~29 AB mag (5 sigma) in the Y_{105}J_{125}H_{160} bands. These remarkable data reach ~0.5 AB mag deeper than ever before, and now are an excellent match to the HUDF optical ACS data. For our search criteria, we use a two-color Lyman-Break selection technique to identify z~8-8.5 Y_{105}-dropouts. We find 5 probable z~8-8.5 candidates. The sources have H_{160}-band magnitudes of ~28.5 AB mag, apparent sizes of ~0.15'' (~0.7 h^{-1} kpc), and very blue UV-continuum slopes (i.e., beta <~-2.5), suggesting that z~8 galaxies are not only dust free but also perhaps have very young ages or low metallicities. The observed number of Y_{105}-dropout candidates is smaller than the 21 and 10 sources expected assuming no evolution from z~6 and z~7, respectively, but is consistent with the 7 expected extrapolating the Bouwens et al. 2008 LF results to z~8. These results provide evidence that the evolution in the LF seen from z~7 to z~3 continues to z~8. The remarkable improvement in the sensitivity of WFC3/IR has enabled HST to cross a threshhold, revealing star-forming galaxies at z~8-9, and even possibly z~10 galaxies. The existence of such galaxies implies that substantial star formation occurred at even higher redshifts z>10, deep into JWST territory.
[Abridged] We present the first comprehensive analysis of Hubble Space Telescope (HST) observations of short-duration gamma-ray burst (GRB) host galaxies. These observations allow us to characterize the galactic and local environments of short GRBs as a powerful constraint on the nature of their progenitors. Using the HST data for 9 short GRB hosts we determine the host morphological properties, measure precise physical and host-normalized offsets relative to the galaxy centers, and study the locations of short GRBs relative to their host light distributions. We find that most short GRB hosts have exponential disk profiles, characteristic of late-type galaxies, but with a median size that is twice as large as that of long GRB hosts, commensurate with their higher luminosities. The observed distribution of projected physical offsets, supplemented by ground-based measurements, has a median of ~5 kpc, about 5 times larger than for long GRBs, and in good agreement with predicted offset distributions for NS-NS binary mergers. For the short GRB population as a whole we find the following robust constraints: (i) >25% have projected offsets of <10 kpc; and (ii) >5% have projected offsets of >20 kpc. We find no clear systematic trends for the offset distribution of short GRBs with and without extended soft emission. While the physical offsets are larger than for long GRBs, the distribution of host-normalized offsets is nearly identical due to the larger size of short GRB hosts. Finally, unlike long GRBs, which are concentrated in the brightest regions of their host galaxies, short GRBs uniformly trace the light distribution of their hosts; this is similar to the distribution of core-collapse supernovae. Based on these results, we conclude that short GRBs are consistent with a progenitor population of NS-NS binaries.
Cool cores of galaxy clusters are thought to be heated by low-power active galactic nuclei (AGN), whose accretion is regulated by feedback. However, the interaction between the hot gas ejected by the AGN and the ambient intracluster medium is extremely difficult to simulate, as it involves a wide range of spatial scales and gas that is Rayleigh-Taylor (RT) unstable. Here we use a subgrid model for RT-driven turbulence to overcome these problems and present the first observationally-consistent hydrodynamical simulations of AGN self-regulation in galaxy clusters. For a wide range of parameter choices the cluster in our three-dimensional simulations regulates itself for at least several Gyrs years. Heating balances cooling through a string of outbreaks with a typical recurrence time of approximately 80 Myrs, a timescale that depends only on the global cluster properties.
We present a sample of 16 robust z~7 z_{850}-dropout galaxies detected by the newly installed WFC3/IR camera on the Hubble Space Telescope. Our analysis is based on the first epoch data of the HUDF09 program covering the Hubble Ultra Deep Field with 60 orbits of Y, J and H observations. These remarkable data cover 4.7 arcmin^2 and are the deepest NIR images ever taken, reaching to ~29 mag AB (5sigma). The 16 z~6.5-7.5 galaxies have been identified based on the Lyman Break technique utilizing (z-Y) vs. (Y-J) colors. They have magnitudes J = 26.1-29.1 (AB), an average apparent half-light radius of ~0.16 arcsec (<~1 kpc), and show very blue colors (beta<~-2.5), in particular at low luminosities. The WFC3/IR data confirms previous NICMOS detections indicating that the dropout selection at z~7 is very reliable. Our data allow a first determination of the faint end slope of the z~7 luminosity function, reaching down to M_{UV} ~ -18, a full magnitude fainter than previous measurements. We find a best-fit value of alpha=-1.86+-0.33, when fixing phi_*=1.1e-3 Mpc^{-3}mag^{-1} and M_*=-19.8 to the previously measured values at z~7 by Bouwens et al. 2008. This steep slope is similar to what is seen at z~2-6 and indicates that low luminosity galaxies could provide adequate flux to reionize the universe. The remarkable depth and resolution of these new images provide insights into the coming power of JWST.
We present a detailed strong lensing analysis of an HST/ACS legacy dataset for the first gravitational lens, Q0957+561. With deep imaging we identify 24 new strongly lensed features, which we use to constrain mass models. We model the stellar component of the lens galaxy using the observed luminosity distribution, and the dark matter halo using several different density profiles. We draw on the weak lensing analysis by Nakajima et al. (2009) to constrain the mass sheet and environmental terms in the lens potential. Adopting the well-measured time delay, we find H_0 = 85 (+14/-13) km/s/Mpc using lensing constraints alone. The principal uncertainties in H_0 are tied to the stellar mass-to-light ratio (a variant of the radial profile degeneracy in lens models). Adding constraints from stellar population synthesis models, we obtain H_0 = 79.3 (+6.7/-8.5) km/s/Mpc. We infer that the lens galaxy has a rising rotation curve and a dark matter distribution with an inner core. Intriguingly, we find the quasar flux ratios predicted by our models to be inconsistent with existing radio measurements, suggesting the presence of substructure in the lens.
Outbursts from active galactic nuclei (AGN) affect the hot atmospheres of isolated giant elliptical galaxies (gE's), as well as those in groups and clusters of galaxies. Chandra observations of a sample of nearby gE's show that the average power of AGN outbursts is sufficient to stop their hot atmospheres from cooling and forming stars, consistent with radio mode feedback models. The outbursts are intermittent, with duty cycles that increases with size.
The physics of X-ray cavities in galaxy clusters is constrained by their observed morphological evolution, which depends on such poorly-understood properties as the turbulent density field and magnetic fields. Here we combine numerical simulations that include subgrid turbulence and software that produces synthetic X-ray observations to examine the evolution of X-ray cavities in the the absence of magnetic fields. Our results reveal an anisotropic size evolution that is very different from simplified, analytical predictions. These differences highlight some of the key issues that must be accurately quantified when studying AGN-driven cavities, and help to explain why the inferred pV energy in these regions appears to be correlated with their distance from the cluster center. Interpreting X-ray observations will require detailed modeling of effects including mass-entrainment, distortion by drag forces, and projection. Current limitations do not allow a discrimination between purely hydrodynamic and magnetically-dominated models for X-ray cavities.
A number of theoretical and simulation results on star and structure formation in galaxy interactions and mergers is reviewed, and recent hydrodynamic simulations are presented. The role of gravity torques and ISM turbulence in galaxy interactions, in addition to the tidal field, is highlighted. Interactions can drive gas inflows towards the central kpc and trigger a central starburst, the intensity and statistical properties of which are discussed. A kinematically decoupled core and a supermassive central black hole can be fueled. Outside of the central kpc, many structures can form inside tidal tails, collisional ring, bridges, including super star clusters and tidal dwarf galaxies. The formation of super star clusters in galaxy mergers can now be directly resolved in hydrodynamic simulations. Their formation mechanisms and long-term evolution are reviewed, and the connection with present-day early-type galaxies is discussed.
We propose that generic magnetic equilibrium of an ideally conducting fluid contains a volume-filling set of singular current layers. Singular current layers should exist inside neutron stars. Residual dissipation in the singular current layers might be the main mechanism for the magnetic field decay. The slow decay of the field might be the clock responsible for triggering the magnetar flares.
We have applied the Tremaine-Weinberg method to 10 late-type barred spiral galaxies using data cubes, in H-alpha emission, from the GHAFAS and FANTOMM Fabry-Perot spectrometers. We have combined the derived bar (and/or spiral) pattern speeds with angular frequency plots to measure the corotation radii for the bars in these galaxies. We base our results on a combination of this method with a morphological analysis designed to estimate the corotation radius to bar-length ratio using two independent techniques on archival near infrared images, and although we are aware of the limitation of the application of the Tremaine-Weinberg method using ionised gas observations, we find consistently excellent agreement between bar and spiral arm parameters derived using different methods. In general, the corotation radius, measured using the Tremaine-Weinberg method, is closely related to the bar length, measured independently from photometry and consistent with previous studies. Our corotation/bar-length ratios and pattern speed values are in good agreement with general results from numerical simulations of bars. In systems with identified secondary bars, we measure higher H-alpha velocity dispersion in the circumnuclear regions, whereas in all the other galaxies, we detect flat velocity dispersion profiles. The excellent agreement between the Tremaine-Weinberg method results and the morphological analysis and bar parameters in numerical simulations, suggests that although the H-alpha emitting gas does not obey the continuity equation, it can be used to derive the bar pattern speed.
We present observations of the 51 Ophiuchi circumstellar disk made with the Keck interferometer operating in nulling mode at N-band. We model these data simultaneously with VLTI-MIDI visibility data and a Spitzer IRS spectrum using a variety of optically-thin dust cloud models and an edge-on optically-thick disk model. We find that single-component optically-thin disk models and optically-thick disk models are inadequate to reproduce the observations, but an optically-thin two-component disk model can reproduce all of the major spectral and interferometric features. Our preferred disk model consists of an inner disk of blackbody grains extending to ~4 AU and an outer disk of small silicate grains extending out to ~1200 AU. Our model is consistent with an inner "birth" disk of continually colliding parent bodies producing an extended envelope of ejected small grains. This picture resembles the disks around Vega, AU Microscopii, and Beta Pictoris, supporting the idea that 51 Ophiuchius may be a Beta Pictoris analog.
We present images of the planetary nebulae (PNe) NGC 7354 and NGC 3242 in
four mid-infrared (MIR) photometric bands centred at 3.6, 4.5, 5.8 and 8.0
microns; the results of observations undertaken using the Spitzer Space
Telescope (SST). The resulting images show the presence of a halo and rings in
NGC 3242, as previously observed through narrow band imaging at visual
wavelengths, as well as evidence for a comparable halo and ring system in NGC
7354. This is the first time that a halo and rings have been observed in the
latter source.
We have analysed the formation of halos as a result of radiatively
accelerated mass loss in the AGB progenitors. Although the models assume that
dust formation occurs in C-rich environments, we note that qualitatively
similar results would be expected for O-rich progenitors as well. The model
fall-offs in halo density are found to result in gradients in halo surface
brightness which are similar to those observed in the visible and MIR.
We present the time evolution of viscously accreting circumstellar disks as they are irradiated by ultraviolet and X-ray photons from a low-mass central star. Our model is a hybrid of a 1D time-dependent viscous disk model coupled to a 1+1D disk vertical structure model used for calculating the disk structure and photoevaporation rates. We find that disks of initial mass 0.1M_o around 1M_o stars survive for 4x10^6 years, assuming a viscosity parameter $\alpha=0.01$, a time-dependent FUV luminosity $L_{FUV}~10^{-2}-10^{-3}$ L_o and with X-ray and EUV luminosities $L_X \sim L_{EUV} ~ 10^{-3}$L_o. We find that FUV/X-ray-induced photoevaporation and viscous accretion are both important in depleting disk mass. Photoevaporation rates are most significant at ~ 1-10 AU and at >~ 30 AU. Viscosity spreads the disk which causes mass loss by accretion onto the central star and feeds mass loss by photoevaporation in the outer disk. We find that FUV photons can create gaps in the inner, planet-forming regions of the disk (~ 1-10 AU) at relatively early epochs in disk evolution while disk masses are still substantial. EUV and X-ray photons are also capable of driving gaps, but EUV can only do so at late, low accretion-rate epochs after the disk mass has already declined substantially. Disks around stars with predominantly soft X-ray fields experience enhanced photoevaporative mass loss. We follow disk evolution around stars of different masses, and find that disk survival time is relatively independent of mass for stars with M <~ 3M_o; for M >~ 3M_o the disks are short-lived(~10^5 years).
We measure the skewness power spectrum of the Cosmic Microwave Background (CMB) anisotropies optimized for a detection of the secondary bispectrum generated by the correlation of the CMB lensing potential with integrated Sachs-Wolfe effect and the Sunyaev-Zel'dovich effect. The covariance of our measurements is generated by Monte-Carlo simulations of Gaussian CMB fields with noise properties consistent with Wilkinson Microwave Anisotropy Probe (WMAP) 5-year data. When interpreting multi-frequency measurements we also take into account the confusion resulting from unresolved radio point sources. We analyze Q, V and W-band WMAP 5-year raw and foreground-cleaned maps using the KQ75 mask out to l_max=600. We find no significant evidence for a non-zero non-Gaussian signal from the lensing-secondary correlation in all three bands and we constrain the overall amplitude of the cross power spectrum between CMB lensing potential and the sum of SZ and ISW fluctuations to be 0.42 \pm 0.86 and 1.19 \pm 0.86 in combined V and W-band raw and foreground-cleaned maps provided by the WMAP team, respectively. The point source amplitude at the bispectrum level measured with this skewness power spectrum is higher than previous measurements of point source non-Gaussianity. We also consider an analysis where we also account for the primordial non-Gaussianity in addition to lensing-secondary bispectrum and point sources. The focus of this paper is on secondary anisotropies. Consequently the estimator is not optimised for primordial non-Gaussianity and the limit we find on local non-Gaussianity from the foreground-cleaned V+W maps is f_NL = -13 \pm 62, when marginalized over point sources and lensing-ISW/SZ constributions to the total bispectrum.
The motion of planetary nebulae (PNe) through the interstellar medium (ISM)
is thought to lead to a variety of observational consequences, including the
formation of bright rims; deformation and fragmentation of the shells; and a
shift of the central stars away from the geometric centres of the envelopes.
These and other characteristics have been noted through imaging in the visual
wavelength regime. We report further observations of such shells taken in the
mid-infrared (MIR), acquired through programs of IRAC imaging undertaken using
the Spitzer Space Telescope (SST).
NGC 2440 and NGC 6629 are shown to possess likely interacting halos, together
with ram-pressure stripped material to one side of their shells. Similarly, the
outer halos of NGC 3242 and NGC 6772 appear to have been fragmented through
Rayleigh-Taylor (RT) instabilities, leading to a possible flow of ISM material
towards the inner portions of their envelopes. If this interpretation is
correct, then it would suggest that NGC 3242 is moving towards the NE; a
suggestion which is also supported through the presence of a 60 microns tail
extending in the opposite direction, and curved bands of H-alpha emission in
the direction of motion - components which may arise through RT instabilities
in the magnetized ISM.
Abridged: The origin and progenitors of short-hard gamma-ray bursts remain a puzzle and a highly debated topic. The most popular model to interpret short-hard bursts is the merger of two compact stellar objects (NS-NS or NS-BH). A closer analysis by Zhang et al. reveals that a small sample of GRBs that have the most robust evidence of the compact star merger origin (Type I) is not a fair representation of the BATSE short-hard GRB sample. This raises the question whether all short-hard GRBs are Type I, and whether there is a strong contamination of GRBs due to massive star core collapses (Type II) in the short-hard GRB sample. We utilize a Monte Carlo approach to determine whether a merger progenitor model can self-consistently account for all the observations of short-hard GRBs, including a sample with redshift measurements in the Swift era (z-known sample) and the CGRO/BATSE sample. We apply various merger time delay distributions invoked in compact star merger models to derive the redshift distributions of Type I GRBs, and then constrain the unknown luminosity function of Type I GRBs. We find that it is difficult to reconcile a merger scenario progenitor with all the data. In order to satisfy both observational constraints, the allowed merger delay time scale is essentially zero, which suggests that most short GRBs should follow the star forming history of the universe. We explore the possibility that the observed short-hard GRB sample is a mix of Type I (which requires a merger time delay to define the z-distribution) and Type II (whose z-distribution tracks the star forming rate) GRBs. We conclude that Type I GRBs only account for a small fraction of short-hard GRBs and that the majority of short-hard GRBs track the star forming history of the universe and are likely of Type II origin.
Motivated by the observations on the intra-cluster light (ICL) and inter-galactic stellar populations, n-body simulations are used to model the galactic merging events as a goal to investigate the production and distribution of gravitational unbound populations (GUPs). Both the parabolic and hyperbolic mergers are considered and each category includes six models with different relative orientations between two galaxies. Our results show that there are more (about a factor of two) GUP after a hyperbolic merging event than after a parabolic one. In general, depending on the relative orientation and also the relative velocity of the two galaxies in a merging pair, a head-on collision of a galaxy pair would only make a tiny fraction (less than one percent) of the initial stellar mass become luminous GUP but a considerable fraction (eight to fourteen percent) of the dark matter become dark GUP.
Using numerical simulations, we show that smooth migration of the giant planets through a planetesimal disk leads to an orbital architecture that is inconsistent with the current one: the resulting eccentricities and inclinations of their orbits are too small. The crossing of mutual mean motion resonances by the planets would excite their orbital eccentricities but not their orbital inclinations. Moreover, the amplitudes of the eigenmodes characterising the current secular evolution of the eccentricities of Jupiter and Saturn would not be reproduced correctly; only one eigenmode is excited by resonance-crossing. We show that, at the very least, encounters between Saturn and one of the ice giants (Uranus or Neptune) need to have occurred, in order to reproduce the current secular properties of the giant planets, in particular the amplitude of the two strongest eigenmodes in the eccentricities of Jupiter and Saturn.
We investigate the dynamical evolution of the terrestrial planets during the planetesimal-driven migration of the giant planets. A basic assumption of this work is that giant planet migration occurred after the completion of terrestrial planet formation, such as in the models that link the former to the origin of the Late Heavy Bombardment. The divergent migration of Jupiter and Saturn causes the g5 eigenfrequency to cross resonances of the form g5=gk with k ranging from 1 to 4. Consequently these secular resonances cause large-amplitude responses in the eccentricities of the terrestrial planets. We show that the resonances g5=g_4 and g5=g3 do not pose a problem if Jupiter and Saturn have a fast approach and departure from their mutual 2:1 mean motion resonance. On the other hand, the resonance crossings g5=g2 and g5=g1 are more of a concern as they tend to yield a terrestrial system incompatible with the current one. We offer two solutions to this problem. The first uses the fact that a secular resonance crossing can also damp the amplitude of a Fourier mode if the latter is large originally. A second scenario involves a 'jumping Jupiter' in which encounters between an ice giant and Jupiter, without ejection of the former, cause the latter to migrate away from Saturn much faster than if migration is driven solely by encounters with planetesimals. In this case, the g5=g2 and g5=g1 resonances can be jumped over, or occur very briefly.
We discuss here a limit on the maximum brightness temperature achievable for an incoherent synchrotron radio source. This limit, commonly referred to in the literature as an inverse Compton limit, prescribes that the brightness temperature for an incoherent synchrotron radio source may not exceed ~10^{12} K, a fact known from observations. However one gets a somewhat tighter limit on the brightness temperatures, T_{b}~10^{11.5} K, independent of the inverse Compton effects, if one employs the condition of equipartition of energy in magnetic fields and relativistic particles in a synchrotron radio source. Pros and cons of the two brightness temperature limits are discussed.
We have observed four low-luminosity active galactic nuclei classified as Type 1 LINERs with the X-ray Telescope (XRT) and the UltraViolet-Optical Telescope (UVOT) onboard Swift, in an attempt to clarify the main powering mechanism of this class of nearby sources. Among our targets, we detect X-ray variability in NGC 3998 for the first time. The light curves of this object reveal variations of up to 30% amplitude in half a day, with no significant spectral variability on this time scale. We also observe a decrease of ~30% over 9 days, with significant spectral softening. Moreover, the X-ray flux is ~40% lower than observed in previous years. Variability is detected in M 81 as well, at levels comparable to those reported previously: a flux increase in the hard X-rays (1-10 keV) of 30% in ~3 hours and variations by up to a factor of 2 within a few years. This X-ray behaviour is similar to that of higher-luminosity, Seyfert-type, objects. Using previous high-angular-resolution imaging data from the Hubble Space Telescope (HST), we evaluate the diffuse UV emission due to the host galaxy and isolate the nuclear flux in our UVOT observations. All sources are detected in the UV band, at levels similar to those of the previous observations with HST. The XRT (0.2-10 keV) spectra are well described by single power-laws and the UV-to-X-ray flux ratios are again consistent with those of Seyferts and radio-loud AGNs of higher luminosity. The similarity in X-ray variability and broad-band energy distributions suggests the presence of similar accretion and radiation processes in low- and high-luminosity AGNs.
We propose that the intermediate luminosity optical transient NGC 300 OT2008-1 was powered by a mass transfer episode from an extreme Asymptotic Giant Branch star to a Main Sequence companion. We find a remarkable similarity of the shapes of the light curves of the several months long NGC 300 OT2008-1 outburst, of the three months long 2002 enigmatic outburst of the B star V838 Mon, and the twenty-years long Great Eruption of the massive binary system Eta Carinae that occurred in the 19th century. Their similar decline properties hint to a common energy source: a gravitational energy that is released by accretion onto a main sequence star. These events populate a specific strip in the total energy vs. outburst duration diagram. The strip is located between novae and supernovae. We add recent transient events to that diagram and find them to occupy the same strip. This suggests that some intermediate luminosity optical transients are powered by accretion onto a compact object (not necessarily a main sequence star). These transients are expected to produce bipolar ejecta as a result of the geometry of the accretion process.
A non-relativistic classification of charged molecular hydrogenic, helium and mixed helium-hydrogenic chains with one or two electrons which can exist in a strong magnetic field $B \lesssim 10^{16} $G is given. It is shown that for both $1e-2e$ cases at the strongest studied magnetic fields the longest hydrogenic chain contains at most five protons indicating to the existence of the $\rm{H}_5^{4+}$ and $\rm{H}_5^{3+}$ ions, respectively. In the case of the helium chains the longest chains can exist at the strongest studied magnetic fields with three and four $\al-$particles for $1e-2e$ cases, respectively. For mixed helium-hydrogenic chains the number of heavy centers can reach five for highest magnetic fields studied. In general, for a fixed magnetic field two-electron chains are more bound than one-electron ones.
We have carried out a survey of compact star clusters (apparent size <3 arcsec) in the southwest part of the M31 galaxy, based on the high-resolution Suprime-Cam images (17.5 arcmin x 28.5 arcmin), covering ~15% of the deprojected galaxy disk area. The UBVRI photometry of 285 cluster candidates (V < 20.5 mag) was performed using frames of the Local Group Galaxies Survey. The final sample, containing 238 high probability star cluster candidates (typical half-light radius r_h ~ 1.5 pc), was selected by specifying a lower limit of r_h > 0.15 arcsec (>0.6 pc). We derived cluster parameters based on the photometric data and multiband images by employing simple stellar population models. The clusters have a wide range of ages from ~5 Myr (young objects associated with 24 um and/or Ha emission) to ~10 Gyr (globular cluster candidates), and possess mass in a range of 3.0 < log(m/M_sol) < 4.3 peaking at m ~ 4000 M_sol. Typical age of these intermediate-mass clusters is in the range of 30 Myr < t < 3 Gyr, with a prominent peak at ~70 Myr. These findings suggest a rich intermediate-mass star cluster population in M31, which appears to be scarce in the Milky Way galaxy.
We use a simulation performed within the Constrained Local UniversE Simulation (CLUES) project to study a realistic Local Group-like object. We employ this group as a numerical laboratory for studying the evolution of the population of its subhaloes from the point of view of the effects it may have on the origin of different types of dwarf galaxies. We focus on the processes of tidal stripping of the satellites, their interaction, merging and grouping before infall. The tidal stripping manifests itself in the transition between the phase of mass accretion and mass loss seen in most subhaloes, which occurs at the moment of infall on to the host halo, and the change of the shape of their mass function with redshift. Although the satellites often form groups, they are loosely bound within them and do not interact with each other. The infall of a large group could however explain the observed peculiar distribution of the Local Group satellites, but only if it occurred recently. Mergers between prospective subhaloes are significant only during an early stage of evolution, i.e. more that 7 Gyr ago, when they are still outside the host haloes. Such events could thus contribute to the formation of more distant early type Milky Way companions. Once the subhaloes enter the host halo the mergers become very rare.
In this paper we present the results of a detailed spectroscopic and photometric analysis of the V=13$^m$.4 low-mass eclipsing binary NSVS 06507557 with an orbital period of 0.515 d. We obtained a series of mid-resolution spectra covering nearly entire orbit of the system. In addition we obtained simultaneous VRI broadband photometry using a small aperture telescope. From these spectroscopic and photometric data we have derived the system's orbital parameters and determined the fundamental stellar parameters of the two components. Our results indicate that NSVS 06507557 consists of a K9 and an M3 pre-main-sequence stars with masses of 0.66$\pm$0.09 \Msun and 0.28$\pm$0.05 \Msun and radii of 0.60$\pm$0.03 and 0.44$\pm$0.02 \Rsun, located at a distance of 111$\pm$9 pc. The radius of the less massive secondary component is larger than that of the zero-age main-sequnce star having the same mass. While the radius of the primary component is in agreement with ZAMS the secondary component appers to be larger by about 35 % with respect to its ZAMS counterpart. Night-to-night intrinsic light variations up to 0$^m$.2 have been observed. In addition, the H$_{\alpha}$, H$_{\beta}$ lines and the forbidden line of [O{\sc i}] are seen in emission. The Li{\sc i} 6708 \AA absorption line is seen in most of the spectra. These features are taken to be the signs of the classic T Tauri stars' characteristics. The parameters we derived are consistent with an age of about 20 Myr according to the stellar evolutionary models. The spectroscopic and photometric results are in agreement with those obtained by theoretical predictions.
We describe the steps involved in performing searches for sources of transient radio emission such as Rotating Radio Transients (RRATs), and present 10 new transient radio sources discovered in a re-analysis of the Parkes Multi-beam Pulsar Survey. Followup observations of each new source as well as one previously known source are also presented. The new sources suggest that the population of transient radio-emitting neutron stars, and hence the neutron star population in general, may be even larger than initially predicted. We highlight the importance of radio frequency interference excision for single-pulse searches. Also, we discuss some interesting properties of individual sources and consider the difficulties involved in precisely defining a RRAT and determining where they fit in with the other known classes of neutron stars.
We show that an upper limit of ~ 10^{12} K on the peak brightness temperature for an incoherent synchrotron radio source, commonly referred to in the literature as an inverse Compton limit, may not really be due to inverse Compton effects. We show that a somewhat tighter limit T_{eq} ~ 10^{11} is actually obtained for the condition of equipartition of energy between radiating particles and magnetic fields which happens to be a configuration of minimum energy for a self-absorbed synchrotron radio source. An order of magnitude change in brightness temperature from T_{eq} in either direction would require departures from equipartition of about eight orders of magnitude, implying a change in total energy of the system up to ~ 10^{4} times the equipartition value. Constraints of such extreme energy variations imply that brightness temperatures may not depart much from T_{eq}. This is supported by the fact that at the spectral turnover, brightness temperatures much lower than ~ 10^{11} K are also not seen in VLBI observations. Higher brightness temperatures in particular, would require in the source not only many orders of magnitude higher additional energy for the relativistic particles but also many order of magnitude weaker magnetic fields. Diamagnetic effects do not allow such extreme conditions, keeping the brightness temperatures close to the equipartition value, which is well below the limit where inverse Compton effects become important.
The analysis of the high-resolution UVES spectra of the CP stars HR 6000 and 46 Aql revealed the presence of an impressive number of unidentified lines, in particular in the 5000 - 5400 A region. Because numerous 4d-4f transitions of FeII lie in this spectral range, and because both stars are iron overabundant, we investigated whether the unidentified lines can be due to FeII. ATLAS12 model atmospheres with parameters [13450K, 4.3] and [12560K, 3.8] were computed for the individual abundances of HR 6000 and 46 Aql, respectively, in order to use the stars as spectroscopic sources to identify FeII lines and to determine FeII gf-values. After having identified several unknown lines in the stellar spectra as due to (3H)4d - (3H)4f transitions of FeII, we derived stellar log gf's for them by comparing observed and computed profiles. The energies of the upper levels were assigned on the basis of both laboratory iron spectra and predicted energy levels. We fixed 21 new levels of FeII with energies between 122910.9 cm^-1 and 123441.1 cm^-1. They allowed us to add 1700 new lines to the Fe II line list in the range 810 - 15011 A.
We present a new maximum likelihood method for the calculation of galaxy luminosity functions from multi-band photometric surveys without spectroscopic data. The method evaluates the likelihood of a trial luminosity function by directly comparing the predicted distribution of fluxes in a multi-dimensional photometric space to the observations, and thus does not require the intermediate step of calculating photometric redshifts. We apply this algorithm to ~27,000 galaxies with m_R<=25 in the MUSYC-ECDFS field, with a focus on recovering the luminosity function of field galaxies at z<1.2. Our deepest LFs reach M_r=-14 and show that the field galaxy LF deviates from a Schechter function, exhibiting a steep upturn at intermediate magnitudes that is due to galaxies of late spectral types.
We give an overview of the science case for spectroscopy of resolved stellar populations beyond the Local Group with the European Extremely Large Telescope (E-ELT). In particular, we present science simulations undertaken as part of the EAGLE Phase A design study for a multi--integral-field-unit, near-infrared spectrograph. EAGLE will exploit the unprecedented primary aperture of the E-ELT to deliver AO-corrected spectroscopy across a large (38.5 sq. arcmin) field, truly revolutionising our view of stellar populations in the Local Volume.
We took advantage of the wealth of information provided by the first ~10000 galaxies of the zCOSMOS-bright survey and its group catalogue to study the complex interplay between group environment and galaxy properties. The classical indicator F_blue (fraction of blue galaxies) proved to be a simple but powerful diagnostic tool. We studied its variation for different luminosity and mass selected galaxy samples. Using rest-frame B-band selected samples, the groups galaxy population exhibits significant blueing as redshift increases, but maintains a lower F_blue with respect both to the global and the isolated galaxy population. However moving to mass selected samples it becomes apparent that such differences are largely due to the biased view imposed by the B-band luminosity selection, being driven by the population of lower mass, bright blue galaxies for which we miss the redder, equally low mass, counterparts. By focusing the analysis on narrow mass bins such that mass segregation becomes negligible we find that only for the lowest mass bin explored (logMass <= 10.6) does a significant residual difference in color remain as a function of environment, while this difference becomes negligible toward higher masses. Our results indicate that red galaxies of logMass >= 10.8 are already in place at z ~ 1 and do not exhibit any strong environmental dependence, possibly originating from so-called 'nature'/internal mechanisms. In contrast, for lower galaxy masses and redshifts lower than z ~ 1, we observe the emergence in groups of a population of 'nurture' red galaxies: slightly deviating from the trend of the downsizing scenario followed by the global galaxy population, and more so with cosmic time. These galaxies exhibit signatures of group-related secular physical mechanisms directly influencing galaxy evolution.
The nonlinear evolution of relativistic magnetic reconnection in sheared magnetic configuration (with a guide field) is investigated by using two-dimensional relativistic two-fluid simulations. Relativistic guide field reconnection features the charge separation and the guide field compression in and around the outflow channel. As the guide field increases, the composition of the outgoing energy changes from enthalpy-dominated to Poynting-dominated. The inertial effects of the two-fluid model play an important role to sustain magnetic reconnection. Implications for the single fluid magnetohydrodynamic approach and the physics models of relativistic reconnection are briefly addressed.
The tidal breakup of binary star systems by the supermassive black hole (SMBH) in the center of the galaxy has been suggested as the source of both the observed sample of hypervelocity stars (HVSs) in the halo of the Galaxy and the S-stars that remain in tight orbits around Sgr A*. Here, we use a post-Newtonian N-body code to study the dynamics of main-sequence binaries on highly elliptical bound orbits whose periapses lie close to the SMBH, determining the properties of ejected and bound stars as well as collision products. Unlike previous studies, we follow binaries that remain bound for several revolutions around the SMBH, finding that in the case of relatively large periapses and highly inclined binaries the Kozai resonance can lead to large periodic oscillations in the internal binary eccentricity and inclination. Collisions and mergers of the binary elements are found to increase significantly for multiple orbits around the SMBH, while HVSs are primarily produced during a binary's first passage. This process can lead to stellar coalescence and eventually serve as an important source of young stars at the galactic center.
We discuss some hitherto puzzling features of the small-scale structure of cosmic strings. We argue that kinks play a key role, and that an important quantity to study is their sharpness distribution. In particular we suggest that for very small scales the two-point correlation function of the string tangent vector varies linearly with the separation and not as a fractional power, as proposed by Polchinski and Rocha [Phys. Rev. D 74, 083504 (2006)]. However, our results are consistent with theirs, because the range of scales to which this linearity applies shrinks as evolution proceeds.
We study the stability properties of isolated star forming dwarf galaxies
which undergo dynamically driven starbursts induced by stellar feedback. Here
we focus on the impact of the adopted ISM model, i.e. either a diffuse or a
clumpy ISM. We apply a one-zone model extended for active dynamical evolution.
We found two major types of repetitive star bursts: one set (type A) of
quasi-periodic starbursts is related to the dynamical timescale of the galaxy.
In that case, the star formation follows the variations of the gas density
induced by decaying virial oscillations. The second set (type B) of starbursts
is characterized by long quiescence periods given by the sum of the dynamical
and the dissipative timescale: after a first burst, the inserted energy leads
to a substantial expansion of the system, by this stopping any significant star
formation activity. A next burst might occur, when the gas reaches high
densities again, i.e. after the gas recollapsed and the energy injected by
stellar feedback is dissipated.
In case of a diffuse ISM model, type A bursts are the most common type due to
the high efficiency of radiative cooling (no type B bursts are found). Bursts
occur then mainly during an initial transitory phase. In case of a clumpy ISM
model (i.e. dissipation by inelastic cloud-cloud collisions), the dissipative
timescale is of the order of the dynamical time or longer. This allows for
both, type A and type B bursts.
The identities of the main processes triggering and quenching star-formation in galaxies remain unclear. A key stage in evolution, however, appears to be represented by post-starburst galaxies. To investigate their impact on galaxy evolution, we initiated a multiwavelength study of galaxies with k+a spectral features in the COSMOS field. We examine a mass-selected sample of k+a galaxies at z=0.48-1.2 using the spectroscopic zCOSMOS sample. K+a galaxies occupy the brightest tail of the luminosity distribution. They are as massive as quiescent galaxies and populate the green valley in the colour versus luminosity (or stellar mass) distribution. A small percentage (<8%) of these galaxies have radio and/or X-ray counterparts (implying an upper limit to the SFR of ~8Msun/yr). Over the entire redshift range explored, the class of k+a galaxies is morphologically a heterogeneous population with a similar incidence of bulge-dominated and disky galaxies. This distribution does not vary with the strength of the Hdelta absorption line but instead with stellar mass in a way reminiscent of the well-known mass-morphology relation. Although k+a galaxies are also found in underdense regions, they appear to reside typically in a similarly rich environment as quiescent galaxies on a physical scale of ~2-8Mpc, and in groups they show a morphological early-to-late type ratio similar to the quiescent galaxy class. With the current data set, we do not find evidence of statistical significant evolution in either the number/mass density of k+a galaxies at intermediate redshift with respect to the local values, or the spectral properties. Those galaxies, which are affected by a sudden quenching of their star-formation activity, may increase the stellar mass of the red-sequence by up to a non-negligible level of ~10%.
The formation and evolution of low-mass stars within dense halos of dark matter (DM) leads to evolution scenarios quite different from the classical stellar evolution. As a result of our detailed numerical work, we describe these new scenarios for a range of DM densities on the host halo, a range of scattering cross sections of the DM particles considered, and for stellar masses from 0.7 to 3 M$_{\odot}$. For the first time, we also computed the evolution of young low-mass stars in their Hayashi track in the pre-main sequence phase and found that, for high DM densities, these stars stop their gravitational collapse before reaching the main sequence, in agreement with similar studies on first stars. Such stars remain indefinitely in an equilibrium state with lower effective temperatures ($|\Delta T_{eff} |>10^{3} $K for a star of one solar mass), the annihilation of captured DM particles in their core being the only source of energy. In the case of lower DM densities, these proto-stars continue their collapse and progress through the main sequence burning hydrogen at a lower rate. A star of 1$ $M$_{\odot}$ will spend a time greater than the current age of the universe consuming all the hydrogen in its core if it evolves in a halo with DM density $\rho_{\chi}=10^{9} $GeV$ $cm$^{-3}$. We also show the strong dependence of the effective temperature and luminosity of these stars on the characteristics of the DM particles and how this can be used as an alternative method for DM research.
It is believed that eta Carinae is actually a massive binary system, with the wind-wind interaction responsible for the strong X-ray emission. Although the overall shape of the X-ray light curve can be explained by the high eccentricity of the binary orbit, other features like the asymmetry near periastron passage and the short quasi-periodic oscillations seen at those epochs, have not yet been accounted for. In this paper we explain these features assuming that the rotation axis of eta Carinae is not perpendicular to the orbital plane of the binary system. As a consequence, the companion star will face eta Carinae on the orbital plane at different latitudes for different orbital phases and, since both the mass loss rate and the wind velocity are latitude dependent, they would produce the observed asymmetries in the X-ray flux. We were able to reproduce the main features of the X-ray light curve assuming that the rotation axis of eta Carinae forms an angle of 29 degrees with the axis of the binary orbit. We also explained the short quasi-periodic oscillations by assuming nutation of the rotation axis, with amplitude of about 5 degrees and period of about 22 days. The nutation parameters, as well as the precession of the apsis, with a period of about 274 years, are consistent with what is expected from the torques induced by the companion star.
We investigate possible origins of the extragalactic radio background reported by the ARCADE 2 collaboration. The surface brightness of the background is several times higher than that which would result from currently observed radio sources. We consider contributions to the background from diffuse synchrotron emission from clusters and the intergalactic medium, previously unrecognized flux from low surface brightness regions of radio sources, and faint point sources below the flux limit of existing surveys. By examining radio source counts available in the literature, we conclude that most of the radio background is produced by radio point sources that dominate at sub microJy fluxes. We show that a truly diffuse background produced by electrons far from galaxes is ruled out because such energetic electrons would overproduce the obserevd X-ray/gamma-ray background through inverse Compton scattering of the other photon fields. Unrecognized flux from low surface brightness regions of extended radio sources, or moderate flux sources missed entirely by radio source count surveys, cannot explain the bulk of the observed background, but may contribute as much as 10%. We consider both radio supernovae and radio quiet quasars as candidate sources for the background, and show that both fail to produce it at the observed level because of insufficient number of objects and total flux, although radio quiet quasars contribute at the level of at least a few percent. We conclude that the most important population for production of the background is likely ordinary starforming galaxies above redshift 1 characterized by an evolving radio far-infrared correlation, which increases toward the radio loud with redshift.
We study the properties of the turbulence driven by the magnetorotational instability (MRI) in a stratified shearing box with outflow boundary conditions and an equation of state determined by self-consistent dissipation and radiation losses. A series of simulations with increasing resolution are performed within a fixed computational box. We achieve numerical convergence with respect to radial and azimuthal resolution. As vertical resolution is improved, the ratio of stress to pressure increases only slowly, but the absolute levels of both the stress and the pressure increase noticeably. The vertical correlation length of the magnetic field within the core of the disk is highly time-dependent, but averaged over time, it is $\simeq 3$ times larger than found in previous unstratifed simulations. This correlation length decreases slowly as vertical resolution increases. We suggest that this greater degree of correlation in stratified than unstratified disks, even near the midplane, is due to field buoyancy. We further show that the undulatory Parker instability drives the magnetic field upwelling at several scaleheights from the midplane that is characteristic of stratified MHD-turbulent disks.
We are undertaking an astrometric search for gas giant planets and brown dwarfs orbiting nearby low mass dwarf stars with the 2.5-m du Pont telescope at the Las Campanas Observatory in Chile. We have built two specialized astrometric cameras, the Carnegie Astrometric Planet Search Cameras (CAPSCam-S and CAPSCam-N), using two Teledyne Hawaii-2RG HyViSI arrays, with the cameras' design having been optimized for high accuracy astrometry of M dwarf stars. We describe two independent CAPSCam data reduction approaches and present a detailed analysis of the observations to date of one of our target stars, NLTT 48256. Observations of NLTT 48256 taken since July 2007 with CAPSCam-S imply that astrometric accuracies of around 0.3 milliarcsec per hour are achievable, sufficient to detect a Jupiter-mass companion orbiting 1 AU from a late M dwarf 10 pc away with a signal-to-noise ratio of about 4. We plan to follow about 100 nearby (primarily within about 10 pc) low mass stars, principally late M, L, and T dwarfs, for 10 years or more, in order to detect very low mass companions with orbital periods long enough to permit the existence of habitable, Earth-like planets on shorter-period orbits. These stars are generally too faint and red to be included in ground-based Doppler planet surveys, which are often optimized for FGK dwarfs. The smaller masses of late M dwarfs also yield correspondingly larger astrometric signals for a given mass planet. Our search will help to determine whether gas giant planets form primarily by core accretion or by disk instability around late M dwarf stars.
We aim to obtain accurate rate coefficients for the collisional excitation of CH+ by He for high gas temperatures. The ab initio coupled-cluster [CCSD(T)] approximation was used to compute the interaction potential energy. Cross sections are then derived in the close coupling (CC) approach and rate coefficients inferred by averaging these cross sections over a Maxwell-Boltzmann distribution of kinetic energies. Cross sections are calculated up to 10'000 cm^-1 for J ranging from 0 to 10. Rate coefficients are obtained at high temperatures up to 2000 K.
Three-dimensional Doppler tomography has been used to study the H$\alpha$ emission sources in the RS Vulpeculae interacting binary. The 2D tomogram of this binary suggested that most of the emission arose from the cool mass losing star with additional evidence of gas flowing close to the predicted trajectory. However, the 3D tomogram revealed surprising evidence of a more pronounced gas stream flow at high $V_z$ velocities from -240 to -360 km s{$^{-1}$}. This behavior is most likely caused by magnetic activity on the cool star since the central velocity plane, defined by $V_z$ = 0 km s{$^{-1}$}, should be coincident with the orbital plane of the binary if the flow is dominated by gravitational forces only. RS Vul has been detected as both an X-ray and a radio source, and it is possible that the RS Vul gas stream may have been deflected by magnetic field lines. This flow is distinctly different from that found in the streamlike state of U CrB, in which the gas flow was confined mostly to the central velocity plane.
Recent models of molecular cloud formation and evolution suggest that such clouds are dynamic and generally exhibit gravitational collapse. We present a simple analytic model of global collapse onto a filament and compare this with our numerical simulations of the flow-driven formation of an isolated molecular cloud to illustrate the supersonic motions and infall ram pressures expected in models of gravity-driven cloud evolution. We apply our results to observations of the Pipe Nebula, an especially suitable object for our purposes as its low star formation activity implies insignifcant perturbations from stellar feedback. We show that our collapsing cloud model can explain the magnitude of the velocity dispersions seen in the $^{13}$CO filamentary structure by Onishi et al. and the ram pressures required by Lada et al. to confine the lower-mass cores in the Pipe nebula. We further conjecture that higher-resolution simulations will show small velocity dispersions in the densest core gas, as observed, but which are infall motions and not supporting turbulence. Our results point out the inevitability of ram pressures as boundary conditions for molecular cloud filaments, and the possibility that especially lower-mass cores still can be accreting mass at significant rates, as suggested by observations.
The four lectures that I gave in the XIII Ciclo de Cursos Especiais at the National Observatory of Brazil in Rio in October 2008 were (1) a brief history of dark matter and structure formation in a LambdaCDM universe; (2) challenges to LambdaCDM on small scales: satellites, cusps, and disks; (3) data on galaxy evolution and clustering compared with simulations; and (4) semi-analytic models. These lectures, themselves summaries of much work by many people, are summarized here briefly.
We extract at redshift z=0 a Milky Way sized object including gas, stars and
dark matter (DM) from a recent, high-resolution cosmological N-body simulation
with baryons. Its resolution is sufficient to witness the formation of a
rotating disk and bulge at the center of the halo potential. The phase-space
structure of the central galactic halo reveals the presence of a dark disk
component, that is co-rotating with the stellar disk. At the Earth's location,
it contributes to around 25% of the total DM local density, whose value is
rho_DM ~ 0.37 GeV/cm^3. The velocity distributions also show strong deviations
from pure Gaussian and Maxwellian distributions, with a sharper drop of the
high velocity tail.
We give a detailed study of the impact of these features on the predictions
for DM signals in direct detection experiments. In particular, the question of
whether the modulation signal observed by DAMA is or is not excluded by limits
set by other experiments (CDMS, XENON and CRESST...) is re-analyzed and
compared to the case of a standard Maxwellian halo, in both the elastic and the
inelastic scattering scenarios. We find that the compatibility between DAMA and
the other experiments is improved. In the elastic scenario, the DAMA modulation
signal is slightly enhanced in the so-called channeling region, as a result of
several effects. For the inelastic scenario, the improvement of the fit is
mainly attributable to the departure from a Maxwellian distribution at high
velocity.
We study the numerical solution of the non-relativistic Schr\"{o}dinger equation for two-electron atoms in ground and excited S-states using pseudospectral (PS) methods of calculation. The calculation achieves convergence rates for the energy, Cauchy error in the wavefunction, and variance in local energy that are exponentially fast for all practical purposes. The method requires three separate subdomains to handle the wavefunction's cusp-like behavior near the two-particle coalescences. The use of three subdomains is essential to maintaining exponential convergence. A comparison of several different treatments of the cusps and the semi-infinite domain suggest that the simplest prescription is sufficient. For many purposes it proves unnecessary to handle the logarithmic behavior near the three-particle coalescence in a special way. The PS method has many virtues: no explicit assumptions need be made about the asymptotic behavior of the wavefunction near cusps or at large distances, the local energy is exactly equal to the calculated global energy at all collocation points, local errors go down everywhere with increasing resolution, the effective basis using Chebyshev polynomials is complete and simple, and the method is easily extensible to other bound states.
We study the effect of short term variations of the evolution of AM CVn systems on their gravitational wave emissions and in particular LISA observations. We model the systems according to their equilibrium mass-transfer evolution as driven by gravitational wave emission and tidal interaction, and determine their reaction to a sudden perturbation of the system. This is inspired by the suggestion to explain the orbital period evolution of the ultra-compact binary systems V407 Vul and RX-J0806+1527 by non-equilibrium mass transfer. The characteristics of the emitted gravitational wave signal are deduced from a Taylor expansion of a Newtonian quadrupolar emission model, and the changes in signal structure as visible to the LISA mission are determined. We show that short term variations can significantly change the higher order terms in the expansion, and thus lead to spurious (non) detection of frequency derivatives. This may hamper the estimation of the parameters of the system, in particular their masses and distances. However, we find that overall detection is still secured as signals still can be described by general templates. We conclude that a better modelling of the effects of short term variations is needed to prepare the community for astrophysical evaluations of real gravitational wave data of AM CVn systems.
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