Clouds containing molecular dark matter in quantities relevant for star formation may exist in minihaloes of the type of cold dark matter included in many cosmological simulations or in the regions of some galaxies extending far beyond their currently known boundaries. We have systematically explored parameter space to identify conditions under which plane-parallel clouds contain sufficient column densities of molecular dark matter that they could be reservoirs for future star formation. Such clouds would be undetected or at least appear by current observational criteria to be uninteresting from the perspective of star formation. We use a time-dependent PDR code to produce theoretical models of the chemistry and emission arising in clouds for our chosen region of parameter space. We then select a subset of model clouds with levels of emission that are low enough to be undetectable or at least overlooked by current surveys. The existence of significant column densities of cold molecular dark matter requires that the background radiation field be several or more orders of magnitude weaker than that in the solar neighbourhood. Lower turbulent velocities and cosmic ray induced ionization rates than typically associated with molecular material within a kpc of the Sun are also required for the molecular matter to be dark. We find that there is a large region within the parameter space that results in clouds that might contain a significant mass of molecular gas whilst remaining effectively undetectable or at least not particularly noticeable in surveys. We note briefly conditions under which molecular dark matter may contain a dynamically interesting mass.
We present observations of the stripped Virgo Cluster spiral NGC 4522, a clear, nearby example of a galaxy currently undergoing ISM-ICM stripping. Utilizing SparsePak integral field spectroscopy on the WIYN 3.5m telescope and GALEX UV photometry, we present an analysis of the outer disk (r > 3 kpc) stellar population of this galaxy, beyond the HI and Halpha truncation radius. We find that the star formation in the gas-stripped outer disk ceased very recently, ~100 Myr ago, in agreement with previous claims that this galaxy is currently being stripped. At the time of this stripping, data and models suggest that the galaxy experienced a modest starburst. The stripping is occurring in a region of the cluster well outside the cluster core, likely because this galaxy is experiencing extreme conditions from a dynamic ICM due to an ongoing sub-cluster merger. The outer disk has a spectrum of a K+A galaxy, traditionally observed in high-redshift cluster galaxies. In the case of NGC 4522, a K+A spectrum is formed by simple stripping of the interstellar gas by the hot intracluster medium. These data show K+A spectra can be created by cluster processes and that these processes likely extend beyond the cluster core.
Astrometric measurements for 25 red dwarf systems are presented, including
the first definitive trigonometric parallaxes for 20 systems within 10 pc of
the Sun, the horizon of the RECONS sample. The three nearest systems that had
no previous trigonometric parallaxes (other than perhaps rough preliminary
efforts) are SO 0253+1652 (3.84 +/- 0.04 pc, the 23rd nearest system), SCR
1845-6357 AB (3.85 +/- 0.02 pc, 24th), and LHS 1723 (5.32 +/- 0.04 pc, 56th).
In total, seven of the systems reported here rank among the nearest 100 stellar
systems. Supporting photometric and spectroscopic observations have been made
to provide full characterization of the systems, including complete VRIJHK
photometry and spectral types. A study of the variability of 27 targets reveals
six obvious variable stars, including GJ 1207, for which we observed a flare
event in the V band that caused it to brighten by 1.7 mag.
Improved parallaxes for GJ 54 AB and GJ 1061, both important members of the
10 pc sample, are also reported. Definitive parallaxes for GJ 1001 A, GJ 633,
and GJ 2130 ABC, all of which have been reported to be within 10 pc, indicate
that they are beyond 10 pc. From the analysis of systems with (previously) high
trigonometric parallax errors, we conclude that parallaxes with errors in
excess of 10 mas are insufficiently reliable for inclusion in the RECONS
sample. The cumulative total of new additions to the 10 pc sample since 2000 is
now 34 systems -- 28 by the RECONS team and six by other groups. This total
represents a net increase of 16% in the number of stellar systems reliably
known to be nearer than 10 pc.
I discuss neutrino production in supernovae (SNe) and the detection of both Galactic core collapse events and the diffuse extra-galactic MeV neutrino background expected from the integrated history of star formation. In particular, I consider what processes might affect our expectations for both. I focus on ``rapid'' rotation, defined as leading to millisecond initial neutron star spin periods. Rotation affects the neutrino luminosity, the average neutrino energy, the duration of the Kelvin-Helmholtz cooling epoch, and the ratios of luminosities and average energies between neutrino species; it can strongly suppresses the anti-electron as well as mu, anti-mu, tau, and anti-tau neutrino fluxes relative to those for the electron neutrinos. As a result, depending on the prevalence of rapid rotation in SN progenitors through cosmic time, this may affect predictions for the MeV neutrino background and the history of nucleosynthetic enrichment. I emphasize connections between the MeV neutrino background and tracers of the star formation rate density at high redshift in other neutrino and photon wavebands.
Flares in Sagittarius A* are produced by hot plasmas within a few Schwarzschild radii of the supermassive black hole at the Galactic center. The recent detection of a correlation between the spectral index and flux during a near infrared (NIR) flare provides a means to conduct detailed investigations of the plasma heating and radiation processes. We study the evolution of the electron distribution function under the influence of a turbulent magnetic field in a hot collisionless plasma. The magnetic field, presumably generated through instabilities in the accretion flow, can both heat the plasma via resonant wave-particle coupling and cool the electrons via radiation. The electron distribution can generally be approximated as relativistic Maxwellian. To account for the observed correlation, we find that the magnetic field needs to be anti-correlated with the electron ''temperature''. NIR and X-ray light curves are produced for a cooling and a heating phase. The model predicts simultaneous flare activity in the NIR and X-ray bands, which can be compared with observations. These results can be applied to MHD simulations to study the radiative characteristics of collisionless plasmas, especially accretion flows in low-luminosity AGNs.
Two-dimensional magnetohydrodynamic simulations are performed using the ZEUS-2D code to investigate the dynamics of a collapsar that generates a GRB jet, taking account of realistic equation of state, neutrino cooling and heating processes, magnetic fields, and gravitational force from the central black hole and self gravity. It is found that neutrino heating processes are not so efficient to launch a jet in this study. It is also found that a jet is launched mainly by B_\phi fields that are amplified by the winding-up effect. However, since the ratio of total energy relative to the rest mass energy in the jet is not so high as several hundred, we conclude that the jets seen in this study are not be a GRB jet. This result suggests that general relativistic effects, which are not included in this study, will be important to generate a GRB jet. Also, the accretion disk with magnetic fields may still play an important role to launch a GRB jet, although a simulation for much longer physical time (\sim 10-100 s) is required to confirm this effect. It is shown that considerable amount of 56Ni is synthesized in the accretion disk. Thus there will be a possibility for the accretion disk to supply sufficient amount of 56Ni required to explain the luminosity of a hypernova. Also, it is shown that neutron-rich matter due to electron captures with high entropy per baryon is ejected along the polar axis. Moreover, it is found that the electron fraction becomes larger than 0.5 around the polar axis near the black hole by \nu_e capture at the region. Thus there will be a possibility that r-process and r/p-process nucleosynthesis occur at these regions. Finally, much neutrons will be ejected from the jet, which suggests that signals from the neutron decays may be observed as the delayed bump of afterglow or gamma-rays.
We present a grid of radiation transfer models of axisymmetric young stellar objects (YSOs), covering a wide range of stellar masses (from 0.1Msun to 50Msun) and evolutionary stages (from the early envelope infall stage to the late disk-only stage). The grid consists of 20,000 YSO models, with spectral energy distributions (SEDs) and polarization spectra computed at ten viewing angles for each model, resulting in a total of 200,000 SEDs. [...]. These models are publicly available on a dedicated WWW server: this http URL . In this paper we summarize the main features of our models, as well as the range of parameters explored. [...]. We examine the dependence of the spectral indices of the model SEDs on envelope accretion rate and disk mass. In addition, we show variations of spectral indices with stellar temperature, disk inner radius, and disk flaring power for a subset of disk-only models. We also examine how changing the wavelength range of data used to calculate spectral indices affects their values. We show sample color-color plots of the entire grid as well as simulated clusters at various distances with typical {\it Spitzer Space Telescope} sensitivities. We find that young embedded sources generally occupy a large region of color-color space due to inclination and stellar temperature effects. Disk sources occupy a smaller region of color-color space, but overlap substantially with the region occupied by embedded sources, especially in the near- and mid-IR. We identify regions in color-color space where our models indicate that only sources at a given evolutionary stage should lie. [...].
We explore the infrared spectrum of a three-dimensional dynamical model of planet HD209458b as a function of orbital phase. The dynamical model predicts day-side atmospheric pressure-temperature profiles that are much more isothermal at pressures less than 1 bar than one-dimensional radiative-convective models have found. The resulting day-side thermal spectra are very similar to a blackbody, and only weak water absorption features are seen at short wavelengths. The dayside emission is consequently in significantly better agreement with ground-based and space-based secondary eclipse data than any previous models, which predict strong flux peaks and deep absorption features. At other orbital phases, absorption due to carbon monoxide and methane is also predicted. We compute the spectra under two treatments of atmospheric chemistry: one uses the predictions of equilibrium chemistry, and the other uses non-equilibrium chemistry, which ties the timescales of methane and carbon monoxide chemistry to dynamical timescales. As a function of orbital phase, we predict planet-to-star flux ratios for standard infrared bands and all Spitzer Space Telescope bands. In Spitzer bands, we predict 2-fold to 15-fold variation in planetary flux as a function of orbital phase with equilibrium chemistry, and 2-fold to 4-fold variation with non-equilibrium chemistry. Variation is generally more pronounced in bands from 3-10 $\mu$m than at longer wavelengths. The orbital phase of maximum thermal emission in infrared bands is 15--45 orbital degrees before the time of secondary eclipse. Changes in flux as a function of orbital phase for HD209458b should be observable with Spitzer, given the previously acheived observational error bars.
We present the result of investigations into two theories to explain the star formation rate-density relationship. For regions of high galaxy density, either there are fewer star forming galaxies, or galaxies capable of forming stars are present but some physical process is suppressing their star formation. We use HI Parkes All Sky Survey's (HIPASS) HI detected galaxies and infrared and radio fluxes to investigate star formation rates and efficiencies with respect to local surface density. For nearby (vel<10000 km\s) HI galaxies we find a strong correlation between HI mass and star formation rate. The number of HI galaxies decreases with increasing local surface density. For HI galaxies (1000<vel<6000 km\s) there is no significant change in the star formation rate or the efficiency of star formation with respect to local surface density. We conclude the SFR-density relation is due to a decrease in the number of HI star forming galaxies in regions of high galaxy density and not to the suppression of star formation.
Neutral fluorine (F I) lines are identified in the optical spectra of cool EHe stars. These are the first identification of F I lines in a star's spectrum, and provide the first measurement of fluorine abundances in EHe stars. The results show that fluorine is overabundant in EHe stars. The overabundance of fluorine provides evidence for the synthesis of fluorine in these stars, that is discussed in the light of asymptotic giant branch (AGB) evolution, and the expectation from accretion of an He white dwarf by a C-O white dwarf.
We investigate structural properties of old, metal-poor globular clusters (GCs) formed at high redshifts (z>6) and located inside and outside virialized galaxy-scale halos in clusters of galaxies with the total masses of M_CL based on high-resolution cosmological simulations with models of GC formation. We mainly derive the parameter dependences of physical properties of intracluster GCs (ICGCs) based on the results of 14 models. Our principle results are summarized as follows. (1) The projected radial density profiles (Sigma_GC) of ICGCs in clusters with different M_CL can be diverse, though ICGCs have inhomogeneous, asymmetric, and somewhat elongated distributions in most models. If Sigma_GC (R) ~ R^alpha, alpha ranges from -1.5 to -2.5 for GCs in clusters. (2) Although total number of GCs within the central 0.05 Mpc (N_GC,0.05) and 0.2 Mpc (N_GC,0.2) are diverse in different clusters, they can depend weakly on M_CL in such a way that both N_GC,0.05 and N_GC,0.2 are likely to be larger for clusters with larger M_CL. (3) Total number of GCs per cluster masses (specific frequency of GCs for clusters of galaxies) are more likely to be larger in more massive clusters, mainly because a larger number of earlier virialized objects can be located in more massive clusters. (4) Spatial distributions of old GCs in clusters can depend on the truncation epoch of GC formation (z_trun) such that they can be steeper and more compact in the models with higher z_trun. (5) The mean metallicity of ICGCs in a cluster can be smaller than that of GCs within the cluster member galaxy-scale halos by ~ 0.3 in [Fe/H]. Metallicity distribution functions (MDFs) of ICGCs show peak values around [Fe/H] ~ -1.6 and do not have remarkable bimodality.
A quadrillion previously unnoticed small bodies beyond Neptune have been spotted as they dimmed X-rays from a distant source. Models of the dynamics of debris in the Solar System's suburbs must now be reworked.
We present results of Very Long Baseline Array (VLBA) observations of PKS 0528+134 at five frequencies (2.3, 5.0, 8.4, 15.4, and 22.2 GHz). These quasi-simultaneous data enable us to study the spectral distribution of Very Long Baseline Interferometer (VLBI) components for the first time in this highly variable source, from which the central compact core is identified. Our observations indicate that there are two bendings for the jet motion at parsec scale. We provide an approximate spatial fit to the curved jet trajectory using the Steffen et al. (\cite{Steffen95}) helical model. We further investigate the proper motions of three jet components, which all show superluminal motion. At high frequencies (15.4 and 22.2 GHz) we detected a new component, which is estimated to be related to a radio burst peaking at about 2000.
Here, it is found that dark energy and dark matter emerge from the gravitational sector, if non-linear term of scalar curvature is added to Einstein-Hilbert lagrangian. An equation of state for dark energy, having the form $p_{\rm de} = - \rho_{\rm de} + f(a) (with $p_{\rm de}(\rho_{\rm de})$ being the pressure(density) for dark energy, $f(a)$ being a function of scale feactor $a(t)$ and $t$ being the cosmic time) is explored. Interestingly, this equation of state leads to a phantom barrier ${\rm w}_{\rm de} = p_{\rm de}/\rho_{\rm de} = - 1$ at $a = a_{\rm w}$. It is found that when $a < a_{\rm w}, {\rm w}_{\rm de} > - 1$ and ${\rm w}_{\rm de} < - 1$ for $a > a_{\rm w},$ showing a transition from non-phantom to phantom phase at $a = a_{\rm w} < a_0 (a_0$ being current scale factor of the universe). PACS no.: 98.80 Cq.
We present results for the spectral distortions of the Cosmic Microwave Background (CMB) arising due to bound-bound transitions during the epoch of cosmological hydrogen recombination at frequencies down to nu~100MHz. We extend our previous treatment of the recombination problem now including the main collisional processes and following the evolution of all the hydrogen sub-states for up to 100 shells. We show that, due to the low baryon density of the Universe, even within the highest considered shell full statistical equilibrium (SE) is not reached and that at low frequencies the recombination spectrum is significantly different when assuming full SE for n>2.We also directly compare our results for the ionization history to the output of the Recfast code, showing that especially at low redshifts rather big differences arise. In the vicinity of the Thomson visibility function the electron fraction differs by roughly -0.6%. Furthermore we shortly discuss the influence of free-free absorption, line broadening due to electron scattering and the generation of CMB angular fluctuations due to scattering of photons within the high shells.
We transformed radial velocities compiled from more than 1400 published sources, including the Geneva--Copenhagen survey of the solar neighborhood (CORAVEL-CfA), into a uniform system based on the radial velocities of 854 standard stars in our list. This enabled us to calculate the average weighted radial velocities for more than 25~000 HIPPARCOS stars located in the local Galactic spiral arm (Orion arm) with a median error of +-1 km/s. We use these radial velocities together with the stars' coordinates, parallaxes, and proper motions to determine their Galactic coordinates and space velocities. These quantities, along with other parameters of the stars, are available from the continuously updated Orion Spiral Arm CAtalogue (OSACA) and the associated database. We perform a kinematic analysis of the stars by applying an Ogorodnikov-Milne model to the OSACA data. The kinematics of the nearest single and multiple main-sequence stars differ substantially. We used distant (r\approx 0.2 kpc) stars of mixed spectral composition to estimate the angular velocity of the Galactic rotation -25.7+-1.2 km/s/kpc, and the vertex deviation,l=13+-2 degrees, and detect a negative K effect. This negative K effect is most conspicuous in the motion of A0-A5 giants, and is equal to K=-13.1+-2.0 km/s/kpc.
We propose a methodology to perform a self-consistent analysis of the
physical properties of the emitting gas of HII galaxies adequate to the data
that can be obtained with the XXI century technology. This methodology requires
the production and calibration of empirical relations between the different
line temperatures that should superseed currently used ones based on very
simple, and poorly tested, photo-ionization model sequences.
As a first step to reach these goals we have obtained simultaneous blue to
far red longslit spectra with the William Herschel Telescope (WHT) of three
compact HII galaxies selected from the Sloan Digital Sky Survey (SDSS) Data
Release 2 (DR2) spectral catalog using the INAOE Virtual Observatory
superserver. Our spectra cover the range from 3200 to 10500 \AA, including the
Balmer jump, the [OII]3727,29 \AA lines, the [SIII]9069,9532 \AA doublet as
well as various weak auroral lines such as [OIII]4363 \AA and [SIII]6312 \AA.
For the three objects we have measured at least four line temperatures:
T([OIII]), T([SIII]), T([OII]) and T([SII]) and the Balmer continuum
temperature T(Bac). These measurements and a careful and realistic treatment of
the observational errors yield total oxygen abundances with accuracies between
5 and 9%. These accuracies are expected to improve as better calibrations based
on more precise measurements, both on electron temperatures and densities, are
produced.
...
We present a new, three-dimensional (3D) plotting library with advanced features, and support for standard and enhanced display devices. The library - S2PLOT - is written in C and can be used by C, C++ and FORTRAN programs on GNU/Linux and Apple/OSX systems. S2PLOT draws objects in a 3D (x,y,z) Cartesian space and the user interactively controls how this space is rendered at run time. With a PGPLOT inspired interface, S2PLOT provides astronomers with elegant techniques for displaying and exploring 3D data sets directly from their program code, and the potential to use stereoscopic and dome display devices. The S2PLOT architecture supports dynamic geometry and can be used to plot time-evolving data sets, such as might be produced by simulation codes. In this paper, we introduce S2PLOT to the astronomical community, describe its potential applications, and present some example uses of the library.
The Dark Energy Survey (DES; operations 2009-2015) will address the nature of dark energy using four independent and complementary techniques: (1) a galaxy cluster survey over 4000 deg2 in collaboration with the South Pole Telescope Sunyaev-Zel'dovich effect mapping experiment, (2) a cosmic shear measurement over 5000 deg2, (3) a galaxy angular clustering measurement within redshift shells to redshift=1.35, and (4) distance measurements to 1900 supernovae Ia. The DES will produce 200 TB of raw data in four bands, These data will be processed into science ready images and catalogs and co-added into deeper, higher quality images and catalogs. In total, the DES dataset will exceed 1 PB, including a 100 TB catalog database that will serve as a key science analysis tool for the astronomy/cosmology community. The data rate, volume, and duration of the survey require a new type of data management (DM) system that (1) offers a high degree of automation and robustness and (2) leverages the existing high performance computing infrastructure to meet the project's DM targets. The DES DM system consists of (1) a grid-enabled, flexible and scalable middleware developed at NCSA for the broader scientific community, (2) astronomy modules that build upon community software, and (3) a DES archive to support automated processing and to serve DES catalogs and images to the collaboration and the public. In the recent DES Data Challenge 1 we deployed and tested the first version of the DES DM system, successfully reducing 700 GB of raw simulated images into 5 TB of reduced data products and cataloguing 50 million objects with calibrated astrometry and photometry.
We have measured the gamma-ray fluxes of the blazars Mrk 421 and Mrk 501 in the energy range between 50 and 350 GeV (1.2 to 8.3 x 10^25 Hz). The detector, called CELESTE, used first 40, then 53 heliostats of the former solar facility "Themis" in the French Pyrenees to collect Cherenkov light generated in atmospheric particle cascades. The signal from Mrk 421 is often strong. We compare its flux with previously published multi-wavelength studies and infer that we are straddling the high energy peak of the spectral energy distribution. The signal from Mrk 501 in 2000 was weak (3.4 sigma). We obtain an upper limit on the flux from 1ES 1426+428 of less than half that of the Crab flux near 100 GeV. The data analysis and understanding of systematic biases have improved compared to previous work, increasing the detector's sensitivity.
The present paper describes the first results of an observational program intended to refine and extend the existing vsini measurements of metal-poor stars, with anemphasis on field evolved stars. The survey was carried out with the FEROS and CORALIE spectrometers. For the vsini measurements, obtained from spectral synthesis, we estimate an uncertainty of about 2.0 km/s. Precise rotational velocities vsini are presented for a large sample of 100 metal-poor stars, most of them evolving off the main-sequence. For the large majority of the stars composing the present sample, rotational velocities have been measured for the first time.
We present Spitzer imaging of the metal-deficient (Z ~30% Z_sun) Local Group dwarf galaxy NGC 6822. On spatial scales of ~130 pc, we study the nature of IR, H alpha, HI, and radio continuum emission. Nebular emission strength correlates with IR surface brightness; however, roughly half of the IR emission is associated with diffuse regions not luminous at H alpha (as found in previous studies). The global ratio of dust to HI gas in the ISM, while uncertain at the factor of ~2 level, is ~25 times lower than the global values derived for spiral galaxies using similar modeling techniques; localized ratios of dust to HI gas are about a factor of five higher than the global value in NGC 6822. There are strong variations (factors of ~10) in the relative ratios of H alpha and IR flux throughout the central disk; the low dust content of NGC 6822 is likely responsible for the different H alpha/IR ratios compared to those found in more metal-rich environments. The H alpha and IR emission is associated with high-column density (> ~1E21 cm^-2) neutral gas. Increases in IR surface brightness appear to be affected by both increased radiation field strength and increased local gas density. Individual regions and the galaxy as a whole fall within the observed scatter of recent high-resolution studies of the radio-far IR correlation in nearby spiral galaxies; this is likely the result of depleted radio and far-IR emission strengths in the ISM of this dwarf galaxy.
We study the spectrum of the cosmic X-ray background (CXB) in energy range $\sim$5-100 keV. Early in 2006 the INTEGRAL observatory performed a series of four 30ksec observations with the Earth disk crossing the field of view of the instruments. The modulation of the aperture flux due to occultation of extragalactic objects by the Earth disk was used to obtain the spectrum of the Cosmic X-ray Background(CXB). Various sources of contamination were evaluated, including compact sources, Galactic Ridge emission, CXB reflection by the Earth atmosphere, cosmic ray induced emission by the Earth atmosphere and the Earth auroral emission. The spectrum of the cosmic X-ray background in the energy band 5-100 keV is obtained. The shape of the spectrum is consistent with that obtained previously by the HEAO-1 observatory, while the normalization is $\sim$10% higher. The CXB spectrum obtained by INTEGRAL agrees well with the measurements of RXTE/PCA, the latest recalculation of HEAO-1 measurements and ASCA and CHANDRA observations. The increase relative to the earlier adopted value of the absolute flux of the CXB near the energy of maximum luminosity (20-50 keV) has direct implications for the energy release of supermassive black holes in the Universe and their growth at the epoch of the CBX origin.
For several years the only experiments sensitive to astrophysical gamma rays with energies beyond the reach of EGRET but below that of the Cherenkov imaging telescopes have been the "solar tower" detectors. They use >2000 m2 mirror areas to sample the Cherenkov wavefront generated by <100 GeV gamma rays, obtaining Crab sensitivities of more than 6$\sigma$ in one ON-source hour. I will review the history of the solar tower Cherenkov experiments from 1992 to the present and their key design features. I will describe some successful analysis strategies, then summarize the principal results obtained.
We present calculations of the Cosmic X-ray background (CXB) reflection by the Earth atmosphere in the 1-1000 keV range and compare it with the reflection by the Moon and the Sun. The calculations use realistic chemical composition of the Earth atmosphere and include all relevant physical processes. An analytic approximation to the Earth X-ray albedo is provided, which should be accurate down to few percent level in the range from few keV up to few hundred keV.
We perform Monte Carlo simulations of cosmic-ray induced hard X-ray emission in the Earth's atmosphere. We find that the shape of the spectrum emergent from the atmosphere in the energy range ~25-300 keV is determined by Compton scattering and photoabsorption and is virtually independent of the incident cosmic ray spectrum. We provide a fitting formula for the spectral intensity as a function of energy, solar modulation level, geomagnetic cutoff rigidity and zenith angle. A recent measurement of the atmospheric hard X-ray emission together with the cosmic X-ray background by the INTEGRAL observatory agrees with our prediction within 10%. This suggests a possibility of using Earth observations for in-orbit calibration of future hard X-ray telescopes. We also demonstrate that the hard X-ray spectra generated by cosmic rays in the crusts of the Moon, Mars and Mercury are expected to significantly differ from that emitted by the Earth's atmosphere.
We present near ultraviolet (NUV:1750 - 2800\AA) and far ultraviolet (FUV: 1350 - 1750\AA) light-curves for flares on 4 nearby dMe-type stars (GJ 3685A, CR Dra, AF Psc and SDSS J084425.9+513830.5) observed with the GALEX satellite. Taking advantage of the time-tagged events recorded with the GALEX photon counting detectors, we present high temporal resolution (<0.01 sec) analysis of these UV flare data. A statistical analysis of 700 seconds of pre-flare quiescence data for both CR Dra and SDSS J084425.9+513830.5 failed to reveal the presence of significant micro-flare activity in time bins of 0.2, 1 and 10 second intervals. Using an appropriate differential emission measure for both the quiescent and flaring state, it is possible to reproduce the observed FUV:NUV flux ratios. A major determinant in reproducing this flux ratio is found to be the value of plasma electron density during the flare. We also searched the count rate data recorded during each of the four flare events for periodicity associated with magneto-hydrodynamic oscillations in the active region coronal loops. Significant oscillations were detected during the flare events observed on all 4 stars, with periodicities found in the 30 to 40 second range. Flare oscillations with this periodicity can be explained as acoustic waves in a coronal loop of length of $\approx 10^{9}$ cm for an assumed plasma temperature of $5-20 \times 10^{6}$K. This suggests a loop length for these M-dwarf flares of less than $1/10^{th}$ of the stellar radii. We believe that this is the first detection of non-solar coronal loop flare oscillations observed at ultraviolet wavelengths.
We present a search for low-mass brown dwarfs in the Pleiades open cluster. The identification of Pleiades members fainter and cooler than those currently known allows us to constrain evolutionary models for L dwarfs and to extend the study of the cluster mass function to lower masses. We conducted a 1.8 deg^2 near-infrared J-band survey at the 3.5m Calar Alto Telescope, with completeness J~19.0. The detected sources were correlated with those of previously available optical I-band images (completeness I~22). Using a J versus I-J colour-magnitude diagram, we identified 18 faint red L-type candidates, with magnitudes 17.4<J<19.7 and colours I-J>3.2. If Pleiades members, their masses would span ~0.040-0.020 M_Sol. We performed follow-up HKs-band imaging to further confirm their cluster membership by photometry and proper motion. Out of 11 IJ candidates with proper motion measurements, we find six cluster members, two non-members and three whose membership is uncertain and depends on the intrinsic velocity dispersion of Pleiades brown dwarfs. This dispersion (>4 mas yr^-1) is at least four times that of cluster stars with masses >1 M_Sol. Five of the seven other IJ candidates are discarded because their J-Ks colours are bluer than those of confirmed members. The J versus I-J sequence of the L-type candidates at J>18 is not as red as theoretical models predict; it rather follows the field L-dwarf sequence translated to the cluster distance. This sequence overlapping, also observed in the J versus J-H and J-K diagrams, suggests that Pleiades and field L dwarfs may have similar spectral energy distributions and luminosities, and thus possibly similar radii. Also, we find alpha=0.5+-0.2 for a power-law approximation dN/dM propor. M^-alpha of the survey mass spectrum in the mass range 0.5-0.026 M_Sol.
Hyperfine structure lines of highly-charged ions may open a new window in
observations of hot rarefied astrophysical plasmas. In this paper we discuss
spectral lines of isotopes and ions abundant at temperatures 10^5-10^7 K,
characteristic for warm-hot intergalactic medium, hot interstellar medium,
starburst galaxies and young and middle-aged supernova remnants. Observations
of these lines might complement soft X-ray observations with
micro-calorimeters, but, in contrast to soft X-rays, they are not obscured by
the Galactic interstellar medium and allow to study observed target bulk and
turbulent motions with much higher spectroscopic and angular resolution.
We estimate feasibility of hyperfine structure emission and absorption line
observations from these astrophysical sources and gamma-ray burst afterglows
using simple theoretical estimates of spectral line absorption cross-section
and emissivity accounting for major processes populating upper hyperfine
sublevel. Using modern and near-future instruments it appears possible both to
observe absorption line of 14-N VII (rest wavelength 5.652 mm) from the
warm-hot intergalactic medium and galactic halos at redshifts above z=0.15 and
emission lines of several less abundant isotopes from young and middle-aged
supernova remnants.
Over the past decade our physical understanding of gamma-ray bursts (GRBs) has progressed rapidly thanks to the discovery and observation of their long-lived afterglow emission. Long-duration (T < 2 s) GRBs are associated with the explosive deaths of massive stars (``collapsars''), which produce accompanying supernovae, while the short-duration (T > 2 s) GRBs arise from a different origin, which has been argued to be the merger of two compact objects, either neutron stars or black holes. Here we present observations of GRB060614, a 100-s long burst discovered by the Swift satellite, which require the invocation of a new explosive process: either a massive ``collapsar'' that powers a GRB without any associated supernova, or a new type of engine, as long-lived as the collapsar but without any such massive stellar host. We also discuss the properties of this burst's redshift z=0.125 host galaxy, which distinguish it from other long-duration GRBs and suggest that an entirely new type of GRB progenitor may be required.
The recent development of numerical schemes for Relativistic MHD (RMHD) allows us to model the acceleration and outflow properties of winds from compact sources. Theoretical models suggest that acceleration and collimation of the flow are extremely inefficient when the speed is close to $c$, in contrast with many observations. Numerical results for an axisymmetric rotator, both in the case of monopolar and dipolar magnetic field will be presented, suggesting that in ideal RMHD acceleration is indeed inefficient. I will also mention numerical challenges and stability problems of present simulations. Finally I will discuss the interaction of a pulsar wind with the surrounding SNR. I will present emission maps based on numerical simulations of the flow inside the nebula and how they can be used in comparison with observations to derive informations about the properties of pulsar winds far away from the source.
We present a sample of 1035 thermonuclear (type-I) X-ray bursts from
observations of 45 accreting neutron stars by the Rossi X-ray Timing Explorer
(RXTE), spanning more than nine years. This sample contains for the first time
confirmed examples of burst ignition in each of the three regimes identified
theoretically: H-ignition of mixed H/He fuel, He-ignition in pure He, and
He-ignition in mixed H/He fuel. We combined bursts from multiple sources to
investigate the variation of the burst rate, energetics, and time scale as a
function of accretion rate. We found a peak burst rate of 0.2-0.3/hr for a
normalised peak flux of gamma approx. 0.02-0.06, corresponding to a source
luminosity of (0.3-2)e37 ergs/cm^2/s.
For 35 sources with bursts exhibiting photospheric radius-expansion, we
estimated the likely distance range arising from the possible range of the
Eddington limit. We found that in general the peak flux of radius-expansion
bursts from individual sources is not constant, varying with a fractional
standard deviation of 14% in the mean. We also examined the bursts with very
short (<~30 min) recurrence times. We found evidence for two distinct
populations of such bursts, one of faint bursts following a much stronger burst
by 6-18 min, and another with comparable intensity bursts separated by 7 min or
more. We describe the properties of bursts observed from 5 new bursters
discovered by RXTE, as well as two new transient outbursts of previously known
sources. Finally, we searched for burst oscillations at the known oscillation
frequencies in 14 sources. We describe the distribution of the oscillation
amplitudes as well as the stages of each burst in which the oscillations are
detected.
We determine stellar parameters for the M dwarf GJ 436 that hosts a Neptune-mass planet. We employ primarily spectral modeling at low and high resolution, examining the agreement between model and observed optical spectra of five comparison stars of type, M0-M3. Modeling high resolution optical spectra suffers from uncertainties in TiO transitions, affecting the predicted strengths of both atomic and molecular lines in M dwarfs. The determination of Teff, gravity, and metallicity from optical spectra remains at ~10%. As molecules provide opacity both in lines and as an effective continuum, determing molecular transition parameters remains a challenge facing models such as the PHOENIX series, best verified with high resolution and photometric spectra. Our analysis of GJ 436 yields an effective temperature of Teff = 3350 +/- 300 K and a mass of 0.44 Msun. New Doppler measurements for GJ 436 with a precision of 3 m/s taken during 6 years improve the Keplerian model of the planet, giving a minimum mass, M sin i = 0.0713 Mjup = 22.7 Mearth, period, P = 2.6439 d, and e = 0.16 +/- 0.02. The noncircular orbit contrasts with the tidally circularized orbits of all close-in exoplanets, implying either ongoing pumping of eccentricity by a more distant companion, or a higher Q value for this low-mass planet. The velocities indeed reveal a long term trend, indicating a possible distant companion.
We study the dynamical evolution of globular clusters containing populations of primordial binaries, using our newly updated Monte Carlo cluster evolution code with the inclusion of direct integration of binary scattering interactions. We describe the modifications we have made to the code, as well as improvements we have made to the core Monte Carlo method. We present several test calculations to verify the validity of the new code, and perform many comparisons with previous analytical and numerical work in the literature. We simulate the evolution of a large grid of models, with a wide range of initial cluster profiles, and with binary fractions ranging from 0 to 1, and compare with observations of Galactic globular clusters. We find that our code yields excellent agreement with direct N-body simulations of clusters with primordial binaries, but yields some results that differ significantly from other approximate methods. Our results for the structural parameters of clusters during the binary-burning phase are outside the range of parameters for observed clusters, implying that either clusters are born significantly more or less centrally concentrated than has been previously considered, or that there are additional physical processes beyond two-body relaxation and binary interactions that affect the structural characteristics of clusters.
Three binaries are now established sources of emission at very high energies (>1e11 eV). They are composed of a massive star and a compact object. The emission can be due to the interaction of the relativistic wind from a young ms pulsar with the stellar wind of the companion, by which rotation-power ends up as non-thermal flux. Variations at VHE energies are explained as due to gamma-gamma absorption and/or changes in shock location along the orbit. Resolved radio emission is due to cooling particles trailing the pulsar.
Spectral properties of super-Eddington accretion flows are investigated by
means of a parallel line-of-sight calculation. The subjacent model, taken from
two-dimensional radiation hydrodynamic simulations by Ohsuga et al. (2005),
consists of a disc accretion region and an extended atmosphere with high
velocity outflows. The non-gray radiative transfer equation is solved,
including relativistic effects, by applying the FLD approximation.
The calculated spectrum is composed of a thermal, blackbody-like emission
from the disc which depends sensitively on the inclination angle, and of high
energy X-ray and gamma-ray emission from the atmosphere. We find mild beaming
effects in the thermal radiation for small inclination angles. If we compare
the face-on case with the edge-on case, the average photon energy is larger by
a factor of ~1.7 due mainly to Doppler boosting, while the photon number
density is larger by a factor of ~3.7 due mainly to anisotropic matter
distribution around the central black hole. This gives an explanation for the
observed X-ray temperatures of ULXs which are too high to be explained in the
framework of intermediate-mass black holes.
While the main features of the thermal spectral component are consistent with
more detailed calculations of slim accretion discs, the atmosphere induces
major changes in the high-energy part, which cannot be reproduced by existing
models. In order to interpret observational data properly, simple approaches
like the Eddington-Barbier approximation cannot be applied.
We constructed hydrodynamical model atmospheres for mid M-type main-, as well as pre-main-sequence (PMS) objects. Despite the complex chemistry encountered in these cool atmospheres a reasonably accurate representation of the radiative transfer is possible, even in the context of time-dependent and three-dimensional models. The models provide detailed information about the morphology of M-type granulation and statistical properties of the convective surface flows. In particular, we determined the efficiency of the convective energy transport, and the efficiency of mixing by convective overshoot. The convective transport efficiency was expressed in terms of an equivalent mixing-length parameter alpha in the formulation of mixing-length theory (MLT) given by Mihalas (1978). Alpha amounts to values around 2 for matching the entropy of the deep, adiabatically stratified regions of the convective envelope, and lies between 2.5 and 3.0 for matching the thermal structure of the deep photosphere. For current spectral analysis of PMS objects this implies that MLT models based on alpha=2.0 overestimate the effective temperature by 100 K and surface gravities by 0.25 dex. The average thermal structure of the formally convectively stable layers is little affected by convective overshoot and wave heating, i.e., stays close to radiative equilibrium conditions. Our models suggest that the rate of mixing by convective overshoot declines exponentially with geometrical distance to the Schwarzschild stability boundary. It increases at given effective temperature with decreasing gravitational acceleration.
The kinematic parameters of the Gould Belt are redetermined using modern data on the motion of nearby young ($\log t<7.91$) open clusters, OB-associations and moving stellar groups. The modelling carried out shows that the residual velocities achieve the maximal values equal to -4 km s$^{-1}$ for rotation (in the direction of the Galactic rotation) and $+4$ km s$^{-1}$ for expansion, when the distance from the kinematic centre is about 300 pc. We assumed the following parameters of the Gould Belt centre: $l_\circ=128^\circ$ and $R_\circ=150$ pc. It is shown that the whole structure of the Gould Belt is moving relative to the local standard of rest with a velocity of $10.7\pm0.7$ km s$^{-1}$ in the direction towards $l=274\pm4^\circ$, $b=-1\pm3^\circ$. Using the rotation velocity, we obtained the virial estimation of the Gould Belt mass as $1.5\times 10^6 M_\odot$.
We created and are constantly updating the Orion Spiral Arm CAtalogue (OSACA) for stars with known coordinates, parallaxes, proper motions and radial velocities. It is shown that there is an effect of contraction appearing in motion of giants of A0--A5 spectral classes which attains a value of K=-13+-2 km/s/kpc. We try to link this effect to the periodic structure of residual velocity field of stars in the solar neighborhood, which is caused by spiral density waves.
Using the IUE data archive, we have examined the SWP-camera echellograms of 74 B0--B2.5e stars for statistically significant fluctuations in the He II ("Halpha") 1640A line profile. In this sample we found that the HeII line is occasionally variable in 10 stars over short to long timescales. The HeII-variable stars discovered are lambda Eri, omega Ori, mu Cen, 6 Cep, HD 67536, psi-1 Ori, eta Cen, pi Aqr, 2 Vul, and 19 Mon. The most frequent two types of variability are an extended blue wing absorption and a weakening of the line along the profile. Other types of variability are a weak emission in the red wing and occasionally a narrow emission feature. In the overwhelming number of cases, the CIV resonance doublet exhibits a similar response; rarely, it can exhibit a variation in the opposite sense. Similar responses are also often seen in the SiIV doublet, and occasionally even the SiIII 1206A line. We interpret the weakenings of HeII and of high-velocity absorptions of CIV to localized decreases in the photospheric temperature, although this may not be a unique interpretation. We discuss the variable blue wing absorptions and red wing emissions in terms of changes in the velocity law and mass flux carried by the wind. In the latter case, recent experimental models by Venero, Cidale, & Ringuelet require that during such events the wind must be heated to 35kK at some distance from the star.
We present predictions for the cosmic metal budget in various phases of baryons from redshift z=6-0, taken from a cosmological hydrodynamic simulation that includes a well-constrained model for enriched galactic outflows. We find that substantial amounts of metals are found in every baryonic phase at all epochs, with diffuse intergalactic gas dominating the metal budget at early epochs and stars and halo gas dominating at recent epochs. We provide a full accounting of metals in the context of the missing metals problem at z~2.5, showing that ~40% of the metals are in galaxies, and the remainder is divided between diffuse IGM gas and shocked gas in halos and filamentary structures. Comparisons with available observations of metallicity and metal mass fraction evolution show broad agreement. We predict stars have a mean metallicity of one-tenth solar already at z=6, which increases slowly to one-half solar today, while stars just forming today have typically solar metallicity. Our HI column density-weighted mean metallicity (comparable to Damped Ly-alpha system metallicities) slowly increases from one-tenth to one-third solar from z=6-1, then falls to one-quarter solar at z=0. The global mean metallicity of the universe tracks ~50% higher than that of the diffuse phase down to z~1, and by z=0 it has a value around one-tenth solar. Metals move towards higher densities and temperatures with time, peaking around the mean cosmic density at z=2 and an overdensity of 100 at z=0. We study how carbon and oxygen ions trace the path of metals in phase space, and show that OIII-OVII lines provide the most practical option for constraining intergalactic medium metals at z<2.
We consider a nine-dimensional parameterization of the dark energy equation of state and compare this with the two-dimensional analysis recently used by the Dark Energy Task Force (DETF) to compare the constraining power of various experimental approaches. We extend the figure of merit analysis used by the DETF to our parameterization and apply it to the DETF data models. The data models constrain 3-4 of our parameters just as well as they constrain the two DETF parameters. While supporting most of the DETF conclusions, our results point to a much higher impact for the larger dark energy experiments than was reported by the DETF. Most of our differences from the DETF can be understood using a simple dimensional rescaling formula.
Identifying binaries among runaway O- and B-type stars offers valuable insight into the evolution of open clusters and close binary stars. Here we present a spectroscopic investigation of 12 known or suspected binaries among field and runaway OB stars. We find new orbital solutions for five single-lined spectroscopic binaries (HD 1976, HD 14633, HD 15137, HD 37737, and HD 52533), and we classify two stars thought to be binaries (HD 30614 and HD 188001) as single stars. In addition, we reinvestigate their runaway status using our new radial velocity data with the UCAC2 proper motion catalogs. Seven stars in our study appear to have been ejected from their birthplaces, and at least three of these runaways are spectroscopic binaries and are of great interest for future study.
In concordance cosmology, first galaxies formed at $z\sim 30$. Although these galaxies may be too small to observe, the extended 21cm absorption regions (dubbed ``21cm absorption halos'') around them can be resolved by future radio surveys. These ``halos'' are close to spherical and thus are ideal sources for measuring the shape distortion ({\it cosmic shear}) caused by gravitational lensing. We investigate the expected lensing signal and consider a variety of noise contributions on the shear measurement. While the radio foreground is overwhelming at these low frequencies ($\sim 55$ MHz), radio surveys much more advanced than SKA/LOFAR, with total collecting area of $\sim 200$ km$^2$ can resolve millions of these 21cm absorption ``halos'' and measure the shear of each ``halo'' with S/N $\sim 1$. The total S/N will be comparable to traditional shear measurement of $\sim$$10^9$ galaxies at $z\sim 1$. Along with several other potentially powerful cosmological applications of these ``halos'', radio surveys aiming at resolving these ``halos'' would be extremely profitable.
Color confinement is only a supposition, which has not been proved in QCD yet. It is proposed here that macroscopic quark gluon plasma in astrophysics could hardly maintain colorless because of causality. The authors expected that the existence of chromatic strange quark stars as well as chromatic strangelets preserved from the QCD phase transition in the early universe could be unavoidable if their colorless correspondents do exist.
The basic geometry of a stellar interferometer with two telescopes consists of a baseline vector and a direction to a star. Two derived vectors are the delay vector, and the projected baseline vector in the plane of the wavefronts of the stellar light. The manuscript deals with the trigonometry of projecting the baseline further outwards onto the celestial sphere. The position angle of the projected baseline is defined, measured in a plane tangential to the celestial sphere, tangent point at the position of the star. This angle represents two orthogonal directions on the sky, differential star positions which are aligned with or orthogonal to the gradient of the delay recorded in the u-v plane. The North Celestial Pole is chosen as the reference direction of the projected baseline angle, adapted to the common definition of the "parallactic" angle.
Extra-galactic point sources are a significant contaminant in cosmic microwave background and Sunyaev-Zel'dovich effect experiments. Deep interferometric observations with the BIMA and OVRO arrays are used to characterize the spatial, spectral, and flux distributions of radio point sources toward galaxy clusters at 28.5 GHz. We compute counts of mJy point source fluxes from 90 fields centered on known massive galaxy clusters and 8 non-cluster fields. We find that source counts in the inner regions of the cluster fields (within 0.5 arcmin of the cluster center) are a factor of 8.9 (+4.3,-2.8) times higher than counts in the outer regions of the cluster fields (radius greater than 0.5 arcmin). Counts in the outer regions of the cluster fields are in turn a factor of 3.3 (+4.1,-1.8) greater than those in the non-cluster fields. Counts in the non-cluster fields are consistent with extrapolations from the results of other surveys. We also compute source counts toward clusters as a function of luminosity in three redshift bins out to z = 1.0 and see no clear evidence for evolution with redshift. We compute spectral indices of mJy sources in cluster fields between 1.4 and 28.5 GHz and find a mean spectral index of alpha = 0.70 with an rms dispersion of 0.34, where flux is proportional to frequency raised to negative alpha. The distribution is skewed, with a median spectral index of 0.76 and 25th and 75th percentiles of 0.55 and 0.95, respectively. This is steeper than the spectral indices of brighter field point sources measured by other surveys.
We perform numerical simulations of black-hole binaries to study the exchange of spin and orbital angular momentum during the last, highly nonlinear, stages of the coalescence process. To calculate the transfer of angular momentum from orbital to spin, we start with a configuration of initially non-spinning holes in a quasicircular orbit. We find that each individual black hole horizon acquires a non-vanishing spin and that this spin is two orders of magnitude smaller than what is needed to tidally lock the binary into a corotation state. We also study the converse transfer from spin into orbital motion. In this case, we start the simulations with parallel, highly-spinning non-boosted black holes. As the collision proceeds, the system acquires a non-head-on orbital motion, due to spin-orbit coupling, that leads to the radiation of angular momentum. We are able to accurately measure the energy and angular momentum losses and model their dependence on the initial spins.
We discuss the motivation for high accuracy relativistic gravitational experiments in the Solar System and complementary cosmological tests. We focus our attention on the issue of distinguishing a generic scalar-theory of gravity as the underlying physical theory from the usual general relativistic picture, where one expects the presence of fundamental scalar fields associated, for instance, to inflation, dark matter and dark energy.
We present the constraints on inflationary parameters in a flat $\Lambda$CDM universe obtained by WMAP three year data release, plus smaller scale CMB and two LSS data sets, 2dF and SDSS (treated separately). We use a Markov Chain Monte Carlo (MCMC) technique combined with an analytic description of the inflationary spectra in terms of the horizon flow functions (HFF). We discuss the constraints both on single field standard inflation and on inflation with the violation of the null energy condition, which leads to a blue spectrum for gravitational waves. Without the consistency condition between the tensor-to-scalar ratio and the tensor slope, the constraints on the tensor amplitude is not significantly changed, but the constraints on the HFFs are significantly relaxed. We show that only when the third HFF $\epsilon_3 \ne 0$ is allowed with a very weak prior, an large negative (at $2 \sigma$) value for the running of the spectral index in standard inflation is found in any set of data we consider, i.e. CMB alone or CMB combined with LSS data sets.
We develop a first-order approximation method for the influence of spin on the motion of extended spinning test masses in a gravitational field. This approach is illustrated for approximately circular equatorial motion in the exterior Kerr spacetime. In this case, the analytic results for the first-order approximation are compared to the numerical integration of the exact system and the limitations of the first-order results are pointed out. Furthermore, we employ our analytic results to illustrate the gravitomagnetic clock effect for spinning particles.
We re-analyze the precision radial velocity (RV) observations of HD160691 (mu Ara) by the Anglo-Australian Planet Search Team. The star is supposed to host two Jovian companions (HD160691b, HD160691c) in long-period orbits (about 630 days and 2500 days, respectively) and a hot-Neptune (HD160691d) in about 9 days orbit. We perform a global search for the best fits in the orbital parameters space with a hybrid code employing the genetic algorithm and simplex method. The stability of Keplerian fits is verified with the N-body model of the RV signal that takes into account the dynamical constraints (so called GAMP method). Our analysis reveals a signature of the fourth, yet unknown and unconfirmed, 0.5 Jupiter-mass planet in about 307 days orbit. In overview, the global architecture of the four-planet configuration recalls the Solar system. All companions of HD160691 move in close to circular orbits. The orbits of the two inner Jovian planets are close to the 2:1 mean motion resonance (MMR). The alternative three-planet system involves two Jovian planets in eccentric orbits (with e about 0.3), close to the 4:1 MMR, but it yields a significantly worse fit to the data.
We present the current estimates of the Galactic merger rate of double-neutron-star (DNS) systems. Using a statistical analysis method, we calculate the probability distribution function (PDF) of the rate estimates, which allows us to assign confidence intervals to the rate estimates. We calculate the Galactic DNS merger rate based on the three known systems B1913+16, B1534+12, and J0737-3039. The discovery of J0737-3039 increases the estimated DNS merger rate by a factor ~6 than what is previously known. The most likely values of DNS merger rate lie in the range 3-190 per Myr depending on different pulsar models. Motivated by a strong correlation between the peak rate estimates and a pulsar luminosity function, we calculate a 'global' probability distribution as a single representation of the parameter space covered by different pulsar population models. We compare the global PDF with the observed supernova Ib/c rate, which sets an upper limit on the DNS merger rate. Finally, we remark on implications of new discoveries such as of J1756-2251, the 4th DNS in the Galactic disk, and J1906+0746, a possible DNS system.
Although the surface of a magnetar is a source of bright thermal X-rays, its spectrum contains substantial non-thermal components. The X-ray emission is pulsed, with pulsed fractions that can be as high as ~ 70%. Several properties of magnetars indicate the presence of persistent, static currents flowing across the stellar surface and closing within the magnetosphere. The charges supporting these currents supply a significant optical depth to resonant cyclotron scattering in the 1-100 keV band. Here we describe a Monte Carlo approach to calculating the redistribution of thermal seed photons in frequency and angle by multiple resonant scattering in the magnetosphere. The calculation includes the full angular dependence of the cyclotron scattering cross section, the relativistic Doppler effect due to the motion of the charges, and allows for an arbitrary particle velocity distribution and magnetic field geometry. We construct synthetic spectra and pulse profiles for arbitrary orientations of the spin axis, magnetic axis, and line of sight, using a self-similar, twisted dipole field geometry, and assuming that the seed photons are supplied by single-temperature black body emission from the stellar surface. Pulse profiles and 1-10 keV spectra typical of AXPs are easily produced by this model, with pulsed fractions of ~ 50%. However, this model cannot reproduce the hard, rising energy spectra that are observed from SGRs during periods of activity, without overproducing the thermal emission peak. This suggests that the 1-100 keV emission of SGRs has a common origin with the hard X-ray emission detected from some AXPs above ~20 keV.
We present an interpretation of the phenomenological relations between the spectral peak, isotropic luminosity and duration of long gamma ray bursts that have been discovered by Amati et al., Ghirlanda et al., Firmani et al., and Liang & Zhang. In our proposed model, a jet undergoes internal dissipation which prevents its bulk Lorentz factor from exceeding 1/theta (theta being the jet opening angle) until it escapes from the core of its progenitor star, at a radius of order 1e10 cm; dissipation may continue at larger radii. The dissipated radiation will be partially thermalized, and we identify its thermal peak (Doppler boosted by the outflow) with E_pk. The radiation comes, in effect, from within the jet photosphere. The non-thermal, high energy part of the GRB emission arises from Comptonization of this radiation by relativistic electrons and positrons outside the effective photosphere. This model can account naturally not only for the surprisingly small scatter in the various claimed correlations, but also for the normalization, as well as the slopes. It then has further implications for the jet energy, the limiting jet Lorentz factor, and the relation of the energy, opening angle and burst duration to the mass and radius of the stellar stellar progenitor. The observed relation between pulse width and photon frequency can be reproduced by inverse-Compton emission at ~ 1e14 cm from the engine, but there are significant constraints on the energy distribution and isotropy of the radiating particles.
We study the reionization histories where ionizing UV photons are emitted from decaying particle, in addition to usual contributions from stars and quasars, and consistent with the fact that the universe is not fully ionized until z = 6 as observed by Sloan Digital Sky Survey. Likelihood analysis of the three-year data of the Wilkinson Microwave Anisotropy Probe severely constrains the decaying particle scenario due especially to polarization data.
We investigate the non-Local Thermodynamic Equilibrium (non-LTE) line formation of neutral carbon in late-type stars in order to remove some of the potential systematic errors in stellar abundance analyses employing C I features. The statistical equilibrium code MULTI was used on a grid of plane-parallel 1D MARCS atmospheric models. Within the parameter space explored, the high-excitation C I lines are stronger in non-LTE due to the combined effect of line source function drop and increased lower level population; the relative importance of the two effects depends on the particular combination of Teff, log g, [Fe/H] and [C/Fe] and on the analysed C I line. As a consequence, the non-LTE abundance corrections are negative and can be substantially so, for example ~-0.4 dex in halo turn-off stars at [Fe/H]~-3. The magnitude of the non-LTE corrections is rather insensitive to whether inelastic H collisions are included or not. Our results have implications on studies of nucleosynthetic processes and on Galactic chemical evolution models. When applying our calculated corrections to recent observational data, the upturn in [C/O] at low metallicity might still be present (thus apparently still necessitating contributions from massive Pop. III stars for the carbon production), but at a lower level and possibly with a rather shallow trend of ~-0.2 dex/dex below [O/H]~-1
CLOVER is an experiment which aims to detect the signature of gravitational waves from inflation by measuring the B-mode polarization of the cosmic microwave background. CLOVER consists of three telescopes operating at 97, 150, and 220 GHz. The 97-GHz telescope has 160 feedhorns in its focal plane while the 150 and 220-GHz telescopes have 256 horns each. The horns are arranged in a hexagonal array and feed a polarimeter which uses finline-coupled TES bolometers as detectors. To detect the two polarizations the 97-GHz telescope has 320 detectors while the 150 and 220-GHz telescopes have 512 detectors each. To achieve the target NEPs (1.5, 2.5, and 4.5x10^-17 W/rtHz) the detectors are cooled to 100 mK for the 97 and 150-GHz polarimeters and 230 mK for the 220-GHz polarimeter. Each detector is fabricated as a single chip to ensure a 100% operational focal plane. The detectors are contained in linear modules made of copper which form split-block waveguides. The detector modules contain 16 or 20 detectors each for compatibility with the hexagonal arrays of horns in the telescopes' focal planes. Each detector module contains a time-division SQUID multiplexer to read out the detectors. Further amplification of the multiplexed signals is provided by SQUID series arrays. The first prototype detectors for CLOVER operate with a bath temperature of 230 mK and are used to validate the detector design as well as the polarimeter technology. We describe the design of the CLOVER detectors, detector blocks, and readout, and present preliminary measurements of the prototype detectors performance.
The concept of a stare-mode astrometric space mission is introduced. The traditionally accepted mode of operation for a mapping astrometric space mission is that of a continuously scanning satellite, like the successful Hipparcos and planned Gaia missions. With the advent of astrometry missions mapping out stars to 20th magnitude, the stare-mode becomes competitive. A stare-mode of operation has several advantages over a scanning missions if absolute parallax and throughput issues can be successfully addressed. Requirements for a stare-mode operation are outlined. The mission precision for a stare-mode astrometric mission is derived as a function of instrumental parameters with examples given. The stare-mode concept has been accepted as baseline for the NASA roadmap study of the Origins Billions Star Survey (OBSS) mission and the Milli-arcsecond Pathfinder Survey (MAPS) micro-satellite proposed project.
We consider the question of whether it is worth building an experiment with the sole purpose of bringing the detectable limit on the tensor-to-scalar ratio down to 10^{-3}. We look at the inflationary models which give a prediction in this region and recap the current situation with the tensor mode, showing that there are only two known models inflation which give definitive predictions in the region 10^{-3}<r<10^{-2}.
We show that the irradiated accretion disk model can account for all the optical and infrared observations of the anomalous X-ray pulsars in the persistent state. Model fits do not constrain the outer disk radii, while placing an upper limit to the inner disk radii, and thus to the strength of the dipole component of the stellar magnetic field. While magnetar fields (B_* > 10^{14} G) in higher multipoles are compatible with the irradiated disk model, magnetic dipole components of magnetar strength are not consistent with optical data.
The MareNostrum Universe is one of the biggest SPH cosmological simulations done so far. It contains more than 2 billion particles (2 times 1024^3) in a 500 Mpc/h cubic volume. This simulation has been performed on the MareNostrum supercomputer at the Barcelona Supercomputer Center. We have obtained more than 0.5 million halos with masses greater than a typical Milky Way galaxy halo. We report results about the halo mass function, the shapes of dark matter and gas distributions in halos, the baryonic fraction in galaxy clusters and groups, baryon oscillations in the dark matter and the halo power spectra as well as the distribution and evolution of the gas fraction at large scales.
We report the detection of nebular emission lines in the optical and mid-infrared spectra of IRAS 17347-3139, a heavily obscured OH/IR star which may be rapidly evolving from the AGB to the PN stage. The presence of emission lines is interpreted as a clear indication that the ionization of its circumstellar envelope has already started. This source belongs to the rare class of objects known as `OHPNe' displaying both OH maser and radio continuum emission. However, unlike the rest of stars in this class, prominent C-rich dust features are detected in its mid-infrared spectrum, which makes the analysis of this star particularly interesting.
It has recently been shown that the correct halo mass function high mass behaviour may be obtained by an appropriate treatment of the all-mass-at-center problem. Here we show that the treatment of this problem leads to an accumulated mass fraction, F, which is not a universal function of the linear variance of the density contrast, but depends also on the shape of the power spectrum. For relevant power spectra F is a universal function of two quantities: the linear variance and a quantity related to the local shape (at the given mass scale) of the power spectrum. This implies, for a given shape of the power spectrum, a redshift dependence of the mass function, which is quite important for a cold dark matter power spectrum. Our results are in good agreement with numerical simulations.
A recent comparison between deep VLA HI observations and dynamical models of the Virgo cluster spiral galaxy NGC 4654 has shown that only a model involving a combination of a tidal interaction and ram pressure can reproduce the data. Deep radio polarization studies, together with detailed MHD modeling, can independently verify those conclusions, that are based on HI observations and dynamical models. We performed deep polarized radio-continuum observations of the Virgo cluster spiral galaxy NGC 4654 with the Effelsberg 100m telescope at 8.35 GHz and the VLA at 4.85 GHz. Detailed 3D MHD simulations were made to determine the large-scale magnetic field and the emission distribution of the polarized radio continuum in the model, during the galaxy evolution within the cluster environment. This direct comparison between the observed and simulated polarized radio continuum emission corroborates the earlier results, that the galaxy had a recent rapid close encounter with NGC 4639 and is undergoing weak ram pressure by the intracluster medium. This combination of deep radio polarization studies and detailed MHD modeling thus gives us unique insight into the interactions of a galaxy with its cluster environment. It represents a diagnostic tool that is complementary to deep HI observations.
On October 28, 2003 an earthward-directed coronal mass ejection (CME) was observed from SOHO/LASCO imagery in conjunction with an X17 solar flare. The CME, traveling at nearly 2000 km/s, impacted the Earth on October 29, 2003 causing ground-based particle detectors to register a counting rate drop known as a Forbush decrease. In addition to affecting the rate of cosmic rays, the CME was also responsible for causing anisotropies in the direction of incidence. Data from Project GRAND, an array of proportional wire chambers, are presented during the time of this Forbush decrease. A simple model for CME propagation is proposed and we present an argument based on gyroradius that shows that a magnetic field of the radius calculated for the ejecta is sufficient to deflect energetic charged particles of an energy detectable by GRAND.
Despite the common picture of an early-type dwarf (dE) as a quiescent galaxy with no star formation and little gas, we identify 23 dEs that have blue central colors caused by recent or ongoing star formation in our sample of 476 Virgo cluster dEs. In addition, 14 objects that were mostly classified as (candidate) BCDs have similar properties. Among the certain cluster members, the dEs with blue centers reach a fraction of more than 15% of the dE population at brighter (B<=16) magnitudes. A spectral analysis of the centers of 16 galaxies reveals in all cases an underlying old population that dominates the mass, with M(old)>=90% for all but one object. Therefore the majority of these galaxies will appear like ordinary dEs within ~one Gigayear or less after the last episode of star formation. Their overall gas content is less than that of dwarf irregular galaxies, but higher than that of ordinary dEs. Their flattening distribution suggests the shape of a thick disk, similar to what has been found for dEs with disk features in Paper I of this series. Their projected spatial distribution shows no central clustering, and their distribution with projected local density follows that of irregular galaxies, indicative of an unrelaxed population. This is corroborated by their velocity distribution, which displays two side peaks characteristic of recent infall. We discuss possible formation mechanisms (ram-pressure stripping, tidally induced star formation, harassment) that might be able to explain both the disk shape and the central star formation of the dEs with blue centers.
We investigate the gas content and baryonic Tully-Fisher relationship for extremely low luminosity dwarf galaxies in the absolute magnitude range -13.5 > Mr > -16. The sample is selected from the Sloan Digital Sky Survey and consists of 101 galaxies for which we have obtained follow-up HI observations using the Arecibo Observatory and Green Bank Telescope. This represents the largest homogeneous sample of dwarfs at low luminosities with well-measured HI and optical properties. The sample spans a range of environments, from dense groups to truly isolated galaxies. The average neutral gas fraction is f_gas=0.6, significantly exceeding that of typical gas-rich galaxies at higher luminosities. Dwarf galaxies are therefore less efficient at turning gas into stars over their lifetimes. The strong environmental dependence of the gas fraction distribution demonstrates that while internal processes can reduce the gas fractions to roughly f_gas=0.4, external processes are required to fully remove gas from a dwarf galaxy. The average rotational velocity of our sample is vrot=50 km/s. Including more massive galaxies from the literature, we fit a baryonic Tully-Fisher slope of M_baryon \propto vrot^(3.70+/- 0.15). This slope compares well with CDM models that assume an equal baryon to dark matter ratio at all masses. While gas stripping or other processes may modify the baryon to dark matter ratio for dwarfs in the densest environments, the majority of dwarf galaxies in our sample have not preferentially lost significant baryonic mass relative to more massive galaxies.
Saturn's magnetic field is remarkably axisymmetric. Its dipole axis is inclined by less than 0.2 deg with respect to its rotation axis. Rotationally driven convection of magnetospheric plasma breaks the axisymmetry of its external magnetic field. Field aligned currents transfer angular momentum from the planet to a tongue of outflowing plasma. This transfer slows the rate of rotation of the ionosphere relative to that of the underlying atmosphere. The currents are the source for the non-axisymmetric components of the field. The common rotation rates of these components and Saturn's kilometric radio (SKR) bursts is that of the plasma near the orbit of Enceladus, and by extension the rotation rate in the ionosphere to which this plasma is coupled. That rate tells us nothing about the rotation rate of Saturn's deep interior. Of that we remain ignorant. Magnetic perturbations with magnitudes similar to those observed by Cassini are produced for Mdot ~ 10^4 g/s, a value similar to estimates for the rate of production of plasma from Saturn's E-ring. Enhancement of the SKR occurs in a narrow range of longitudes where the tip of the outgoing plasma stream connects to the auroral ionosphere via field lines that are bowed outwards by currents that supply the plasma's centripetal acceleration. (abridged)
We present observations of two LMC supernova remnants (SNRs), DEM L238 and DEM L249, with the Chandra and XMM-Newton X-ray satellites. Bright central emission, surrounded by a faint shell, is present in both remnants. The central emission has an entirely thermal spectrum dominated by strong Fe L-shell lines, with the deduced Fe abundance in excess of solar and not consistent with the LMC abundance. This Fe overabundance leads to the conclusion that DEM L238 and DEM L249 are remnants of thermonuclear (Type Ia) explosions. The shell emission originates in gas swept up and heated by the blast wave. A standard Sedov analysis implies about 50 solar masses in both swept-up shells, SNR ages between 10,000 and 15,000 yr, low (< 0.05 cm^-3) preshock densities, and subluminous explosions with energies of 3x10^50 ergs. The central Fe-rich supernova ejecta are close to collisional ionization equilibrium. Their presence is unexpected, because standard Type Ia SNR models predict faint ejecta emission with short ionization ages. Both SNRs belong to a previously unrecognized class of Type Ia SNRs characterized by bright interior emission. Denser than expected ejecta and/or a dense circumstellar medium around the progenitors are required to explain the presence of Fe-rich ejecta in these SNRs. Substantial amounts of circumstellar gas are more likely to be present in explosions of more massive Type Ia progenitors. DEM L238, DEM L249, and similar SNRs could be remnants of ``prompt'' Type Ia explosions with young (~100 Myr old) progenitors.
The host stars of extrasolar planets tend to be metal-rich. We have examined the data for these stars for evidence of trends in other galactic parameters, without success. However, several ESP hosts are likely to be members of the thick disk population, indicating that planet formation has occurred throughout the full lifetime of the Galactic disk. We briefly consider the radial metallicity gradient and age-metallicity relation of the Galactic disk, and complete a back-of-the envelope estimate of the likely number of solar-type stars with planetary companions with 6 < R < 10 kpc.
Solutions of the stationary axisymmetric Einstein equations describing the interior of circularly rotating dust are investigated in order to study their potential applicability as galaxy models. It is shown that such interior solutions cannot be used as global metrics without becoming unphysical in certain regions of space. Although definite results concerning the non-existence of a continuation into a vacuum can only be derived for interior solutions describing rigidly rotating dust, the present analysis exhibits that the case of non-rigidly rotating dust would in general also be inadequate as a physically reasonable galaxy model.
We present images and slitless spectra which were obtained in HST surveys of Planetary Nebulae (PNe) in both the Large and Small Magellanic Clouds, using the Space Telescope Imaging Spectrograph. These new data on 59 PNe (54 in the LMC and five in the SMC) permit us to determine the nebular dimensions and morphology in the monochromatic light of several emission lines: Halpha, [N II] lambda 6583 and [O III] lambda 5007, plus others of varying ionization, including [O I], He I, and [S II]. We describe the nebular morphology and related features in detail. This survey, when combined with similar data from our prior HST programs and other archived PN images, brings the total of nebulae imaged with HST to 114 in the LMC and 35 in the SMC. We describe various basic properties for the sample, including sizes, morphologies, densities, and completeness. Trends in [O III] lambda 5007 flux, surface brightness, and electron density with physical radius suggest that many nebulae, particularly those with bipolar morphology, may be optically thick even at large size. Bipolars also show the most extreme values of [N II]/Halpha flux ratios, which is a rough indicator N enrichment.
We present optical and near-infrared images, H I 21 cm emission maps, optical spectroscopy, and Hubble Space Telescope/Space Telescope Imaging Spectrograph ultraviolet spectroscopy of the QSO/galaxy pair SBS 1122+594/IC 691. The QSO sight line lies at a position angle of 27 degrees from the minor axis of the nearby dwarf starburst galaxy IC 691 (cz_gal = 1204+-3 km/s, L_B ~ 0.09 L*, current star formation rate = 0.08-0.24 solar masses per year) and 33 kpc (6.6 arcmin) from its nucleus. We find that IC 691 has an H I mass of M_HI = (3.6+-0.1) x 10^8 solar masses and a dynamical mass of M_dyn = (3.1+-0.5) x 10^10 solar masses. The UV spectrum of SBS 1122+594 shows a metal-line (Ly-alpha + C IV) absorber near the redshift of IC 691 at cz_abs = 1110+-30 km/s. Since IC 691 is a dwarf starburst and the SBS 1122+594 sight line lies in the expected location for an outflowing wind, we propose that the best model for producing this metal-line absorber is a starburst wind from IC 691. We place consistent metallicity limits on IC 691 ([Z/Zsun] ~ -0.7) and the metal-line absorber ([Z/Zsun] < -0.3). We also find that the galaxy's escape velocity at the absorber location is v_esc = 80+-10 km/s and derive a wind velocity of v_w = 160+-50 km/s. Thus, the evidence suggests that IC 691 produces an unbound starburst wind that escapes from its gravitational potential to transport metals and energy to the surrounding intergalactic medium.
We present a measurement of the D/H abundance ratio in a metal-poor damped Lyman alpha (DLA) system along the sightline of QSO SDSS1558-0031. The DLA system is at redshift z = 2.70262, has a neutral column density of log(NHI)=20.67+/-0.05 cm^2, and a gas-phase metallicity [O/H]= -1.49 which indicates that deuterium astration is negligible. Deuterium absorption is observed in multiple Lyman series with a column density of log(NDI)=16.19+/-0.04 cm^2, best constrained by the deuterium Lyman-11 line. We measure log(D/H) = -4.48+/-0.06, which when combined with previous measurements along QSO sightlines gives a best estimate of log(D/H) = -4.55+/-0.04, where the 1-sigma error estimate comes from a jackknife analysis of the weighted means. Using the framework of standard big bang nucleosynthesis, this value of D/H translates into a baryon density of Omega_b h^2 = 0.0213 +/- 0.0013 +/- 0.0004 where the error terms represent the 1-sigma errors from D/H and the uncertainties in the nuclear reaction rates respectively. Combining our new measurement with previous measurements of D/H, we no longer find compelling evidence for a trend of D/H with NHI.
The cosmic neutrino background (CNB) consists of low-energy relic neutrinos which decoupled from the cosmological fluid at a redshift z ~ 10^{10}. Despite being the second-most abundant particles in the universe, direct observation remains a distant challenge. Based on the measured neutrino mass differences, one species of neutrinos may still be relativistic with a thermal distribution characterized by the temperature T ~ 1.9K. We show that the temperature distribution on the sky is anisotropic, much like the photon background, experiencing Sachs-Wolfe and integrated Sachs-Wolfe effects.
On the basis of a large scale 'adiabatic', namely non-radiative and non-dissipative, cosmological smooth particle hydrodynamic simulation we compare the entropy profiles of the gas and the dark matter (DM) in galaxy clusters. The quantity K_g = T_g \rho_g^{-2/3} provides a measure for the entropy of the intra-cluster gas. By analogy with the thermodynamic variables of the gas the velocity dispersion of the DM is associated with a formal temperature and thereby K_DM = \sigma_DM^2 \rho_DM^{-2/3} is defined. This DM entropy is related to the DM phase space density by K_DM \propto Q_DM^{-2/3}. In accord with other studies the DM phase space density follows a power law behaviour, Q_DM \propto r^{-1.82}, which corresponds to K_DM \propto r^{1.21}. The simulated intra-cluster gas has a flat entropy core within (0.8 \pm 0.4) R_s, where R_s is the NFW scale radius. The outer profile follows the DM behaviour, K_g \propto r^{1.21}, in close agreement with X-ray observations. Upon scaling the DM and gas densities by their mean cosmological values we find that outside the entropy core a constant ratio of K_g / K_{DM} = 0.71 \pm 0.18 prevails. By extending the definition of the gas temperature to include also the bulk kinetic energy the ratio of the DM and gas extended entropy is found to be unity for r > 0.8 R_s. The constant ratio of the gas thermal entropy to that of the DM implies that observations of the intra-cluster gas can provide an almost direct probe of the DM.
In the 1970s, the Narrabri intensity interferometer was used to measure 32 stellar diameters; some as small as 0.4 milli-arc-seconds (mas). The interferometer consisted of a pair of 6.5m telescopes with relatively crude optics, similar to those currently in use as Atmospheric Cherenkov Telescopes (ACT). We explore the possibility of implementing a modern intensity interferometer on an ACT array. Developments in fast digital signal processing technology now make such a system relatively easy to implement, and provide improved sensitivity. Allowing measurements at short wavelength (<400nm), with long baselines (> 100m), which are still challenging for Michelson interferometers, present ACT arrays could be used to probe angular structures as small as ~0.2mas, and smaller with large array projects already being discussed. This would provide measurements of stellar diameters, binary systems, circumstellar environments and, possibly, stellar surface features. ACT arrays could be used as intensity interferometers during bright moon periods, providing valuable scientific output for little expense and no impact on the gamma-ray observing schedule.
The trajectory of a primary gamma-ray detected with an array of at least four atmospheric Cherenkov imaging telescopes can be reconstructed from the shower image centroid positions and geometrical considerations independent of the primary energy. Using only the image centroid positions some cosmic-ray discrimination is also possible. This minimal approach opens the possibility of pushing the analysis threshold to lower values, close to the hardware threshold.
We study the cascading of fast MHD modes in magnetically dominated plasma by performing one-dimensional (1D) dynamical simulations. We find that the cascading becomes more efficient as an angle between wave vector and underlying magnetic field increases and fast mode becomes more compressive. We also consider imbalanced turbulence, in which wave packets propagating in one-direction have more power than those in the opposite direction. Unlike imbalanced Alfvenic turbulence, the imbalanced fast mode turbulence shows faster cascading as the degree of imbalance increases. We found that the spectral index quickly reaches stationary value of -2. Thus we conclude that the dissipation of fast mode, at least in 1D case, happens not due to weak or strong turbulent cascading, but mostly due to nonlinear steepening.
Analyses of spectra obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite, together with spectra from the Copernicus and IMAPS instruments, reveal an unexplained very wide range in the observed deuterium/hydrogen (D/H) ratios for interstellar gas in the Galactic disk beyond the Local Bubble. We argue that spatial variations in the depletion of deuterium onto dust grains can explain these local variations in the observed gas-phase D/H ratios. We present a variable deuterium depletion model that naturally explains the constant measured values of D/H inside the Local Bubble, the wide range of gas-phase D/H ratios observed in the intermediate regime (log N(H I} = 19.2-20.7), and the low gas-phase D/H ratios observed at larger hydrogen column densities. We consider empirical tests of the deuterium depletion hypothesis: (i) correlations of gas-phase D/H ratios with depletions of the refractory metals iron and silicon, and (ii) correlation with the molecular hydrogen rotational temperature. Both of these tests are consistent with deuterium depletion from the gas phase in cold, not recently shocked, regions of the ISM, and high gas-phase D/H ratios in gas that has been shocked or otherwise heated recently. We argue that the most representative value for the total (gas plus dust) D/H ratio within 1 kpc of the Sun is >=23.1 +/- 2.4 (1 sigma) parts per million (ppm). This ratio constrains Galactic chemical evolution models to have a very small deuterium astration factor, the ratio of primordial to total (D/H) ratio in the local region of the Galactic disk, which we estimate to be f_d <= 1.19 +/-0.16 (1 sigma) or <= 1.12 +/- 0.14 (1 sigma) depending on the adopted light element nuclear reaction rates.
We present ground-based infrared K- (2-2.5 micron) and L-band (2.8-4.1 micron) spectroscopy, as well as interferometric observations at 3mm, for the individual merging components (A, B, and C) of the luminous infrared galaxy Arp 299. We investigate the presence and location of the putative buried active galactic nucleus (AGN) inferred from previous X-ray observations at E > 10 keV. Our sub-arcsec-resolution infrared spectra clearly reveal that the putative buried AGN resides in the nucleus B1 (a subcomponent of B), based on a very low equivalent width of 3.3 micron polycyclic aromatic hydrocarbon emission, a weak 2.3 micron CO absorption feature, and a large time variation of the K- and L-band continuum fluxes. In component C, we find strong 3.1 micron ice absorption at L and weak 2.3 micron CO absorption at K, as expected in a buried AGN; however, a centrally concentrated young super star cluster is an alternative possibility because of the modest infrared luminosity and non-galaxy-nucleus nature of this component. The infrared K- and L-band spectra of the infrared brightest nucleus, A, are typical of a normal starburst with no explicit AGN signatures. Our interferometric observations simultaneously obtain HCN (J=1-0) and HCO+ (J=1-0) emission lines with 4 arcsec resolution, and we find the HCN to HCO+ brightness-temperature ratios to be as low as those found in starburst nuclei in all the major merging components of Arp 299. The low ratio even in the AGN-hosting nucleus B may be due to the presence of a large amount of high-density molecular gas whose chemistry is dominated by coexisting starbursts and/or shocks, rather than by the central strong X-ray-emitting AGN.
Suzaku deep observations have discovered two highly significant nonthermal X-ray sources, Suzaku J1804$-$2142 (Src 1) and Suzaku J1804$-$2140 (Src 2), positionally coincident with the unidentified TeV $\gamma$-ray source, HESS J1804$-$216. The X-ray sources are not time variable and show no counterpart in other wavebands, except for the TeV source. Src 1 is unresolved at Suzaku spatial resolution, whereas Src 2 is extended or composed of multiple sources. The X-ray spectra are highly absorbed, hard, and featureless, and are well fitted by absorbed power-law models with best-fit photon indices and absorption columns of $-0.3_{-0.5}^{+0.5}$ and $0.2_{-0.2}^{+2.0}\times 10^{22}$ cm$^{-2}$ for Src 1, and $1.7_{-1.0}^{+1.4}$ and $1.1_{-0.6}^{+1.0}\times 10^{23}$ cm$^{-2}$ for Src 2. The measured X-ray absorption to the latter source is significantly larger than the total Galactic neutral hydrogen column in that direction. The unabsorbed 2--10 keV band luminosities are $7.5\times 10^{32}(d/{\rm 5 kpc})^2$ ergs s$^{-1}$ (Src 1) and $1.3\times 10^{33}(d/{\rm 5 kpc})^2$ ergs s$^{-1}$ (Src 2), where $d$ is the source distance. Among the handful of TeV sources with known X-ray counterparts, HESS J1804$-$216 has the largest ratio of TeV $\gamma$-ray to hard X-ray fluxes. We discuss the nature of the emission and propose the Suzaku sources as plausible counterparts to the TeV source, although further observations are necessary to confirm this.
"Rotating RAdio Transients" (RRATs) are a newly discovered astronomical phenomenon, characterised by occasional brief radio bursts, with average intervals between bursts ranging from minutes to hours. The burst spacings allow identification of periodicities, which fall in the range 0.4 to 7 seconds. The RRATs thus seem to be rotating neutron stars, albeit with properties very different from the rest of the population. We here present the serendipitous detection with the Chandra X-ray Observatory of a bright point-like X-ray source coincident with one of the RRATs. We discuss the temporal and spectral properties of this X-ray emission, consider counterparts in other wavebands, and interpret these results in the context of possible explanations for the RRAT population.
This thesis investigates the detection of ultra high energy (E > 1 EeV) cosmic neutrinos using acoustic sensors immersed in water. The method is based on the thermoacoustic model describing the production of microsecond bipolar acoustic pulses by neutrino-induced particle cascades. These cascades locally heat the medium which leads to rapid expansion and a short sonic pulse detectable in water with hydrophones over distances of several kilometres. This makes acoustic detection an approach complementary to todays optical Cerenkov and radio Cerenkov detectors, and could help to reduce the respective systematic uncertainties. In this work a complete simulation / reconstruction chain for a submarine acoustic neutrino telescope is developed, and the sensitivity of such a detector to a diffuse flux of ultra highenergy cosmic neutrinos is estimated.
It is now accepted that long duration gamma-ray bursts (GRBs) are produced during the collapse of a massive star. The prevailing model (the collapsar) predicts that a broad-lined and luminous Type Ic core-collapse supernova (SN) accompanies every long-duration GRB. This association has been confirmed in observations of several nearby GRBs. Here we present observations of two nearby long-duration GRBs that challenge the prevailing model. In the GRBs 060505 and 060614 we demonstrate that no SN emission accompanied these bursts, down to limits hundreds of times fainter than the archetypal SN 1998bw that accompanied GRB 980425, and fainter than any Type Ic SN ever observed. Multi-band observations of the early afterglows, as well as spectroscopy of the GRB 060505 host galaxy, exclude the possibility of significant dust obscuration. Both GRBs are long-duration bursts, and we show that they lie in actively star-forming regions, typical of long GRBs. The absence of a SN to such deep limits is qualitatively different from all previous nearby long GRBs and suggests a new phenomenological type of massive stellar death.
Two sequences of solar images obtained by the Transition Region and Coronal Explorer in three UV passbands are studied using wavelet and Fourier analysis and compared to the photospheric magnetic flux measured by the Michelson Doppler Interferometer on the Solar Heliospheric Observatory to study wave behaviour in differing magnetic environments. Wavelet periods show deviations from the theoretical cutoff value and are interpreted in terms of inclined fields. The variation of wave speeds indicates that a transition from dominant fast-magnetoacoustic waves to slow modes is observed when moving from network into plage and umbrae. This implies preferential transmission of slow modes into the upper atmosphere, where they may lead to heating or be detected in coronal loops and plumes.
In this paper we present a correlation between the peak spectral energy of gamma-ray bursts (GRBs) and the peak bolometric luminosity of the underlying supernovae (SNe), based on a sample of four pairs of GRBs-SNe with spectroscopically confirmed connection. Combining it with the well-known relation between the peak spectral energy and the isotropic equivalent energy of GRBs, we obtain an upper limit on the isotropic energy of GRBs, which is \approx 10^{52} erg (L_{SN,peak}/10^{43} erg s^{-1})^{10}, where L_{SN,peak} is the peak bolometric luminosity of the SNe. Our results suggest that the critical parameter determining the GRB-SN connection is the peak luminosity of SNe, rather than the feature of the SN spectra and/or the SN explosion energy as commonly hypothesized. Since it is generally believed that the peak luminosity of SNe powered by radioactive decays is related to the amount of 56Ni produced in the SN explosion, the mass of 56Ni may be a key physical factor for understanding the nature of GRBs and their connection with SNe. Application of our relation to Type Ibc SNe with normal peak luminosities indicates that if those normal SNe have GRBs accompanying them, the GRBs would be extremely soft and sub-energetic in gamma-rays, and hence easier to detect with X-ray or UV detectors than with gamma-ray detectors.
Cosmological long-duration gamma-ray bursts (LGRBs) are thought to originate from the core collapse to black holes of stripped massive stars. Those with sufficient rotation form a centrifugally-supported torus whose collapse powers the GRB. We investigate the role of tidal locking within a tight binary as a source of the necessary angular momentum. We find that the binary orbit must be no wider than a few solar radii for a torus to form upon core collapse. Comparing this criterion to the observed population of binaries containing two compact objects suggests that rotation may have been important in the formation of up to 50% of the observed systems. As these systems created a neutron star and not a black hole they presumably did not produce highly luminous GRBs. We suggest instead that they make the subset of GRBs in the relatively local universe which have much lower luminosity.
We discuss the possible appearance of strange exotic multi-quark states in the interior of neutron stars and signals for the existence of strange quark matter in the core of compact stars. We show how the in-medium properties of possible pentaquark states are constrained by pulsar mass measurements. The possibility of generating the observed large pulsar kick velocities by asymmetric emission of neutrinos from strange quark matter in magnetic fields is outlined.
We review recent findings that the universe on its largest scales shows hints of the violation of statistical isotropy, in particular alignment with the geometry and direction of motion of the solar system, and missing power at scales greater than 60 degrees. We present the evidence, attempts to explain it using astrophysical, cosmological or instrumental mechanisms, and prospects for future understanding.
Aims: To determine whether ``vortex creep'' in neutron stars, the slow motion of neutron vortices with respect to pinning sites in the core or inner crust, is consistent with observations of long-period precession. Methods: Using the concept of vortex drag, I discuss the precession dynamics of a star with imperfectly-pinned (i.e., "creeping'') vortices. Results: The precession frequency is far too high to be consistent with observations, indicating that the standard picture of the inner core (superfluid neutrons in co-existence with type II, superconducting protons) should be reconsidered. There is a slow precession mode, but it is highly over-damped and cannot complete even a single cycle. Moreover, the vortices of the inner crust must be able to move with little dissipation with respect to the solid.
Using XMM we detect faint unresolved X-ray emission from the Compact Steep Spectrum radio galaxy 3C303.1. We detect a thermal component at kT = 0.8 keV which seems likely to be produced in the ISM of the host galaxy. There is evidence for a second component in the spectrum whose nature is currently ambiguous. Plausible hypotheses for the second component include (1) hot gas shocked by the expansion of the radio source, and (2) Synchrotron self-Compton emission from the southern radio lobe if the magnetic field is below the equipartition value by a factor of about 3.5.
We present new optical and X-ray observations to show that the X-ray source 1 WGA J1346.5--6255 previously associated with the SNR G309.2--0.6 can be unequivocally identified with the emission line star HD 119682 located in the foreground open cluster NGC 5281. Images from Chandra in the X-ray band as well as from Magellan in the narrow optical H-alpha band show a coincidence of the source positions within 0.5". The X-ray source appears extremely hot for an OB-star identified as of O9.7e type. XMM-Newton spectra show plasma temperatures of 1 keV and >8 keV with an X-ray luminosity of 6.2E32 +/-0.1E32 erg/s. The optical and X-ray properties are very reminiscent of the prototype emission line star Gamma-Cas. We discuss the ramifications of this similarity with respect to very early type emission line stars as a new class of hard X-ray sources.
Gamma-ray bursts (GRBs) are short, intense flashes of soft gamma radiation coming from the distant Universe. There is now convincing evidence that long-duration GRBs (those lasting more than ~2 s) originate from the deaths of massive stars. This conclusion is mostly based on a handful of outstanding associations between GRBs and supernovae (SNe) of type Ib/c. In particular, the GRB-associated SNe belong to a peculiar subclass, characterized by high luminosity at maximum light (M_V ~ -19, with a dispersion of ~1 mag) and high expansion velocity of the ejecta (v_max ~ 30000 km/s). Such SNe are often labelled "hypernovae". Here we report deep optical observations of GRB 060614 showing no emerging SN brighter than M_V = -13.9, up to 50 days after the burst. Any SN associated with this GRB was therefore at least 100 times fainter at optical wavelengths than the other SNe associated with GRBs. This fact allows us to set a stringent upper limit to the expansion velocity of the SN ejecta v < 3500 km/s. This is exceptionally low, about an order of magnitude smaller than all GRB-associated SNe discovered so far. This is the first clear evidence that long-duration GRBs can be either associated with SNe which do not have hypernova properties or they can be produced by different phenomena.
We have converted the Palomar 60-inch telescope (P60) from a classical night assistant-operated telescope to a fully robotic facility. The automated system, which has been operational since September 2004, is designed for moderately fast (t <~ 3 minutes) and sustained (R <~ 23 mag) observations of gamma-ray burst afterglows and other transient events. Routine queue-scheduled observations can be interrupted in response to electronic notification of transient events. An automated pipeline reduces data in real-time, which is then stored on a searchable web-based archive for ease of distribution. We describe here the design requirements, hardware and software upgrades, and lessons learned from roboticization. We present an overview of the current system performance as well as plans for future upgrades.
Theoretical models predict that the initial metallicity of the progenitor of a Type Ia supernova (SN Ia) affects the peak of the supernova light curve. This can cause a deviation from the standard light curve calibration employed when using SNe Ia as standardizable distance candles and, if there is a systematic evolution of the metallicity of SN Ia progenitors, could affect the determination of cosmological parameters. Here we show that this metallicity effect can be substantially larger than has been estimated previously, when the neutronisation in the immediate pre-explosion phase in the CO white dwarf is taken into account, and quantitatively assess the importance of metallicity evolution for determining cosmological parameters. We show that, in principle, a moderate and plausible amount of metallicity evolution could mimic a lambda-dominated, flat Universe in an open, lambda-free Universe. However, the effect of metallicity evolution appears not large enough to explain the high-z SN Ia data in a flat Universe, for which there is strong independent evidence, without a cosmological constant. We also estimate the systematic uncertainties introduced by metallicity evolution in a lambda-dominated, flat Universe. We find that metallicity evolution may limit the precision with which Omega_m and w can be measured and that it will be difficult to distinguish evolution of the equation of state of dark energy from metallicity evolution, at least from SN Ia data alone.
The formation of brown dwarfs via encounters between proto-stars has been confirmed with high-resolution numerical simulations with a restricted treatment of the thermal conditions. The new results indicate that young brown dwarfs (BDs) formed this way are disk-like and often reside in multiple systems. The newly-formed proto-BDs disks are up to 18 AU in size and spin rapidly making small-scale bipolar outflows, fragmentation and the possible formation of planetary companions likely as have recently been observed for BDs. The object masses range from 2 to 73 Jupiter masses, distributed in a manner consistent with the observed sub-stellar initial mass function. The simulations usually form multiple BDs on eccentric orbits about a star. One such system was hierarchical, a BD binary in orbit around a star, which may explain recently observed hierarchical systems. One third of the BDs were unbound after a few thousand years and interactions among orbiting BDs may eject more or add to the number of binaries. Improvements over prior work include resolution down to a Jupiter mass, self-consistent models of the vertical structure of the initial disks and careful attention to avoid artificial fragmentation.
Based on a new approach on modeling the magnetically dominated outflows from AGNs (Li et al. 2006), we study the propagation of magnetic tower jets in gravitationally stratified atmospheres (such as a galaxy cluster environment) in large scales ($>$ tens of kpc) by performing three-dimensional magnetohydrodynamic (MHD) simulations. We present the detailed analysis of the MHD waves, the cylindrical radial force balance, and the collimation of magnetic tower jets. As magnetic energy is injected into a small central volume over a finite amount of time, the magnetic fields expand down the background density gradient, forming a collimated jet and an expanded ``lobe'' due to the gradually decreasing background density and pressure. Both the jet and lobes are magnetically dominated. In addition, the injection and expansion produce a hydrodynamic shock wave that is moving ahead of and enclosing the magnetic tower jet. This shock can eventually break the hydrostatic equilibrium in the ambient medium and cause a global gravitational contraction. This contraction produces a strong compression at the head of the magnetic tower front and helps to collimate radially to produce a slender-shaped jet. At the outer edge of the jet, the magnetic pressure is balanced by the background (modified) gas pressure, without any significant contribution from the hoop stress. On the other hand, along the central axis of the jet, hoop stress is the dominant force in shaping the central collimation of the poloidal current. The system, which possesses a highly wound helical magnetic configuration, never quite reaches a force-free equilibrium state though the evolution becomes much slower at late stages. The simulations were performed without any initial perturbations so the overall structures of the jet remain mostly axisymmetric.
We present a high-resolution Keck/ESI spectrum of GRB, which exhibits four absorption systems at z=1.04329, 1.95260, 1.96337, and 1.98691. The two highest redshift systems, separated by about 2400 km/s, have been previously suspected as kinematic features arising in the circumstellar wind around the progenitor star. However, the high column densities of low-ionization species (including possibly neutral hydrogen) in the blue-shifted system, are inconsistent with the expected highly ionized state of the circumstellar wind from the massive progenitor star, even prior to the GRB explosion. This conclusion is also supported by the lack of detectable absorption from fine-structure transitions of SiII and FeII. Instead we conclude that the two redshift systems are similar to multiple DLAs found in QSO sight lines with a similar velocity separation and chemical abundance of [Cr/Fe] and [Zn/Fe]. The absorption system at z=1.96337 is likely an intervening low-mass galaxy, possibly related to the GRB host as part of a forming large-scale structure.
Radial velocity (RV) planet searches are increasingly finding planets with small velocity amplitudes, with long orbital periods, or in multiple planet systems. Bayesian inference has the potential to improve the interpretation of existing observations, the planning of future observations and ultimately inferences concerning the overall population of planets. In recent years, the refinement of Markov chain Monte Carlo (MCMC) algorithms has made it practical to accurately characterize orbital parameters and their uncertainties from RV observations of single-planet and weakly interacting multiple-planet systems. Unfortunately, MCMC is not sufficient for Bayesian model selection, i.e., comparing the marginal posterior probability of models, as is necessary to determine how strongly the observational data favor a model with n+1 planets over a model with just n planets. Many of the obvious estimators for the marginal posterior probability suffer from poor convergence properties. We compare several estimators of the marginal likelihood and feature those that display desirable convergence properties based on the analysis of a sample data set for HD 88133b. We find that methods based on importance sampling are most efficient, provided that a good analytic approximation of the posterior probability distribution is available. We present a simple algorithm for using a sample from the posterior to construct a mixture distribution that approximates the posterior and can be used for importance sampling and Bayesian model selection. We conclude with some suggestions for the development and refinement of computationally efficient and robust estimators of marginal posterior probabilities.
Errors in the kinematic distances, under the assumption of circular gas orbits, were estimated by performing synthetic observations of a model disk galaxy. It was found that the error is < 0.5 kpc for most of the disk when the measured rotation curve was used, but larger if the real rotation curve is applied. In both cases, the error is significantly larger at the positions of the spiral arms. The error structure is such that, when kinematic distances are used to develope a picture of the large scale density distribution, the most significant features of the numerical model are significantly distorted or absent, while spurious structure appears. By considering the full velocity field in the calculation of the kinematic distances, most of the original density structures can be recovered.
This presentation is a Moriond version of our recent paper (Kravtsov, Vikhlinin & Nagai astro-ph/0603205) where we discussed X-ray proxies for the total cluster mass, including the spectral temperature (Tx), gas mass measured within r500 (Mg), and the new proxy, Yx, which is a simple product of Tx and Mg. We use mock Chandra images constructed for a sample of clusters simulated with high resolution in the concordance LambdaCDM cosmology. The simulated clusters exhibit tight correlations between the considered observables and total mass. The normalizations of the M500-Tx, Mg-Tx, and M500-Yx relations agree to better than =~ 10-15% with the current observational measurements of these relations. Our results show that Yx is the best mass proxy with a remarkably low scatter of only =~ 5-7% in M500 for a fixed Yx, at both low and high redshifts and regardless of whether clusters are relaxed or not. In addition, we show that redshift evolution of the Yx-M500 relation is close to the self-similar prediction, which makes Yx a very attractive mass indicator for measurements of the cluster mass function from X-ray selected samples.
We present a detailed study of the small frequency separations as diagnostics of the mass of the convective core and evolutionary stage of solar-type stars. We demonstrate how the small separations can be combined to provide sensitive tests for the presence of convective overshoot at the edge of the core. These studies are focused on low degree oscillation modes, the only modes expected to be detected in distant stars. Using simulated data with realistic errors, we find that the mass of the convective core can be estimated to within 5% if the total stellar mass is known. Systematic errors arising due to uncertainty in the mass could be up to 20%. The evolutionary stage of the star, determined in terms of the central hydrogen abundance using our proposed technique, however, is much less sensitive to the mass estimate.
We present a sample of 1777 bright (9<B<14) metal-poor candidates selected from the Hamburg/ESO Survey (HES). Despite saturation effects present in the red portion of the HES objective-prism spectra, the data were recoverable and quantitative selection criteria could be applied to select the sample. Analyses of medium-resolution (~2 A) follow-up spectroscopy of the entire sample, obtained with several 2 to 4 m class telescopes, yielded 145 new metal-poor stars with metallicity [Fe/H]<-2.0, of which 79 have [Fe/H]<-2.5, and 17 have [Fe/H]<-3.0. We also obtained C/Fe estimates for all these stars. From this, we find a frequency of C-enhanced ([C/Fe]>1.0) metal-poor ([Fe/H]<-2.0) giants of 9% +- 2%, which is lower than previously reported. However, the frequency raises to similar (>20%) and higher values with increasing distance from the Galactic plane. Although the numbers of stars at low metallicity are falling rapidly at the lowest metallicities, there is evidence that the fraction of carbon-enhanced metal-poor stars is increasing rapidly as a function of declining metallicity. For ~60 objects, high-resolution data have already been obtained; one of these, HE 1327-2326, is the new record holder for the most iron-deficient star known.
We present critical, long-wavelength observations of Eta Carinae in the submillimetre using SCUBA on the JCMT at 850 and 450 um to confirm the presence of a large mass of warm dust around the central star. We fit a two-component blackbody to the IR-submm spectral energy distribution and estimate between 0.3-0.7 solar masses of dust exists in the nebula depending on the dust absorption properties and the extent of contamination from free-free emission at the SCUBA wavelengths. These results provide further evidence that Eta Carinae's circumstellar nebula contains > 10 solar masses of gas, although this may have been ejected on a longer timescale than previously thought.
We report long-slit spectroscopic observations of the Herbig Ae/Be star Elias 1 in the 3.2-3.6 mum region covering the C-H stretch emission features of hydrogen-terminated diamonds and PAHs. The data were recorded at UKIRT using UIST and yield information on the profiles and intensities of the bands as a function of offset along the N-S and E-W axes centred on the close binary. The diamond and nearby IR continuum emission arises from a symmetrical inner core region (< 0.34 arcsec or 48 AU). The 3.3 mum PAH emission is extended along the E-W axis up to ~ 100 AU each side of the star. This result supports a suggestion of Haas, Leinert & Richichi (1997) of an E-W oriented bipolar nebula in Elias 1.
We present here simultaneous INTEGRAL/RXTE observations of Sco X-1, and in particular a study of the hard X-ray emission of the source and its correlation with the position in the Z-track of the X-ray color-color diagram. We find that the hard X-ray (above about 30 keV) emission of Sco X-1 is dominated by a power-law component with a photon index of ~3. The flux in the power-law component slightly decreases when the source moves in the color-color diagram in the sense of increasing inferred mass accretion rate from the horizontal branch to the normal branch/flaring branch vertex. It becomes not significantly detectable in the flaring branch, where its flux has decreased by about an order of magnitude. These results present close analogies to the behavior of GX 17+2, one of so-called Sco-like Z sources. Finally, the hard power law in the spectrum of Sco X-1 does not show any evidence of a high energy cutoff up to 100 - 200 keV, strongly suggesting a non-thermal origin of this component.
We present the detection and modeling of more than 70 far-IR pure rotational lines of water vapor, including the 18O and 17O isotopologues, towards Orion KL. Observations were performed with the Long Wavelength Spectrometer Fabry-Perot (LWS/FP; R~6800-9700) on board the Infrared Space Observatory (ISO) between ~43 and ~197 um. The water line profiles evolve from P-Cygni type profiles (even for the H2O18 lines) to pure emission at wavelengths above ~100 um. We find that most of the water emission/absorption arises from an extended flow of gas expanding at 25+-5 kms^-1. Non-local radiative transfer models show that much of the water excitation and line profile formation is driven by the dust continuum emission. The derived beam averaged water abundance is 2-3x10^-5. The inferred gas temperature Tk=80-100 K suggests that: (i) water could have been formed in the "plateau" by gas phase neutral-neutral reactions with activation barriers if the gas was previously heated (e.g. by shocks) to >500 K and/or (ii) H2O formation in the outflow is dominated by in-situ evaporation of grain water-ice mantles and/or (iii) H2O was formed in the innermost and warmer regions (e.g. the hot core) and was swept up in ~1000 yr, the dynamical timescale of the outflow.
Low metallicity very massive stars with an initial mass between 140 Mo and 260 Mo can be subdivided into two groups: those between 140 Mo and 200 Mo which produce a relatively small amount of Fe, and those with a mass between 200 Mo and 260 Mo where the Fe-yield ejected during the supernova explosion is enormous. We first demonstrate that the inclusion of the second group into a chemical evolutionary model for the Solar Neighbourhood predicts an early temporal evolution of Fe which is at variance with observations whereas it can not be excluded that the first group could have been present. We then show that a low metallicity binary with very massive components (with a mass corresponding to the first group) can be an efficient site of primary 14N production through the explosion of a binary component that has been polluted by the pair instability supernova ejecta of its companion. When we implement these massive binary 14N yields in a chemical evolution model, we conclude that very massive close binaries may be important sites of 14N enrichment during the early evolution of the Galaxy.
We analyse high-resolution archival UVES data of turnoff and subgiant stars in the nearby globular cluster NGC 6397 ([Fe/H] = -2). Balmer-profile analyses are performed to derive reddening-free effective temperatures. Due to the limited S/N and uncertainties related to blaze removal, we find the data quality insufficient to exclude the existence of gravitational settling. If the newly derived effective temperatures are taken as a basis for an abundance analysis, the photospheric iron (Fe II) abundance in the turnoff stars is 0.11 dex lower than in the (well-mixed) subgiants.
We have carried-out multi-configuration Breit-Pauli AUTOSTRUCTURE calculations for the dielectronic recombination (DR) of Fe^{8+} - Fe^{12+} ions. We obtain total DR rate coefficients for the initial ground-level which are an order of magnitude larger than those corresponding to radiative recombination (RR), at temperatures where Fe 3p^q (q=2-6) ions are abundant in photoionized plasmas. The resultant total (DR+RR) rate coefficients are then an order of magnitude larger than those currently in use by photoionized plasma modeling codes such as CLOUDY, ION and XSTAR. These rate coefficients, together with our previous results for q=0 and 1, are critical for determining the ionization balance of the M-shell Fe ions which give rise to the prominent unresolved-transition-array X-ray absorption feature found in the spectrum of many active galactic nuclei. This feature is poorly described by CLOUDY and ION, necessitating an ad hoc modification to the low-temperature DR rate coefficients. Such modifications are no longer necessary and a rigorous approach to such modeling can now take place using these data.
KASCADE-Grande is a new extensive air shower experiment co-located to the KASCADE site at Forschungszentrum Karlsruhe. The multi-detector system allows to investigate the energy spectrum, composition, and anisotropies of cosmic rays with unprecedented prevision in the energy range from 10^{14}-10^{18} eV. The primary goals besides investigating the origin of the knee at E ~ 3 * 10^{15} eV, are to verify the existence of the second knee at E ~ 10^{17} eV and to measure the composition in the expected transition region of galactic to extragalactic cosmic rays. The performance of the apparatus and shower reconstruction methods will be discussed on the basis of detailed Monte Carlo simulations and first data. First results based on slightly more than a year of data taking are presented.
The astrophysical p process, which is responsible for the origin of the proton rich stable nuclei heavier than iron, was investigated using a full nuclear reaction network for a type II supernova explosion when the shock front passes through the O/Ne layer. Calculations were performed with a multi-layer model adopting the seed of a pre-explosion evolution of a 25 solar mass star. The reaction flux was calculated to determine the main reaction path and branching points responsible for synthesizing the proton rich nuclei. In order to investigate the impact of nuclear reaction rates on the predicted p-process abundances, extensive simulations with different sets of collectively and individually modified neutron-, proton-, alpha-capture and photodisintegration rates have been performed. These results are not only relevant to explore the nuclear physics related uncertainties in p-process calculations but are also important for identifying the strategy and planning of future experiments.
We investigate the evolution and the statistical properties of the Lya absorbers of the intergalactic medium (IGM) in the largely unexplored redshift range z=0.5-1.9. We use high-resolution (R > 30,000) UV (STIS) and optical (VLT/UVES and Keck/HIRES) spectra of nine bright quasars with z_em < 1.94. The main results for the combined Lya line sample are summarized as follows: 1. The evolution of the number density of the absorbers can be described by the power law dn/dz ~ (1+z)^gamma. The number density of the low column density lines decreases with decreasing z with gamma=0.74+-0.31 in the interval z=0.7-1.9. A comparison with results at higher redshifts shows that it is decelerated in the explored redshift range and turns into a flat evolution for z -> 0. The stronger absorbers thin out faster (gamma=1.50+-0.45). The break in their evolution predicted for z=1.5-1.7 cannot be seen down to z=0.7. On the other hand, a comparison with values from the literature for the local number density gives a hint that this break occurs at lower redshift. 2. The distribution of the column densities of the absorbers is complete down to N_HI=10^12.90 cm^-2. It can be approximated by a single power law with the exponent beta=1.60+-0.03 over almost three orders of magnitude. beta is redshift independent. 3. The Lya lines with lower column densities as well as the higher column density lines show marginal clustering with a 2 sigma significance over short distances (Delta v < 200 km/s and Delta v < 100 km/s, respectively). We do not see any difference in the clustering with either column density or redshift.
In its first part, this paper summarizes recent work on the mass and shape of the Galactic dark halo. The second part presents a review of the large-scale structure of the Milky Way, and of the evidence that the inner Galaxy is dominated by baryonic matter. This is briefly compared with the predictions of LCDM and MOND. Finally, a summary is given of bulge formation from clumpy, gas-rich disks, a process which may give rise to old, disk-like, alpha-rich bulges similar to the Galactic bulge.
[Abridged] Over the past two decades observations and theoretical simulations have established a global frame-work of galaxy formation and evolution in the young Universe. Galaxies formed as baryonic gas cooled at the centres of collapsing dark matter halos. Mergers of halos led to the build up of galaxy mass. A major step forward in understanding these issues requires well resolved physical information on individual galaxies at high redshift. Here we report adaptive optics, spectroscopic observations of a representative luminous star forming galaxy when the Universe was only twenty percent of its age. The superior angular resolution of these data reveals the physical and dynamical properties of a high redshift galaxy in unprecedented detail. A large and massive rotating proto-disk is channelling gas towards a growing central stellar bulge hosting an accreting massive black hole.
The existence of deconfined quark matter in the superdense interior of neutron stars is a key question that has drawn considerable attention over the past few decades. Quark matter can comprise an arbitrary fraction of the star, from 0 for a pure neutron star to 1 for a pure quark star, depending on the equation of state of matter at high density. From an astrophysical viewpoint, these two extreme cases are generally expected to manifest different observational signatures. An intermediate fraction implies a hybrid star, where the interior consists of mixed or homogeneous phases of quark and nuclear matter, depending on surface and Coulomb energy costs, as well as other finite size and screening effects. In this brief review article, we discuss what we can deduce about quark matter in neutron stars in light of recent exciting developments in neutron star observations. We state the theoretical ideas underlying the equation of state of dense quark matter, including color superconducting quark matter. We also highlight recent advances stemming from re-examination of an old paradigm for the surface structure of quark stars and discuss possible evolutionary scenarios from neutron stars to quark stars, with emphasis on astrophysical observations.
In order to simulate the effects of energetic charged particles present in the solar wind colliding with the cometary gaseous formic acid molecule (HCOOH), laboratory experiments have been performed. The absolute ionisation and dissociation cross sections for this molecule interacting with solar wind particles were measured employing fast electrons in the energy range of 0.5 to 2 keV and energetic protons with energies varying from 0.128 to 2 MeV. Despite the fact that both projectiles lead to a very similar fragmentation pattern, differences in the relative intensities of the fragments were observed. Formic acid survives about 4-5 times more to the proton beam than to the energetic electron collision.The minimum momentum transfer in the electron impact case was estimated to be 3-38% larger than the minimum momentum transfer observed with the equivelocity protons. The UV photodissociation rates and half-lives for HCOOH are roughly closer to the values obtained with energetic electrons. It is consequently important to take electron impact data into account when developing chemical models to simulate the interplanetary conditions.
The evidence for a shortage of exosolar planets with semimajor axes -1.1 <= log (a/AU) <= -0.2 is investigated. It is shown that this valley results from a gap in the radial distribution of planets, orbiting stars with masses M >= 1.2 Msolar (the high-mass sample, HMS). No underabundance is found for planets orbiting stars with smaller masses. The observational data also indicate that within the HMS population it is preferentially the more massive planets with M sin(i) >= 0.8 M_J that are missing. Monte-Carlo simulations of planet formation and migration are presented that reproduce the observed shortage of planets in the observed radius regime. A dependence on the disk depletion timescale tau_dep is found. The gap is more pronounced for tau_dep = 10^6 - 10^7 yrs than for tau_dep = 3*10^6 - 3*10^7 yrs. This might explain the observed trend with stellar mass if disks around stars with masses M* >= 1.2 Msolar have shorter depletion timescales than those around less massive stars. Possible reasons for such a dependence are a decrease of disk size and an increase of stellar EUV flux with stellar mass.
We establish that global solar p-mode frequencies can be measured with sufficient precision on time scales as short as nine days to detect activity-related shifts. Using ten years of GONG data, we report that mode-mass and error-weighted frequency shifts derived from nine days are significantly correlated with the strength of solar activity and are consistent with long duration measurements from GONG and the MDI/SOHO instrument. However, the correlation varies from year to year and appears to be a complex phenomena. For the short-duration observations, the analysis indicates a higher sensitivity to activity. We also find that magnetic indices behave differently in the rising and falling phases of the activity cycle.
A variety of on-orbit imaging and spectroscopic observations are used to characterize the Charge Transfer Efficiency (CTE) of the Charge-Coupled Device (CCD) of the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope. A set of formulae is presented to correct observations of point sources for CTE-related loss of signal. For data taken in imaging mode, the CTE loss is parametrized in terms of the location of the source on the CCD, the source signal level within the measurement aperture, the background level, and the time of observation. For spectroscopic data, it is found that one additional parameter is needed to provide an adequate calibration of the CTE loss, namely the signal in the point spread function located between the signal extraction box and the read-out amplifier. The effect of the latter parameter is significant for spectra taken using the G750L or G750M gratings of STIS. The algorithms presented here correct flux calibration inaccuracies due to CTE losses as large as 30% to within ~ 1.5% RMS throughout the wavelength range covered by the STIS CCD modes. This uncertainty is similar to the Poisson noise associated with a source detected at a signal level of about 2500 electrons per resolution element. Using bi-directional CCD readouts, centroid shifts incurred due to CTE loss are also derived. A tight correlation is found between the CTE loss and the centroid shift (both for imaging and spectroscopic modes), thus enabling one to correct for both effects of imperfect charge transfer to STIS CCD observations.
Disk galaxies in a cluster of galaxies are moving in hot gas filling the cluster. Generally, they are moving at transonic or supersonic velocities. If ram-pressure stripping is insufficient to destroy the gas disk, the galaxies should be affected by the wind of the surrounding hot gas similar to an airfoil. In this paper, I consider whether the aerodynamic interaction can be strong enough to force a disk galaxy to deviate from the orbit that it would have been in. I find that while the lift force is not effective, the drag force could affect face-on disk galaxies in poor clusters on long timescales.
Recently, the Heidelberg-Moscow double beta decay experiment has claimed a detection for a neutrino mass with high significance. Here we consider the impact of this measurement on the determination of the dark energy equation of state. By combining the Heidelberg-Moscow result with the WMAP 3-years data and other cosmological datasets we constrain the equation of state to -1.67< w <-1.05 at 95% c.l., ruling out a cosmological constant at more than 95% c.l.. Interestingly enough, coupled neutrino-dark energy models may be consistent with such equation of state. While future data are certainly needed for a confirmation of the controversial Heildelberg-Moscow claim, our result shows that future laboratory searches for neutrino masses may play a crucial role in the determination of the dark energy properties.
We have carried out a sensitive high-resolution imaging survey of stars in the young (6-8 Myr), nearby (97 pc) compact cluster around eta Chamaeleontis to search for stellar and sub-stellar companions. Given its youth and proximity, any sub-stellar companions are expected to be luminous, especially in the near infrared, and thus easier to detect next to their parent stars. Here, we present VLT/NACO adaptive optics imaging with companion detection limits for 17 eta Cha cluster members, and follow-up VLT/ISAAC near-infrared spectroscopy for companion candidates. The widest binary detected is ~0.2", corresponding to the projected separation 20 AU, despite our survey being sensitive down to sub-stellar companions outside 0.3", and planetary mass objects outside 0.5". This implies that the stellar companion probability outside 0.3" and the brown dwarf companion probability outside 0.5" are less than 0.16 with 95% confidence. We compare the wide binary frequency of eta Cha to that of the similarly aged TW Hydrae association, and estimate the statistical likelihood that the wide binary probability is equal in both groups to be < 2e-4. Even though the eta Cha cluster is relatively dense, stellar encounters in its present configuration cannot account for the relative deficit of wide binaries. We thus conclude that the difference in wide binary probability in these two groups provides strong evidence for multiplicity properties being dependent on environment. In two appendices we derive the projected separation probability distribution for binaries, used to constrain physical separations from observed projected separations, and summarize statistical tools useful for multiplicity studies.
We determine what aspects of the density field surrounding galaxies most affect their properties. For Sloan Digital Sky Survey galaxies, we measure the group environment, meaning the host group luminosity and the distance from the group center (hereafter, ``groupocentric distance''). For comparison, we measure the surrounding density field on scales ranging from 100 kpc/h to 10 Mpc/h. We use the relationship between color and group environment to test the null hypothesis that only the group environment matters, searching for a residual dependence of properties on the surrounding density. Generally, red galaxies are slightly more clustered on small scales (about 100--300 kpc/h) than the null hypothesis predicts, possibly indicating that substructure within groups has some importance. At large scales (> 1 Mpc/h), the actual projected correlation functions of galaxies are biased at less than the 5% level with respect to the null hypothesis predictions. We exclude strongly the converse null hypothesis, that only the surrounding density (on any scale) matters. These results generally encourage the use of the halo model description of galaxy bias, which models the galaxy distribution as a function of host halo mass alone. We compare these results to proposed galaxy formation scenarios within the Cold Dark Matter cosmological model.
We report the discovery of an eclipsing X-ray binary in M101, the first such system to be discovered outside the Local Group. Based on a sequence of 25 Chandra observations that sample a wide range of orbital phases, we find a period of 32.688 $\pm$ 0.002 hours, which we interpret as an orbital period. The folded light curve exhibits an eclipse lasting about 8 hours, suggesting a compact orbit in an nearly edge-on configuration. The X-ray binary has an average luminosity of $L_X(0.3-8 {\rm keV}) \approx 1.3\times10^{38}$ erg/sec, with only one out of the 25 observations significantly lower in flux than the average light curve.The presence of the eclipse and the $\sim$ 1.4-day orbital period suggests that this source is an analog of the well studied eclipsing X-ray binary pulsars Her X-1 or LMC X-4. Combining the Chandra data and the HST ACS/WFC images, we have identified several possible optical counterparts, including an O5-O3 star with V = 25.0. Follow-up optical monitoring observations should be able to identify the donor and further constrain the orbital properties.
We investigate the interaction between a giant planet and a viscous circumstellar disk by means of high-resolution, two-dimensional hydrodynamical simulations. We consider planet masses that range from 1 to 3 Jupiter masses (Mjup) and initial orbital eccentricities that range from 0 to 0.4. We find that a planet can cause eccentricity growth in a disk region adjacent to the planet's orbit, even if the planet's orbit is circular. Disk-planet interactions lead to growth in a planet's orbital eccentricity. The orbital eccentricities of a 2 Mjup and a 3 Mjup planet increase from 0 to 0.11 within about 3000 orbits. Over a similar time period, the orbital eccentricity of a 1 Mjup planet grows from 0 to 0.02. For a case of a 1 Mjup planet with an initial eccentricity of 0.01, the orbital eccentricity grows to 0.09 over 4000 orbits. Radial migration is directed inwards, but slows considerably as a planet's orbit becomes eccentric. If a planet's orbital eccentricity becomes sufficiently large, e > ~0.2, migration can reverse and so be directed outwards. The accretion rate towards a planet depends on both the disk and the planet orbital eccentricity and is pulsed over the orbital period. Planet mass growth rates increase with planet orbital eccentricity. For e~0.2 the mass growth rate of a planet increases by approximately 30% above the value for e=0. For e > ~0.1, most of the accretion within the planet's Roche lobe occurs when the planet is near the apocenter. Similar accretion modulation occurs for flow at the inner disk boundary which represents accretion toward the star.
New NTT/SOFI imaging and spectroscopy of the Wolf-Rayet population in Westerlund 1 are presented. Narrow-band near-IR imaging together with follow up spectroscopy reveals four new WR stars, of which three were independently identified recently by Groh et al., bringing the confirmed WR content to 24 (23 excluding source S) [..] A quantitative near-IR spectral classification scheme for WR stars is presented and applied to members of Westerlund 1. Late subtypes are dominant, with no subtypes earlier than WN5 or WC8 for the nitrogen and carbon sequences, respectively. A qualitative inspection of the WN stars suggests that most (75%) are highly H-deficient. The WR binary fraction is high (>62%), on the basis of dust emission from WC stars, in addition to a significant WN binary fraction from hard X-ray detections according to Clark et al. We exploit the large WN population of Westerlund 1 to reassess its distance (~5.0kpc) and extinction (A_Ks ~ 0.96 mag), such that it is located at the edge of the Galactic bar, [..]. The observed ratio of WR stars to red and yellow hypergiants, N(WR)/N(RSG+YHG)~3, favours an age of 4.5-5.0 Myr, with individual WR stars descended from progenitors of initial mass ~ 40-55 Msun. Qualitative estimates of current masses for non-dusty, H-free WR stars are presented, revealing 10-18 Msun, such that ~75% of the initial stellar mass has been removed via stellar winds or close binary evolution. We present a revision to the cluster turn-off mass for other Milky Way clusters in which WR stars are known, based upon the latest temperature calibration for OB stars. Finally, comparisons between the observed WR population and subtype distribution in Westerlund 1 and instantaneous burst evolutionary synthesis models are presented.
We present a study of the natal core harboring the class 0 protostar GF9-2 in the filamentary dark cloud GF 9 (d = 200 pc). GF9-2 stands unique in the sense that it shows H2O maser emission, a clear signpost of protostar formation, whereas it does not have a high-velocity large-scale molecular outflow evidenced by our deep search for CO wing emission. These facts indicate that GF9-2 core is early enough after star formation so that it still retains some information of initial conditions for collapse. Our 350 um dust continuum emission image revealed the presence of a protostellar envelope in the center of a molecular core. The mass of the envelope is ~0.6 Msun from the 350 um flux density, while LTE mass of the core is ~3 Msun from moleuclar line observations. Combining visibility data from the OVRO mm-array and the 45m telescope, we found that the core has a radial density profile of $\rho(r)\propto r^{-2}$ for 0.003 < r/pc < 0.08 region. Molecular line data analysis revealed that the velocity width of the core gas increases inward,while the outermost region maintains a velocity dispersion of a few times of the ambient sound speed. The broadened velocity width can be interpreted as infall. Thus, the collapse in GF9-2 is likely to be described by an extension of the Larson-Penston solution for the period after formation of a central star. We derived the current mass accretion rate of ~3E-05 Msun/year from infall velocity of ~ 0.3 km/s at r~ 7000 AU. All results suggest that GF9-2 core has been undergoing gravitational collapse for ~ 5000 years since the formation of central protostar(s), and that the unstable state initiated the collapse ~2E+05 years (the free-fall time) ago.
Context: We investigate mid-infrared and X-ray properties of the dusty torus
in unification scenarios for active galactic nuclei.
Aims: We use the relation between mid IR and hard X-ray luminosities to
constrain AGN unification scenarios.
Methods: With VISIR at the VLT, we have obtained the currently highest
angular resolution (0".35 FWHM) narrow-band mid infrared images of the nuclei
of 8 nearby Seyfert galaxies. Combining these observations with X-ray data from
the literature we study the correlation between their mid IR and hard X-ray
luminosities.
Results: We find that the rest frame 12.3 mircon (L_MIR) and 2-10 keV (L_X)
luminosities are correlated at a highly significant level. The best fit
power-law to our data is log L_MIR \propto (1.60 \pm 0.22) log L_X, showing a
much smaller dispersion than earlier studies.
Conclusions: The similarity in the og L_MIR / log L_X ratio between Sy1s and
Sy2s even using high angular resolution MIR data implies that the similarity is
intrinsic to AGN and not caused by contamination from extra-nuclear emission.
This supports clumpy torus models. The exponent of the correlation constrains
the inner geometry of the torus.
A planet is an end product of disk accretion around a primary star or substar. I quantify this definition by the degree to which a body dominates the other masses that share its orbital zone. Both theoretical and observational measures of dynamical dominance reveal a gap of five orders of magnitude separating the eight planets of our solar system from the populations of asteroids and comets. This simple definition dispenses with upper and lower mass limits for a planet. It reflects the tendency of disk evolution in a mature system to produce a small number of relatively large bodies (planets) in non-intersecting or resonant orbits, which prevent collisions between them.
We review theoretical developments in studies of dense matter and its phase structure of relevance to compact stars. Observational data on compact stars, which can constrain the properties of dense matter, are presented critically and interpreted.
A description of the light curves of microlensed Type Ia supernovae (SNe Ia) as extended and expanding sources is presented. We give examples of what microlensing by stellar-mass Chang-Refsdal lenses can do to a small percentage of supernova light curves. We find that in addition to overall brightening, significant changes in light--curve shapes can also occur. Peaks can be distorted, plateaus can appear, and even secondary peaks can be formed. The effects of both the relative motion of the lens and the supernova and the expansion of the supernova are given and compared. The effects of relative motion are more pronounced when a distant supernova ($z_s \sim 1$) impacts well within the Einstein ring of a nearby microlens ($z_d \sim 0.05$) and are less important for more distant deflectors. We also find that the increase in shear that comes with increased deflector distance tends to reduce the time variability of microlensing. We briefly discuss the probability of observing these effects.
We present the results of a coronagraphic imaging search for extra-solar planets around the young main-sequence stars, epsilon Eri and Vega. Concentrating the stellar light into the core of the point spread function by the adaptive optic system and blocking the core by the occulting mask in the coronagraph, we have achieved the highest sensitivity for point sources in close vicinity of the both central stars. Nonetheless we had no confidential detection of a point source around the stars. The observations give the upper limits on the masses of the planets to 4 -- 6 Jupiter mass and 5 -- 10 Jupiter mass at a few arcsecond from epsilon Eri and Vega, respectively. Diffuse structures are also not detected around both stars.
We present a physical framework that can account for most of the observed spectral properties of the prompt gamma-ray burst emission. This includes the variety of spectral shapes, shape evolutions, and spectral correlations between flux and spectral peak, within bursts described by Borgonovo & Ryde, and among bursts described by Amati/Ghirlanda. In our proposed model the spectral peak is given by the photospheric emission from a relativistic outflow for which the horizon length is much smaller than the radial width. The observed duration of the thermal flash will be given by the radial light-crossing time. This then gives that the typical emission site is at ~10e11 cm from the center, with a Lorentz factor of ~300. This emission is accompanied by non-thermal emission from dissipation locations outside the photosphere. The relative strength of these two components depend on injection effects at the central engine leading to varying relative location of the saturation and photospheric radii. The total emission can then reproduce the observed variety. The spectral correlations are found by assuming that the amount of energy dissipated depends non-linearly on the averaged particle density. Beside the spectral correlations this also gives a description of how the relative strength of the thermal component varies with temperature within a burst.
All the confirmed Soft Gamma-ray Repeaters have been observed with the EPIC instrument on the XMM-Newton satellite. We review the results obtained in these observations, providing the most accurate spectra on the persistent X-ray emission in the 1-10 keV range for these objects, and discuss them in the context of the magnetar interpretation.
We probe the scale dependence of the primordial spectrum in the light of the three-year WMAP (WMAP3) alone and WMAP3 in combination with the other cosmological observations such as galaxy clustering and Type Ia Supernova (SNIa). We pay particular attention to the combination with the Lyman $\alpha$ (Ly$\alpha$) forest. Different from the first-year WMAP (WMAP1), WMAP3's preference on the running of the scalar spectral index on the large scales is now fairly independent of the low CMB multipoles $\ell$. A combination with the galaxy power spectrum from the Sloan Digital Sky Survey (SDSS) prefers a negative running to larger than 2$\sigma$, regardless the presence of low $\ell$ CMB ($2\le \ell \le 23$) or not. On the other hand if we focus on the Power Law $\Lambda$CDM cosmology with only six parameters (matter density $\Omega_m h^2$, baryon density $\Omega_b h^2$, Hubble Constant $H_0$, optical depth $\tau$, the spectral index, $n_s$, and the amplitude, $A_s$, of the scalar perturbation spectrum) when we drop the low $\ell$ CMB contributions WMAP3 is consistent with the Harrison-Zel'dovich-Peebles scale-invariant spectrum ($n_s=1$ and no tensor contributions) at $\sim 1\sigma$. When assuming a simple power law primordial spectral index or a constant running, in case one drops the low $\ell$ contributions ($2\le \ell \le 23$) WMAP3 is consistent with the other observations better, such as the inferred value of $\sigma_8$. We also find, using a spectral shape with a minimal extension of the running spectral index model, LUQAS$+$ CROFT Ly$\alpha$ and SDSS Ly$\alpha$ exhibit somewhat different preference on the spectral shape.
We investigate the suggestion that there are stellar populations in some globular clusters with enhanced helium (Y from 0.28 to 0.40) compared to the primordial value. We assume that a previous generation of massive Asymptotic Giant Branch (AGB) stars have polluted the cluster. Two independent sets of AGB yields are used to follow the evolution of helium and CNO using a Salpeter initial mass function (IMF) and two top-heavy IMFs. In no case are we able to produce the postulated large Y ~ 0.35 without violating the observational constraint that the CNO content is nearly constant.
We propose a quantitative concept for the lower planetary boundary, requiring that a planet must keep its atmosphere in vacuum. The solution-set framework of Pecnik and Wuchterl (2005) enabled a clear and quantitative criterion for the discrimination of a planet and a minor body. Using a simple isothermal core-envelope model, we apply the proposed planetary criterion to the large bodies in the Solar System.
We investigate the environmental dependence of galaxy population properties in a complete volume-limited sample of 91566 galaxies in the redshift range 0.05 <= z <= 0.095 and with M_r <= -20.0, selected from the Sloan Digital Sky Survey (SDSS) Data Release 4. Our aim is to search for systematic variations in the properties of galaxies with the local galaxy density. In particular, we analize how the (u - r) color index and the morphological type of galaxies (the latter evaluated through the SDSS Eclass and FracDev parameters) are related to the environment and to the luminosity of galaxies, in order to find hints that can be related to the presence of a ''void'' galaxy population. Void galaxies are identified through a highly selective criterion, which takes also into account redshift and allows us to exclude from the sample all the galaxies that are not really close to the center of underdense regions. We study the (u - r) color distribution for galaxies in different luminosity bins, and we look for correlations of color with the environment, the luminosity, and the morphological type of the galaxies. We find that galaxies in underdense regions (voids) have lower luminosity (M_r > -21) and are bluer than cluster galaxies. The transition from overdense to underdense environments is smooth, the fraction of late-type galaxies decreases while the fraction of early-type galaxies increases smoothly from underdense to dense environments. We do not find any sudden transition in the galaxy properties with density, which, according to a suggestion by Peebles (2001), should mark the transition to a population of "void" galaxies in LCDM models. On the contrary, our results suggest a continuity of galaxy properties, from voids to clusters.
Aims: We discuss the applicability and reliability of the shapelet technique for scientific image analysis. Methods: We quantify the effects of non-orthogonality of sampled shapelet basis functions and misestimation of shapelet parameters. We perform the shapelet decomposition on artificial galaxy images with underlying shapelet models and galaxy images from the GOODS survey, comparing the publicly available IDL implementation with our new C++ implementation. Results: Non-orthogonality of the sampled basis functions and misestimation of the shapelet parameters can cause substantial misinterpretation of the physical properties of the decomposed objects. Additional constraints and image preprocessing have to be incorporated in order to achieve reliable decomposition results.
As part of the DART project we have used the ESO/2.2m Wide Field Imager in conjunction with the VLT/FLAMES GIRAFFE spectrograph to study the detailed properties of the resolved stellar population of the Fornax dwarf spheroidal galaxy out to and beyond its tidal radius. We re-derived the structural parameters of the Fornax dwarf spheroidal using our wide field imaging covering the galaxy out to its tidal radius, and analysed the spatial distribution of the Fornax stars of different ages as selected from Colour-Magnitude Diagram analysis. We have obtained accurate velocities and metallicities from spectra in the CaII triplet wavelength region for 562 Red Giant Branch stars which have velocities consistent with membership in Fornax dwarf spheroidal. We have found evidence for the presence of at least three distinct stellar components: a young population (few 100 Myr old) concentrated in the centre of the galaxy, visible as a Main Sequence in the Colour-Magnitude Diagram; an intermediate age population (2-8 Gyr old); and an ancient population (> 10Gyr),which are distinguishable from each other kinematically, from the metallicity distribution and in the spatial distribution of stars found in the Colour-Magnitude Diagram. From our spectroscopic analysis we find that the ``metal rich'' stars ([Fe/H]> -1.3) show a less extended and more concentrated spatial distribution, and display a colder kinematics than the ``metal poor'' stars ([Fe/H<-1.3). There is tentative evidence that the ancient stellar population in the centre of Fornax does not exhibit equilibrium kinematics. This could be a sign of a relatively recent accretion of external material.
We calculate the thermal conductivity of electrons produced by electron-electron Coulomb scattering in a strongly degenerate electron gas taking into account the Landau damping of transverse plasmons. The Landau damping strongly reduces this conductivity in the domain of ultrarelativistic electrons at temperatures below the electron plasma temperature. In the inner crust of a neutron star at temperatures T < 1e7 K this thermal conductivity completely dominates over the electron conductivity due to electron-ion (electron-phonon) scattering and becomes competitive with the the electron conductivity due to scattering of electrons by impurity ions.
Persistent high-energy emission of magnetars is produced by a plasma corona around the neutron star, with total energy output of ~10^{36}erg/s. The corona forms as a result of occasional starquakes that twist the external magnetic field of the star and induce electric currents in the closed magnetosphere. Once twisted, the magnetosphere cannot untwist immediately because of its self-induction. The self-induction electric field lifts particles from the stellar surface, accelerates them, and initiates avalanches of pair creation in the magnetosphere. The created plasma corona maintains the electric current demanded by curl(B) and regulates the self-induction e.m.f. by screening. This corona persists in dynamic equilibrium: it is continually lost to the stellar surface on the light-crossing time and replenished with new particles. In essence, the twisted magnetosphere acts as an accelerator that converts the toroidal field energy to particle kinetic energy. The voltage along the magnetic field lines is maintained near threshold for ignition of pair production, in the regime of self-organized criticality. The voltage is found to be about 1 GeV which is in agreement with the observed dissipation rate \~10^{36}erg/s. The coronal particles impact the solid crust, knock out protons, and regulate the column density of the hydrostatic atmosphere of the star. The transition layer between the atmosphere and the corona is the likely source of the observed 100-keV emission from magnetars. The corona also emits curvature radiation up to 10^{14}Hz and can supply the observed IR-optical luminosity.
We calculate the current spatial distribution of projectile delivery to the Earth and Moon using numerical orbital dynamics simulations of candidate impactors drawn from a debiased Near-Earth-Object (NEO) model. Surprisingly, we find that the average lunar impact velocity is 20 km/s, which has ramifications for converting observed crater densities to impactor size distributions. We determine that current crater production on the leading hemisphere of the Moon is 1.29 +/- 0.01 that of the trailing when considering the ratio of craters within 30 degrees of the apex to those within 30 degrees of the antapex and that there is virtually no nearside-farside asymmetry. As expected, the degree of leading-trailing asymmetry increases when the Moon's orbital distance is decreased. We examine the latitude distribution of impactor sites and find that for both the Earth and Moon there is a small deficiency of time-averaged impact rates at the poles. The ratio between deliveries within 30 degrees of the pole to that of a 30 degree band centered on the equator is nearly unity for Earth (<1%)(0.992 +/- 0.001) but detectably non-uniform for the Moon (~10%)(0.912 +/- 0.004). The terrestrial arrival results are examined to determine the degree of AM/PM asymmetry to compare with meteorite fall times (of which there seems to be a PM excess). Our results show that the impact flux of objects derived from the NEOs in the AM hours is ~2 times that of the PM hemisphere, further supporting the assertion that meteorite-dropping objects are recent ejections from the main asteroid belt rather than young fragments of NEOs.
The magnetorotational instability (MRI) is generally accepted as the physical cause of turbulence and enhanced transport of angular momentum in accretion disks, which are one of the most basic and ubiquitous astrophysical structures. We show, by means of a perturbative weakly nonlinear analysis, that the axisymmetric MRI of a viscous, resistive, incompressible rotating shear flow in a thin channel gives rise to a Landau equation for the disturbance amplitude. For small magnetic Prandtl number (${\cal P}_{\rm m}$), the saturation amplitude is $\propto \sqrt{{\cal P}_{\rm m}}$ and the resulting momentum transport scales as ${\cal R}^{-1}$, where $\cal R$ is the hydrodynamic Reynolds number. Simplifying assumptions, such as linear shear base flow, mathematically expedient boundary conditions and continuous spectrum of the vertical linear modes, are used to facilitate this analysis. The asymptotic results are shown to comply with numerical calculations using a spectral code. They suggest that the transport due to the nonlinearly developed MRI may be very small, in experimental setups with ${\cal P}_{\rm m} \ll 1$.
We present a numerical study of the formation of dense cloud complexes and of stars within them via the collision of two opposite streams of self-gravitating, thermally bistable diffuse interstellar gas. We find that: a) The clouds are NOT in a state of equilibrium. Instead, they are continually evolving, increasing their mass and gravitational energy Eg, until the latter becomes comparable to the turbulent energy Ek, at which time global, and later local, collapse set in. b) After this time, the cloud begins to contract gravitationally as a whole, producing a simultaneous increase in |Eg| and Ek, satisfying a near-equipartition condition |Eg|~2Ek, a result that explains the apparent ``virialized'' state of MCs. c) Longer inflow durations delay the onset of both global and local collapse, by maintaining a constant turbulent velocity dispersion in the cloud. d) The star formation rate is large from the beginning, without any period of slow and accelerating star formation. e) At the onset of star formation, the column densities of the local star- forming clumps are typically 0.5-2 X 10^{21} pc, very similar to reported values of the column density required for molecule formation, suggesting that locally molecular gas and star formation occur nearly simultaneously. At that time, the bulk of the cloud is still expected to remain atomic. Within their framework and assumptions, our simulations thus support the scenario of rapid star formation AFTER MCs are formed, although long (> 15 Myr) accumulation periods are probably spent in the atomic phase, during which the clouds build up their gravitational energy.
We study the kinematics of gaseous disks in triaxial dark matter halos using the closed-loop orbit solutions in non-axisymmetric potentials. The orbits are in general non-circular and, for given triaxiality, their ellipticity depends on the ratio of escape to circular velocities, V_esc^2/V_c^2. This ratio increases steeply towards the center for cold dark matter (CDM) halo density profiles, implying that even minor deviations from spherical symmetry may induce large deviations from circular orbits in the velocity field of a gaseous disk, especially near the center. This result suggests that caution should be exercised when interpreting constraints on the presence of density cusps in the dark halo derived from the innermost velocity profile. Simulated long-slit rotation curves vary greatly in shape, depending primarily on the viewing angle of the disk and on its orientation relative to the principal axes of the potential. "Solid-body" rotation curves - typically interpreted as a signature of a constant density core in the dark matter distribution - are often obtained when the slit samples velocities near the major axis of the closed loop orbits. Triaxial potentials imprint specific symmetries in 2D velocity fields, generally inducing "twists" in the isovelocity contours and anti-symmetric patterns in opposite quadrants. We suggest that triaxial halos may be responsible for the variety of shapes of long-slit rotation curves of low surface brightness (LSB) galaxies, as well as for the complex central kinematics of LSBs, which are sometimes ascribed to the presence of "radial motions" in the gas. We argue that LSB rotation curves might be reconciled with the structure of CDM halos once the effects of halo triaxiality on the dynamics of gaseous disks are properly taken into account.
We study primordial nucleosynthesis abundance yields for assumed ranges of cosmological lepton numbers, sterile neutrino mass-squared differences and active-sterile vacuum mixing angles. We fix the baryon-to-photon ratio at the value derived from the cosmic microwave background (CMB) data and then calculate the deviation of the 2H, 4He, and 7Li abundance yields from those expected in the zero lepton number(s), no-new-neutrino-physics case. We conclude that high precision (< 5% error) measurements of the primordial 2H abundance from, e.g., QSO absorption line observations coupled with high precision (< 1% error) baryon density measurements from the CMB could have the power to either: (1) reveal or rule out the existence of a light sterile neutrino if the sign of the cosmological lepton number is known; or (2) place strong constraints on lepton numbers, sterile neutrino mixing properties and resonance sweep physics. Similar conclusions would hold if the primordial 4He abundance could be determined to better than 10%.