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Know Your Neighborhood: A Detailed Model Atmosphere Analysis of Nearby White Dwarfs
We present improved atmospheric parameters of nearby white dwarfs lyingwithin 20 pc of the Sun. The aim of the current study is to obtain thebest statistical model of the least-biased sample of the white dwarfpopulation. A homogeneous analysis of the local population is performedcombining detailed spectroscopic and photometric analyses based onimproved model atmosphere calculations for various spectral typesincluding DA, DB, DC, DQ, and DZ stars. The spectroscopic technique isapplied to all stars in our sample for which optical spectra areavailable. Photometric energy distributions, when available, are alsocombined to trigonometric parallax measurements to derive effectivetemperatures, stellar radii, as well as atmospheric compositions. Arevised catalog of white dwarfs in the solar neighborhood is presented.We provide, for the first time, a comprehensive analysis of the massdistribution and the chemical distribution of white dwarf stars in avolume-limited sample.
| Effect of different stellar galactic environments on planetary discs - I. The solar neighbourhood and the birth cloud of the Sun
We have computed trajectories, distances and times of closest approachesto the Sun by stars in the solar neighbourhood with known position,radial velocity and proper motions. For this purpose, we have used afull potential model of the Galaxy that reproduces the local z-force,the Oort constants, the local escape velocity and the rotation curve ofthe Galaxy. From our sample, we constructed initial conditions, withinobservational uncertainties, with a Monte Carlo scheme for the 12 mostsuspicious candidates because of their small tangential motion. We findthat the star Gliese 710 will have the closest approach to the Sun, witha distance of approximately 0.34 pc in 1.36 Myr in the future. We showthat the effect of a flyby with the characteristics of Gliese 710 on a100 au test particle disc representing the Solar system is negligible.However, since there is a lack of 6D data for a large percentage ofstars in the solar neighbourhood, closer approaches may exist. Wecalculate parameters of passing stars that would cause notable effectson the solar disc. Regarding the birth cloud of the Sun, we performedexperiments to reproduce roughly the observed orbital parameters such aseccentricities and inclinations of the Kuiper belt. It is now known thatin Galactic environments, such as stellar formation regions, the stellardensities of new born stars are high enough to produce close encounterswithin 200 au. Moreover, in these Galactic environments, the velocitydispersion is relatively low, typically ?˜ 1-3 kms-1. We find that with a velocity dispersion of ˜1 kms-1 and an approach distance of about 150 au, typical ofthese regions, we obtain approximately the eccentricities andinclinations seen in the current Solar system. Simple analyticalcalculations of stellar encounters effects on the Oort Cloud arepresented.
| Candidate subdwarfs and white dwarfs from the 2MASS, Tycho-2, XPM and UCAC3 catalogues
Photometry from the Two-Micron All-Sky Survey (2MASS), United StatesNaval Observatory CCD Astrograph Catalog (UCAC3) and SuperCosmoscatalogues, together with proper motions from the Tycho-2, KharkivProper Motions (XPM) and UCAC3 catalogues, is used to select all-skysamples of 28 candidate white dwarfs, 1826 evolved and 7641 unevolvedsubdwarfs for R from 9-17 mag. The samples are separated frommain-sequence stars with an admixture of less than 10 per cent, owing toan analysis of the distribution of the stars in colour index versusreduced proper-motion diagrams for various latitudes using related MonteCarlo simulations. It is shown that the XPM and UCAC3 catalogues havethe same level of proper-motion accuracy. Most of the selected starshave at least six-band photometry. This allows us to eliminate someadmixtures and reveal some binaries. Empirical calibrations of absolutemagnitude versus colour index and reduced proper motion for Hipparcosstars give us distances and a three-dimensional (3D) distribution forall the selected stars. It is shown that the subdwarf samples are almostcomplete for the Tycho-2 stars, i.e. to 11 mag or 150 pc from the Sun.For fainter stars from the XPM and UCAC3 catalogues, the subdwarfsamples are complete only to 20-60 per cent because of the selectionmethod and incompleteness of the catalogues. Some conclusions can bemade, however, especially for Tycho-2 stars with known radial velocitiesand metallicities. The subdwarfs show some concentration in the GalacticCentre hemisphere, with voids due to extinction in the Gould belt andthe Galactic plane. Some as yet unexplained overdensities of evolvedsubdwarfs are seen in several parts of the sky. For 176 stars withradial velocities, the 3D motion and Galactic orbits are calculated. For57 stars with Fe/H we find relations of the metallicity with colourindex, asymmetric drift velocity and orbital eccentricity. All the dataare consistent with the suggestion that most unevolved subdwarfs belongto the low-metallicity halo with large asymmetric drift, whereas evolvedsubdwarfs have various metallicities and velocities and include bothdisc and halo stars. The lower limit of the local mass density ofunevolved subdwarfs, estimated as 2 × 10-5M&sun; pc-3, appears twice as high as traditionalestimates. The selected stars are listed in the new catalogue ofcandidate subdwarfs and white dwarfs from the 2MASS, Tycho-2, XPM andUCAC3 catalogues (hereafter SDWD catalogue) for future spectroscopicconfirmation of the subluminous status of these stars, because themajority of them are now classified for the first time.
| A selection of hot subluminous stars in the GALEX survey - I. Correlation with the Guide Star Catalog
We assembled a catalogue of bright, hot subdwarf and white dwarf starsextracted from a joint ultraviolet, optical and infrared source list.The selection is secured using colour criteria that correlate well witheffective temperatures Teff? 12 000 K. We built aNUV- V versus V - J diagram for ?60 000 bright sourcesusing the Galaxy Evolution Explorer (GALEX) NUV magnitude(NUV < 14), the associated Guide Star Catalog (GSC2.3.2)photographic quick-V magnitude and the Two-Micron All-Sky Survey J and Hmagnitudes. This distillation process delivered a catalogue of ?700sources with NUV- V < 0.5 comprising ˜160 known hotsubdwarf stars and another ˜60 known white dwarf stars. A reducedproper-motion diagram built using the proper-motion measurementsextracted from the Naval Observatory Merged Astrometric Dataset allowedus to identify an additional ˜120 new hot subdwarf candidates and˜10 hot white dwarf candidates. We present a spectroscopic studyof a subset of 52 subdwarfs, 48 of them analysed here for the firsttime, and with nine objects brighter than V ˜ 12. Our sample ofspectroscopically confirmed hot subdwarfs comprises 10 sdO-type starsand 42 sdB-type stars suitable for pulsation and binary studies. We alsopresent a study of 50 known white dwarfs selected in the GALEX surveyand six new white dwarfs from our catalogue of subluminous candidates.Ultraviolet, optical and infrared synthetic magnitudes employed in theselection and analysis of white dwarf stars are listed in appendix.Based on observations made with ESO telescopes at the La Silla ParanalObservatory under programmes 82.D-0750, 83.D-0540 and085.D-0866.Visiting Astronomer, Kitt Peak National Observatory, NationalOptical Astronomy Observatory, which is operated by the Association ofUniversities for Research in Astronomy (AURA) under cooperativeagreement with the National Science Foundation.
| Identification of blue high proper motion objects in the Tycho-2 and 2MASS catalogues using Virtual Observatory tools
Aims: With available Virtual Observatory tools, we looked for newbright blue high proper motion objects in the entire sky: white dwarfs,hot subdwarfs, runaway OB stars, and early-type stars in nearby youngmoving groups. Methods: We performed an all-sky cross-matchbetween the optical Tycho-2 and near-infrared 2MASS catalogues withAladin, and selected objects with proper motions ? > 50 masyr-1 and colours VT - Ks < -0.5 magwith TOPCAT. We also collected multi-wavelength photometry, constructedthe spectral energy distributions and estimated effective temperaturesfrom fits to atmospheric models with VOSA for the most interestingtargets. Results: We assembled a sample of 32 bright blue highproper motion objects, including ten sdO/B subdwarfs, nine DA whitedwarfs, five young early-type stars (two of which are runaway stars),two blue horizontal branch stars, one star with poor information, andfive objects reported for the first time in this work. These last fiveobjects have magnitudes BT ? 11.0-11.6 mag, effectivetemperatures Teff ? 24 000-30 000 K, and are located inthe region of known white dwarfs and hot subdwarfs in a reduced propermotion-colour diagram. We confirmed the hot subdwarf nature of one ofthe new objects, Albus 5, with public far-ultraviolet spectroscopic dataobtained with FUSE.
| Accurate Coordinates and 2MASS Cross Identifications for (Almost) All Gliese Catalog Star
We provide precise J2000, epoch 2000 coordinates, andcross-identifications to sources in the 2MASS Point Source Catalog fornearly all stars in the Gliese, Gliese-Jahreiss, and Woolley catalogs ofnearby stars. The only Gliese objects where we were not successful aretwo Gliese sources that are actually QSOs; two proposed companions tobrighter stars, which we believe do not exist; four stars included inone of the catalogs but identified there as only optical companions; oneprobable plate flaw; and two stars that simply remain unrecovered. Forthe 4251 recovered stars, 2693 have coordinates based on Hipparcospositions, 1549 have coordinates based on 2MASS data, and 9 havepositions from other astrometric sources. All positions have beencalculated at epoch 2000 using proper motions from the literature, whichare also given here.
| UBV(RI)C JHK observations of Hipparcos-selected nearby stars
We present homogeneous, standardized UBV(RI)C photometry forover 700 nearby stars selected on the basis of Hipparcos parallaxes.Additionally, we list JHK photometry for about half of these stars, aswell as L photometry for 86 of the brightest. A number of stars withpeculiar colours or anomalous locations in various colour-magnitudediagrams are discussed.
| Searching for stars closely encountering with the solar system
Based on a new version of the Hipparcos catalog and currently availableradial velocity data, we have searched for stars that either haveencountered or will encounter the solar neighborhood within less than 3pc in the time interval from ?2 Myr to +2 Myr. Nine new candidateswithin 30 pc of the Sun have been found. To construct the stellar orbitsrelative to the solar orbit, we have used the epicyclic approximation.We show that, given the errors in the observational data, theprobability that the well-known star HIP 89 825 (GL 710) encounteringwith the Sun most closely falls into the Oort cloud is 0.86 in the timeinterval 1.45 ± 0.06 Myr. This star also has a nonzeroprobability, 1 × 10?4, of falling into the regiond < 1000 AU, where its influence on Kuiper Belt objects becomespossible.
| The White Dwarfs Within 20 Parsecs of the Sun: Kinematics and Statistics
We present the kinematical properties, distribution of spectroscopicsubtypes, and stellar population subcomponents of the white dwarfswithin 20 pc of the Sun. We find no convincing evidence of halo whitedwarfs in the total 20 pc sample of 129 white dwarfs nor is thereconvincing evidence of genuine thick disk subcomponent members within 20parsecs. Virtually, the entire 20 pc sample likely belongs to the thindisk. The total DA to non-DA ratio of the 20 pc sample is 1.6, amanifestation of deepening envelope convection which transforms DA starswith sufficiently thin H surface layers into non-DAs. The addition offive new stars to the 20 pc sample yields a revised local space densityof white dwarfs of (4.9 ± 0.5) × 10–3pc–3 and a corresponding mass density of (3.3 ±0.3) × 10–3 M sunpc–3. We find that at least 15% of the white dwarfswithin 20 parsecs of the Sun (the DAZ and DZ stars) have photosphericmetals that possibly originate from accretion of circumstellar material(debris disks) around them. If this interpretation is correct, thissuggests the possibility that the same percentage have planets orasteroid-like bodies orbiting them.
| High-resolution UVES/VLT spectra of white dwarfs observed for the ESO SN Ia Progenitor Survey. III. DA white dwarfs
Context: The ESO Supernova Ia Progenitor Survey (SPY) tookhigh-resolution spectra of more than 1000 white dwarfs and pre-whitedwarfs. About two thirds of the stars observed are hydrogen-dominated DAwhite dwarfs. Here we present a catalog and detailed spectroscopicanalysis of the DA stars in the SPY. Aims: Atmospheric parameterseffective temperature and surface gravity are determined for normal DAs.Double-degenerate binaries, DAs with magnetic fields or dM companions,are classified and discussed. Methods: The spectra are comparedwith theoretical model atmospheres using a ?2 fittingtechnique. Results: Our final sample contains 615 DAs, which showonly hydrogen features in their spectra, although some aredouble-degenerate binaries. 187 are new detections or classifications.We also find 10 magnetic DAs (4 new) and 46 DA+dM pairs (10 new).Based on data obtained at the Paranal Observatory ofthe European Southern Observatory for programmes 165.H-0588 and167.D-0407.
| Polarisation of very-low-mass stars and brown dwarfs. I. VLT/FORS1 optical observations of field ultra-cool dwarfs
Context: Ultra-cool dwarfs of the L spectral type (T_eff = 1400-2200 K)are known to have dusty atmospheres. Asymmetries of the dwarf surfacemay arise from rotationally-induced flattening and dust-cloud coverage,and may result in non-zero linear polarisation through dust scattering.Aims: We aim to study the heterogeneity of ultra-cool dwarfs'atmospheres and the grain-size effects on the polarisation degree in asample of nine late M, L and early T dwarfs. Methods: We obtainlinear polarimetric imaging measurements using FORS1 at the Very LargeTelescope, in the Bessel I filter, and for a subset in the Bessel R andthe Gunn z filters. Results: We measure a polarisation degree of(0.31±0.06)% for LHS102BC. We fail to detect linear polarisationin the rest of our sample, with upper-limits on the polarisation degreeof each object of 0.09% to 0.76% (95% of confidence level), depending onthe targets and the bands. For those targets we do not find evidence oflarge-scale cloud horizontal structure in our data. Together withprevious surveys, our results set the fraction of ultra-cool dwarfs withdetected linear polarisation to 30+10_-6% (1-? errors).From the whole sample of well-measured objects with errors smaller than0.1%, the fraction of ultra-cool dwarfs with polarisation degree largerthan 0.3% is smaller than 16% (95% confidence level). Conclusions: For three brown dwarfs, our observations indicatepolarisation degrees different (at the 3-? level) than previouslyreported, giving hints of possible variations. Our results fail tocorrelate with the current model predictions for ultra-cool dwarfpolarisation for a flattening-induced polarisation, or with thevariability studies for a polarisation induced by an heterogeneous cloudcover. This stresses the intricacy of each of those tasks, but may ariseas well from complex and dynamic atmospheric processes.Based on observations collected at the European Observatory, Paranal,Chile,under programmes 075.C-0653(A) and 077.C-0819(A).
| Precise Estimates of the Synthetic Spectral Distances to DA White Dwarfs
Using recent photometric calibrations, we develop greatly improveddistance estimates for DA white dwarfs using multi-band syntheticphotometry based on spectroscopic temperatures and gravities. Very goodcorrelations are shown to exist between our spectroscopically basedphotometric distance estimates and those derived from trigonometricparallaxes. We investigate the uncertainties involved in our distanceestimates, as well as discuss the circumstances where such techniquesare most likely to fail. We apply our techniques to the large sample ofSloan Digital Sky Survey DA white dwarfs where automated fitting of H IBalmer profiles yields spectrometric temperatures and gravities. Wedetermine simple empirical corrections to these temperatures andgravities with respect to published slit spectroscopy. After applyingthese T eff-log g corrections as well as appropriateinterstellar extinction corrections, where necessary, we derivespectroscopically based photometric distances for 7062 DA stars fromthis sample.
| a New Look at the Local White Dwarf Population
We have conducted a detailed new survey of the local population of whitedwarfs lying within 20 pc of the Sun. A new revised catalog of localwhite dwarfs containing 122 entries (126 individual degenerate stars) ispresented. This list contains 27 white dwarfs not included in a previouslist from 2002, as well as new and recently published trigonometricparallaxes. In several cases new members of the local white dwarfpopulation have come to light through accurate photometric distanceestimates. In addition, a suspected new double degenerate system (WD0423+120) has been identified. The 20 pc sample is currently estimatedto be 80% complete. Using a variety of recent spectroscopic,photometric, and trigonometric distance determinations, we re-compute aspace density of 4.8 ± 0.5 × 10 3 pc3 corresponding to a mass density of 3.2 ± 0.3 ×10 3 M sun pc 3 from the completeportion of the sample within 13 pc. We find an overall mean mass for thelocal white dwarfs of 0.665 M sun, a value larger than mostother non-volume-limited estimates. Although the sample is small, wefind no evidence of a correlation between mass and temperature in whichwhite dwarfs below 13,000 K are systematically more massive than thoseabove this temperature. Within 20 pc 25% of the white dwarfs are inbinary systems (including double degenerate systems). Approximately 6%are double degenerates and 6.5% are Sirius-like systems. The fraction ofmagnetic white dwarfs in the local population is found to be 13%.
| The Ratio of Helium- to Hydrogen-Atmosphere White Dwarfs: Direct Evidence for Convective Mixing
We determine the ratio of helium- to hydrogen-atmosphere white dwarfstars as a function of effective temperature from a model atmosphereanalysis of the infrared photometric data from the Two Micron All SkySurvey combined with available visual magnitudes. Our study surpassesany previous analysis of this kind, both in terms of the accuracy of theTeff determinations and the size of the sample. We observethat the ratio of helium- to hydrogen-atmosphere white dwarfs increasesgradually from a constant value of ~0.25 between Teff=15,000and 10,000 K to a value twice as large in the range 10,000K>Teff>8000 K, suggesting that convective mixing, whichoccurs when the bottom of the hydrogen convection zone reaches theunderlying convective helium envelope, is responsible for this gradualtransition. The comparison of our results with an approximate model usedto describe the outcome of this convective mixing process implieshydrogen mass layers in the rangeMH/Mtot=10-10 to 10-8 forabout 15% of the DA stars that survived the DA-to-DB transition nearTeff~30,000 K, the remainder having presumably more massivelayers above MH/Mtot~10-6.
| A Comparative Study of Optical and Ultraviolet Effective Temperatures for DA White Dwarfs from the IUE Archive
We present a comparative study of effective temperatures determined fromthe hydrogen Balmer lines and from the UV energy distribution for 140 DAwhite dwarfs drawn from the International Ultraviolet Explorer (IUE)archive. Our results indicate that the optical and UV temperatures ofthe majority of stars below Teff~40,000 K and within ~75 pcare in fairly good agreement given the uncertainties. At highertemperatures and/or larger distances, however, significant discrepanciesare observed. Several mechanisms are investigated to account for thesediscrepancies, including the effect of interstellar reddening, thepresence of metals in the photosphere, and the existence of unresolvedbinary white dwarfs. The results of our analysis reveal thatwavelength-dependent extinction is the most natural explanation for theobserved temperature differences. We also attempt to predict thedifferences in optical and UV temperatures expected from unresolveddegenerate binaries by performing an exhaustive simulation of compositemodel spectra. In light of these simulations, we then discuss some knowndouble degenerates and identify new binary candidates by restricting ouranalysis to stars located within 75 pc, where the effect of interstellarreddening is significantly reduced.
| Albus 1: A Very Bright White Dwarf Candidate
We have serendipitously discovered a previously unknown, bright source(BT=11.75+/-0.07 mag) with a very blueVT-Ks color, which we have named Albus 1. Aphotometric and astrometric study using Virtual Observatory tools hasshown that it possesses an appreciable proper motion and magnitudes andcolors very similar to those of the well-known white dwarf G191-B2B. Weconsider Albus 1 as a DA-type white dwarf located at about 40 pc. If itsnature is confirmed, Albus 1 would be the sixth brightest isolated whitedwarf in the sky, which would make it an excellent spectrophotometricstandard.
| A Spitzer White Dwarf Infrared Survey
We present mid-infrared photometry of 124 white dwarf stars with theSpitzer Space Telescope. Objects were observed simultaneously at 4.5 and8.0 ?m with sensitivities better than 0.1 mJy. This data set can beused to test models of white dwarf atmospheres in a new wavelengthregime, as well as to search for planetary companions and debris disks.
| Spectropolarimetric Survey of Hydrogen-rich White Dwarf Stars
We have conducted a survey of 61 southern white dwarfs searching formagnetic fields using Zeeman spectropolarimetry. Our objective is toobtain a magnetic field distribution for these objects and, inparticular, to find white dwarfs with weak fields. We found one possiblecandidate (WD 0310-688) that may have a weak magnetic field of-6.1+/-2.2 kG. Next, we determine the fraction and distribution ofmagnetic white dwarfs in the solar neighborhood and investigate theprobability of finding more of these objects based on the currentincidence of magnetism in white dwarfs within 20 pc of the Sun. We havealso analyzed the spectra of the white dwarfs to obtain effectivetemperatures and surface gravities.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Calibration of Synthetic Photometry Using DA White Dwarfs
We have calibrated four major ground-based photometric systems withrespect to the Hubble Space Telescope (HST) absolute flux scale, whichis defined by Vega and four fundamental DA white dwarfs. Thesephotometric systems include the Johnson-Kron-Cousins UBVRI, theStrömgren uvby filters, the Two Micron All Sky SurveyJHKs, and the Sloan Digital Sky Survey ugriz filters.Synthetic magnitudes are calculated from model white dwarf spectrafolded through the published filter response functions; these magnitudesin turn are absolutely calibrated with respect to the HST flux scale.Effective zero-magnitude fluxes and zero-point offsets of each systemare determined. In order to verify the external observationalconsistency, as well as to demonstrate the applicability of thesedefinitions, the synthetic magnitudes are compared with the respectiveobserved magnitudes of larger sets of DA white dwarfs that havewell-determined effective temperatures and surface gravities and span awide range in both of these parameters.
| Simulating observable comets. III. Real stellar perturbers of the Oort cloud and their output
Context: .This is the third of a series of papers on simulating themechanisms acting currently on the Oort cloud and producing the observedlong-period comets.Aims.In this paper we investigate the influence ofcurrent stellar perturbers on the Oort cloud of comets under thesimultaneous galactic disk tide. We also analyse the past motion of theobserved long-period comets under the same dynamical model to verify thewidely used definition of dynamically new comets. Methods.The action ofnearby stars and the galactic disk tide on the Oort cloud was simulated.The original orbital elements of all 386 long-period comets of qualityclasses 1 and 2 were calculated, and their motion was followednumerically for one orbital revolution into the past, down to theprevious perihelion. We also simulated the output of the close futurepass of GJ 710 through the Oort cloud. Results.The simulated flux of theobservable comets resulting from the current stellar and galacticperturbations, as well as the distribution of perihelion direction, wasobtained. The same data are presented for the future passage of GJ 710.A detailed description is given of the past evolution of aphelion andperihelion distances of the observed long-period comets. Conclusions. Weobtained no fingerprints of the stellar perturbations in the simulatedflux and its directional structure. The mechanisms producing observablecomets are highly dominated by galactic disk tide because all currentstellar perturbers are too weak. Also the effect of the close passage ofthe star GJ 710 is very difficult to recognise on the background of theGalactic-driven observable comets. For the observed comets we found only45 to be really dynamically "new" according to our definition based onthe previous perihelion distance value.
| 3D kinematics of white dwarfs from the SPY project. II.
We present the kinematics of a sample of 398 DA white dwarfs from theSPY project (ESO SN Ia Progenitor surveY) and discuss kinematic criteriafor distinguishing of thin-disk, thick-disk, and halo populations. Thisis the largest homogeneous sample of white dwarfs for which 3D spacemotions have been determined. Since the percentage of old stars amongwhite dwarfs is higher than among main-sequence stars, they arepresumably valuable tools in studies of old populations, such as thehalo and the thick disk. Studies of white-dwarf kinematics can help todetermine the fraction of the total mass of our Galaxy contained in theform of thick-disk and halo white dwarfs, an issue which is still underdiscussion. Radial velocities and spectroscopic distances obtained bythe SPY project were combined with our measurements of proper motions toderive 3D space motions. Galactic orbits and further kinematicparameters were computed. We calculated individual errors of kinematicparameters by means of a Monte Carlo error propagation code. Ourkinematic criteria for assigning population membership were deduced froma sample of F and G stars taken from the literature, for which chemicalcriteria can be used to distinguish between a thin-disk, a thick-diskand a halo star. Our kinematic population classification scheme is basedon the position in the U-V-velocity diagram, the position in theJz-eccentricity diagram, and the Galactic orbit. We combinedthis with age information and found seven halo and 23 thick-disk whitedwarfs in this brightness limited sample. Another four rather cool whitedwarfs probably also belong to the thick disk. Correspondingly 2% of thewhite dwarfs belong to the halo and 7% to the thick disk. The masscontribution of the thick-disk white dwarfs is found to be substantial,but is insufficient to account for the missing dark matter.
| Late-epoch optical and near-infrared observations of the GRB 000911 afterglow and its host galaxy
We present the results of an optical and near-infrared (NIR) monitoringcampaign of the counterpart of Gamma-Ray Burst (GRB) 000911, located atredshift z = 1.06, from 5 days to more than 13 months after explosion.Our extensive dataset is a factor of 2 larger and spans a time interval~4 times longer than the ones considered previously for this GRBafterglow; this allows a more thorough analysis of its light curve andof the GRB host galaxy properties. The afterglow light curves show asingle power-law temporal decline, modified at late times by light froma host galaxy with moderate intrinsic extinction, and possibly by anemerging supernova (SN). The afterglow evolution is interpreted withinthe classical “fireball” scenario as a weakly collimatedadiabatic shock propagating in the interstellar medium. The presence ofa SN light curve superimposed on the non-thermal afterglow emission isinvestigated: while in the optical bands no significant contribution tothe total light is found from a SN, the NIR J-band data show an excesswhich is consistent with a SN as bright as the known hypernova SN1998bw.If the SN interpretation is true, this would be the farthestGRB-associated SN, as well as the farthest core-collapse SN, discoveredto date. However, other possible explanations of this NIR excess arealso investigated. Finally, we studied the photometric properties of thehost, and found that it is likely to be a slightly reddened,subluminous, extreme starburst compact galaxy, with luminosity ~0.1L*, an age of ~0.5 Gyr and a specific Star Formation Rate(SFR) of ?30 M_&sun; yr-1 (L/L*)-1.This is the highest specific SFR value for a GRB host inferred fromoptical/NIR data.
| Rotation velocities of white dwarfs. III. DA stars with convective atmospheres
The sharp H? NLTE line cores of hydrogen-rich (DA) white dwarfsallow their projected rotational velocities to be determined. Highresolution optical spectra of 22 stars obtained with the Keck Itelescope are matched by synthetic spectra computed from a grid of NLTEmodel atmospheres. In this paper, the third in a series on white dwarfrotation, we concentrate preferentially on DA white dwarfs withconvective atmospheres, i.e. with Teff < 14 000 K.Previous analyses found DA white dwarfs hotter than 14 000 K to be veryslow rotators and rarely show any spectroscopically detectable rotation.For 19 of our programme stars we were able to derive projectedrotational velocities or upper limits. No rotation could be detected forseven stars in our sample. However twelve stars show significant linebroadening. In the case of the ZZ Ceti star G 117-B15A, the observedH? line profile cannot be matched by a rotationally broadenedprofile as its line core is too narrow. Combining our results with thosefrom two similar studies, we have obtained information on the rotationor other line broadening mechanisms (such as caused by magnetic fields)of 56 DA white dwarfs. The fraction of rotating DA white dwarfs whoseline profiles can be matched for a vanishing projected rotation velocityis high for hot white dwarfs with radiative atmospheres (25 out of 28).Amongst the cool white dwarfs with presumedly convective atmospheres,only for a few stars (8 out of 22) has no additional line broadening tobe invoked to explain their observed H? line profiles. Weconjecture that the physics of H? line formation in convective DAwhite dwarf atmospheres is not yet sufficiently well understood andadditional observational and theoretical efforts have to be made.
| A population model of the solar neighbourhood thin disc white dwarfs
We present a model of the solar neighbourhood (d < 100pc) white dwarf(WD) population and show the resulting effective temperature and massdistribution. Our model parameters, in particular the WD coolingtime-scale, are constrained by the models of Chabrier et al. The localinitial mass function and star formation rate (SFR, per unit volume inthe Galactic plane) of Schröder & Pagel are used for thecreation of the synthetic stars from which the WDs originate, as well asthe above authors' grid of evolutionary tracks. Furthermore, we considerin detail the significant depletion of the older (and cooler) WDs bydilution into the column, as caused by the dynamics of the `thin disc'.To verify our synthetic sample, especially the WD cooling time-scale, westudy the temperature distribution of a small but nearly complete,volume-limited sample of observed WDs, which we characterize by a singleand simple indicator R6300: the number ratio of WDs withTeff < 6300K over those with Teff > 6300K.After determination of the bias owing to a residual incompleteness withcool WDs, we find a corrected value of R6300 of 0.68(+/-0.24). This is in good agreement with our WD population model: itstemperature distribution yields R6300= 0.77.For a spherical volume around the Sun, with d < 100pc, our populationmodel suggests a total of about 13700 WDs (omiting part of the WDs witha binary system origin, an estimated 7 +/- 2 per cent of the totalcount). A subsample, limited to magnitude 19.0 (as expected for theEuropean Space Agency mission Gaia), would contain about 7750 WDs ofthis population model. With a less deep magnitude limit of B= 16.0, moretypical of current observed WD samples (SN Ia Progenitor surveY; SPY),the number of objects is reduced to only ~1350. We use this specificsynthetic subsample to test the completeness of the prospective SPY WDsample, which (with over 50 per cent of the candidates now observed) isalready the largest sample of WDs with high-resolution spectra. We findthat within d < 100pc and for B < 16.0 SPY will deliver a fairlycomplete (almost 80 per cent) sample.
| Discovery of kilogauss magnetic fields in three DA white dwarfs
We have detected longitudinal magnetic fields between 2 and 4 kG inthree (WD 0446-790, WD 1105-048, WD 2359-434) out of a sample of 12normal DA white dwarfs by using optical spectropolarimetry done with theVLT Antu 8 m telescope equipped with FORS1. With the exception of 40 EriB (4 kG) these are the first positive detections of magnetic fields inwhite dwarfs below 30 kG. Although suspected, it was not clear whether asignificant fraction of white dwarfs contain magnetic fields at thislevel. These fields may be explained as relics from magnetic fields inthe main-sequence progenitors considerably enhanced by magnetic fluxconservation during the shrinkage of the core. A detection rate of 25%(3/12) may indicate now for the first time that a substantial fractionof white dwarfs have a weak magnetic field. This result, if confirmed byfuture observations, would form a cornerstone for our understanding ofthe evolution of stellar magnetic fields.Based on observations collected at the European Southern Observatory,Paranal, Chile, under programme ID 70.D-0259.
| Intrinsic and dust-induced polarization in gamma-ray burst afterglows: The case of GRB 021004
Polarization measurements for the optical counterpart to GRB 021004 arepresented and discussed. Our observations were performed with the TNGand the VLT-UT3 (Melipal) during the first and fourth night after thegamma-ray burst discovery. We find robust evidence for temporalevolution of the polarization, which is therefore, at least partially,intrinsic to the optical transient. We do not find convincing evidenceof wavelength dependence for the intrinsic polarization of thetransient, in agreement with current polarization models for opticalafterglows. We discuss the role of dust, both in our galaxy and in thehost, in modifying the transmitted polarization vector, showing how asizable fraction of the observed polarized flux is due to Galacticselective extinction, while it is not possible to single out any clearcontribution from dust in the host galaxy. We discuss how our datacompare to those obtained by different groups showing that atwo-component model is required to describe the complete dataset. Thisis not surprising given the complex lightcurve of GRB 021004.Based on observations made with ESO telescopes at the ParanalObservatory under programme Id 70.D-0111, on data from the ESO/ST-ECFScience Archive Facility and on observations made with the TNG underprogramme TAC 8_01(47).
| An Archive of IUE Low-Dispersion Spectra of the White Dwarf Stars
We have produced an archive of the ultraviolet low-dispersion spectrafor the full set of white dwarf stars observed with the InternationalUltraviolet Explorer (IUE) satellite over the course of its 18 yrmission. This archive contains the spectra of 322 individual degeneratestars which have been processed to optimize the signal-to-noise for eachstar. In particular, all spectra have been corrected for residualtemporal and thermal effects and placed on the Hubble Space TelescopeFaint Object Spectrograph absolute flux scale using procedures describedby Massa & Fitzpatrick. Wherever possible, multiple observations ofindividual stars have been co-added to further enhance signal-to-noiseand have been combined into a single spectrum including the full 1150 to3150 Å wavelength region observed by IUE. The contents of thisspectral archive are described and the details of data reductionprocedures are provided, along with the url for access to the electronicfiles of the processed spectra.
| A Determination of the Local Density of White Dwarf Stars
The most recent version of the Catalog of Spectroscopically IdentifiedWhite Dwarfs lists 2249 white dwarf stars. Among these stars are 109white dwarfs that have either reliable trigonometric parallaxes orcolor-based distance moduli that place them at a distance within 20 pcof the Sun. Most of these nearby white dwarfs are isolated stars, but 28(25% of the sample) are in binary systems, including such well-knownsystems as Sirius A/B and Procyon A/B. There are also three doubledegenerate systems in this sample of the local white dwarf population.The sample of local white dwarfs is largely complete out to 13 pc, andthe local density of white dwarf stars is found to be5.0+/-0.7×10-3 pc-3, with a correspondingmass density of 3.4+/-0.5×10-3Msolarpc-3.
| Imaging planets around nearby white dwarfs
We suggest that Jovian planets will survive the late stages of stellarevolution, and that white dwarfs will retain planetary systems in wideorbits (>~5au). Utilizing evolutionary models for Jovian planets, weshow that infrared imaging with 8-m class telescopes of suitable nearbywhite dwarfs should allow us to resolve and detect companions >~3MJUP . Detection of massive planetary companions to nearbywhite dwarfs would prove that such objects can survive the final stagesof stellar evolution, place constraints on the frequency ofmain-sequence stars with planetary systems dynamically similar to ourown and allow direct spectroscopic investigation of their compositionand structure.
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