Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
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.
| 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.
| 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.
| Common Proper Motion Companions to Nearby Stars: Ages and Evolution
A set of 41 nearby stars (closer than 25 pc) is investigated which havevery wide binary and common proper motion (CPM) companions at projectedseparations between 1000 and 200,000 AU. These companions are identifiedby astrometric positions and proper motions from the NOMAD catalog.Based mainly on measures of chromospheric and X-ray activity, ageestimation is obtained for most of 85 identified companions.Color-absolute magnitude diagrams are constructed to test whether CPMcompanions are physically related to the primary nearby stars and havethe same age. Our carefully selected sample includes three remote whitedwarf companions to main-sequence stars and two systems (55 Cnc and GJ777A) of multiple planets and distant stellar companions. Ten new CPMcompanions, including three of extreme separations, are found. Multiplehierarchical systems are abundant; more than 25% of CPM components arespectroscopic or astrometric binaries or multiples themselves. Two newastrometric binaries are discovered among nearby CPM companions, GJ 264and HIP 59000, and preliminary orbital solutions are presented. TheHyades kinematic group (or stream) is presented broadly in the sample,but we find few possible thick-disk objects and no halo stars. Itfollows from our investigation that moderately young (age<~1 Gyr)thin-disk dwarfs are the dominating species in the near CPM systems, ingeneral agreement with the premises of the dynamical survival paradigm.
| 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%.
| 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.
| The mass and luminosity functions and the formation rate of DA white dwarfs in the Sloan Digital Sky Survey
Aims.The SDSS Data Release 1 includes 1833 DA white dwarfs (WDs) andforms the largest homogeneous sample of WDs. This sample provides thebest opportunity to study the statistical properties of WDs. Methods:We adopt a recently established theoretical model to calculate the massand distance of each WD using the observational data. Then we adopt abin-correction method to correct for selection effects and use the 1/Vweight-factor method to calculate the luminosity function, thecontinuous mass function and the formation rate of these WDs. Results:The SDSS DA WD sample is incomplete and suffers seriously from selectioneffects. After corrections for the selection effects, only 531 WDsremain. From this final sample we derive the most up-to-date luminosityfunction and mass function, in which we find a broad peak of WD massescentered around 0.58 Mȯ. The DA WD space density iscalculated as 8.81×10-5 pc-3 and theformation rate is 2.579×10-13 pc-3yr-1. Conclusions: The statistical properties of the SDSS DAWD sample are generally in good agreement with previous observationaland theoretical studies, and provide us information on the formation andevolution of WDs. However, a larger and more complete all-sky WD sampleis still needed to explain some subtle disagreements and unresolvedissues.Full Table 2 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/466/627
| The fraction of DA white dwarfs with kilo-Gauss magnetic fields
Context: Weak magnetic fields have been searched for on only a smallnumber of white dwarfs. Current estimates find that about 10% of allwhite dwarfs have fields in excess of 1 MG; according to previousstudies this number increases up to about 25% in the kG regime. Aims:Our aim is to improve on these statistics by a new sample of ten whitedwarfs in order to determine the ratio of magnetic to field-free whitedwarfs. Methods: Mean longitudinal magnetic fields strengths weredetermined by means of high-precision circular polarimetry of Hβand Hγ with the FORS1 spectrograph of the VLT "Kueyen" 8 mtelescope. Results: In one of our objects (LTT 7987), we detected astatistically significant (97% confidence level) longitudinal magneticfield varying between (-1± 0.5) kG and (+1± 0.5) kG. Thiswould be the weakest magnetic field ever found in a white dwarf, butsystematic errors cannot completely be ruled out at this level ofaccuracy. We also observed the sdO star EC 11481-2303 but could notdetect a magnetic field. Conclusions: . VLT observations withuncertainties typically of 1000 G or less suggest that 15-20% of WDshave kG fields. Together with previous investigations, the fraction ofkG magnetic fields in white dwarfs amounts to about 11-15%, which isclose to the current estimations for highly magnetic white dwarfs (>1MG).Based on observations made with ESO Telescopes at the La Silla orParanal Observatories under programme ID 073.D-0356. Figures A.1 and A.2are only available in electronic form at http://www.aanda.org
| 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.
| Scalar dark matter and cold stars
In a medium composed of scalar particles with non-zero mass, the rangeof Van-der-Waals-type scalar mediated interactions among nucleonsbecomes infinite when the medium makes a transition to a Bose Einsteincondensed phase. We explore this phenomenon in an astrophysical context.Namely, we study the effect of a scalar dark matter background on theequilibrium of degenerate stars. In particular we focus on white dwarfsand the changes induced in their masses and in their radii.
| 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.
| Low-Luminosity Companions to White Dwarfs
This paper presents results of a near-infrared imaging survey forlow-mass stellar and substellar companions to white dwarfs. A wide-fieldproper-motion survey of 261 white dwarfs was capable of directlydetecting companions at orbital separations between ~100 and 5000 AUwith masses as low as 0.05 Msolar, while a deep near-fieldsearch of 86 white dwarfs was capable of directly detecting companionsat separations between ~50 and 1100 AU with masses as low as 0.02Msolar. Additionally, all white dwarf targets were examinedfor near-infrared excess emission, a technique capable of detectingcompanions at arbitrarily close separations down to masses of 0.05Msolar. No brown dwarf candidates were detected, whichimplies a brown dwarf companion fraction of <0.5% for white dwarfs.In contrast, the stellar companion fraction of white dwarfs as measuredby this survey is 22%, uncorrected for bias. Moreover, most of the knownand suspected stellar companions to white dwarfs are low-mass starswhose masses are only slightly greater than the masses of brown dwarfs.Twenty previously unknown stellar companions were detected, five ofwhich are confirmed or likely white dwarfs themselves, while 15 areconfirmed or likely low-mass stars. Similar to the distribution of coolfield dwarfs as a function of spectral type, the number of coolunevolved dwarf companions peaks at mid-M type. Based on the presentwork, relative to this peak, field L dwarfs appear to be roughly 2-3times more abundant than companion L dwarfs. Additionally, there is noevidence that the initial companion masses have been altered bypost-main-sequence binary interactions.
| Deep Photometry of the Globular Cluster M5: Distance Estimates from White Dwarf and Main-Sequence Stars
We present deep VI photometry of stars in the globular cluster M5 (NGC5904) based on images taken with the Hubble Space Telescope. Theresulting color-magnitude diagram reaches below V~27 mag, revealing theupper 2-3 mag of the white dwarf cooling sequence and main-sequencestars 8 mag and more below the turnoff. We fit the main sequence tosubdwarfs of known parallax to obtain a true distance modulus of(m-M)0=14.45+/-0.11 mag. A second distance estimate based onfitting the cluster white dwarf sequence to field white dwarfs withknown parallax yielded (m-M)0=14.67+/-0.18 mag. We discussthe nature of the difference between the two distance estimates andsuggest approaches for reducing the uncertainty in white dwarf fittingestimates for future studies. We couple our distance estimates withextensive photometry of the cluster's RR Lyrae variables to provide acalibration of the RR Lyrae absolute magnitude yieldingMV(RR)=0.42+/-0.10 mag at [Fe/H]=-1.11 dex. We provideanother luminosity calibration in the form of reddening-free Wasenheitfunctions. Comparison of our calibrations with predictions based onrecent models combining stellar evolution and pulsation theories showsencouraging agreement, and the existing differences may provide usefulfeedback to the models.
| Toward an Empirical Determination of the ZZ Ceti Instability Strip
We present atmospheric parameters for a large sample of DA white dwarfsthat are known to be photometrically constant. For each star, wedetermine the effective temperature and surface gravity by comparinghigh signal-to-noise ratio optical spectra to the predictions ofdetailed model atmosphere calculations. We also report the successfulprediction and detection of photometric variability in G232-38 based onsimilar Teff and logg determinations. The atmosphericparameters derived for this sample of constant stars, as well as thosefor the known sample of bright ZZ Ceti stars (now boosted to a total of39), have been obtained in a highly homogeneous way. We combine them tostudy the empirical red and blue edges, as well as the purity of the ZZCeti instability strip. We find that the red edge is rather wellconstrained, whereas there exists a rather large range of possibilitiesfor the slope of the blue edge. Furthermore, the ZZ Ceti instabilitystrip that results from our analysis contains no nonvariable whitedwarfs. Our sample of constant stars is part of a much broaderspectroscopic survey of bright (V<17) DA white dwarfs, which we haverecently undertaken. We also present here some preliminary results ofthis survey. Finally, we revisit the analysis by Mukadam et al. of thevariable and nonvariable DA stars uncovered as part of the Sloan DigitalSky Survey. Their erroneous conclusion of an instability stripcontaining several nonvariable stars is traced back to the lowsignal-to-noise ratio spectroscopic observations used in that survey.
| The Formation Rate and Mass and Luminosity Functions of DA White Dwarfs from the Palomar Green Survey
Spectrophotometric observations at high signal-to-noise ratio wereobtained of a complete sample of 348 DA white dwarfs from the PalomarGreen (PG) Survey. Fits of observed Balmer lines to synthetic spectracalculated from pure-hydrogen model atmospheres were used to obtainrobust values of Teff, logg, masses, radii, and cooling ages.The luminosity function of the sample, weighted by 1/Vmax,was obtained and compared with other determinations. Incompleteness ofthe sample due to selection by photographic mu-mbcolor and magnitude limits was found to be a serious problem, and anattempt is made to correct for this. The mass distribution of the whitedwarfs is derived, after important corrections for the radii of thewhite dwarfs in this magnitude-limited survey and for the cooling timescales. This distribution has (1) a ``peak'' component centered near 0.6Msolar, (2) a low-mass component centered near 0.4Msolar, and (3) a high-mass component above about 0.8Msolar. The formation rate of DA white dwarfs from the PG isestimated to be0.6×10-12pc-3yr-1. Of these, 75%are from the peak component, 10% from the low-mass component, and 15%from the high-mass component. The low-mass component requires binaryevolution for 100% of the objects, with a degenerate companion likely inthe majority of cases. Comparison with predictions from a theoreticalstudy of the white dwarf formation rate for single stars indicates that>=80% of the high-mass component requires a different origin,presumably mergers of lower mass double degenerate stars. The need for abinary channel may not be as great for the massive, very hot whitedwarfs found in the EUV all-sky surveys. In an Appendix, we even suggestthat an enhanced density of the massive white dwarfs at lower Galacticlatitudes might be due to some of them being the progeny of B stars inGould's Belt. In order to estimate the recent formation rate of allwhite dwarfs in the local Galactic disk, corrections for incompletenessof the PG, addition of the DB-DO white dwarfs, and allowance for starshidden by luminous binary companions had to be applied to enhance therate. An overall formation rate of white dwarfs recently in the localGalactic disk of(1+/-0.25)×10-12pc-3yr-1 isobtained. Admittedly, the systematic errors in this estimate aredifficult to quantify. Two recent studies of samples of nearby Galacticplanetary nebulae lead to estimates around twice as high. Difficultiesin reconciling these determinations are discussed.
| 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.
| 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.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| Wide Binary Systems and the Nature of High-Velocity White Dwarfs
We present measured radial velocities and complete space motions for 116white dwarf stars with M dwarf companions. Thirteen pairs have``halo-like'' velocities. According to a recent study by Oppenheimer etal., all these high-velocity white dwarfs should be considered part ofthe dark matter heavy halo of the Galaxy, based on their kinematics.Based on the near-solar abundance levels of the M dwarf companions, weconclude that 12 of our 13 high-velocity white dwarfs are actually partof the high-velocity tail of the thick disk, rather than the dark matterhalo of the Galaxy, in agreement with the results of a recent study of514 M dwarfs performed by Reid, Sahu, & Hawley. We find only onepotential stellar halo white dwarf (LP 164-52) in our sample. The Mdwarf companion of LP 164-52 is a metal-poor, intermediate subdwarf withhigh-velocity UVW-space motions. In view of the similarity to oursample, we conclude that the majority of the dark matter halo whitedwarfs identified by Oppenheimer et al. are most likely to be members ofthe thick disk, and hence their contribution to the dark matter contentof the halo is very much overestimated. Our results suggest that theassignment of population membership solely on incomplete kinematicalinformation is not definitive and that a more robust examination ofsuspected halo white dwarfs must be performed. Based on observationsobtained with the Apache Point Observatory 3.5 m telescope, which isowned and operated by the Astrophysical Research Consortium, and theSARA Observatory at Kitt Peak, which is owned and operated by theSoutheastern Association for Research in Astronomy.
| Revised Coordinates and Proper Motions of the Stars in the Luyten Half-Second Catalog
We present refined coordinates and proper-motion data for the highproper-motion (HPM) stars in the Luyten Half-Second (LHS) catalog. Thepositional uncertainty in the original Luyten catalog is typicallygreater than 10" and is often greater than 30". We have used the digitalscans of the POSS I and POSS II plates to derive more accurate positionsand proper motions of the objects. Out of the 4470 candidates in the LHScatalog, 4323 objects were manually reidentified in the POSS I and POSSII scans. A small fraction of the stars were not found because of thelack of finder charts and digitized POSS II scans. The uncertainties inthe revised positions are typically ~2" but can be as high as ~8" in afew cases, which is a large improvement over the original data.Cross-correlation with the Tycho-2 and Hipparcos catalogs yielded 819candidates (with mR<~12). For these brighter sources, theposition and proper-motion data were replaced with the more accurateTycho-2/Hipparcos data. In total, we have revised proper-motionmeasurements and coordinates for 4040 stars and revised coordinates for4330 stars. The electronic version of the paper5 contains the updated information on all 4470stars in the LHS catalog.
| 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.
| Low resolution spectroscopy of ISOGAL sources: Search for early-type stars with infrared excess
An analysis of low resolution spectra and infrared data of 29ISOGAL-DENIS sources with mid-IR excess is presented. Eight ISOGALsources from our sample with 7-15 mu m excesses are found to be B andA-type stars, some of them with emission lines. Two ISOGAL sources,J175614.4-240831 (B3-4IIIe) and J173845.3-312403 (B7IIIe), show a bumpbetween 5000 and 6000 Å which may be attributed to extended redemission (ERE). Some of the B, A and F-type giants with a large infraredexcess might be in the post-AGB phase. For about 50% of the sources inthis preliminary study, a nearby second (or even multiple) component wasfound. Such sources, in particular two B-stars, are not discussed whenthe probability of the optical spectrum being associated with the ISOGALsource is low. These results confirm that the DENIS-ISOGAL I-J/K-[15]diagram is the most suitable diagram to distinguish between early (AB)and late spectral types (KM). It provides the most useful tool tosystematically search for nearby early-type stars with an infraredexcess among the background of distant AGB stars in ISOGAL fields of theGalactic disk. This is paper No. 9 in a refereed journal based on datafrom the ISOGAL project. Based on observations with ISO, an ESA projectwith instruments funded by ESA Member States (especially the PIcountries: France, Germany, The Netherlands and the UK) and with theparticipation of ISAS and NASA. Based on observations collected at theEuropean Southern Observatory, La Silla Chile (63.L-0319).
| The White Dwarf Distance to the Globular Cluster 47 Tucanae and its Age
We present a new determination of the distance (and age) of the Galacticglobular cluster 47 Tucanae (NGC 104) based on the fit of its whitedwarf (WD) cooling sequence with the empirical fiducial sequence oflocal WD with known trigonometric parallax, following the methoddescribed by Renzini and collaborators. Both the cluster and the localWDs were imaged with HST WFPC2 using the same instrument setup. Weobtain an apparent distance modulus of (m-M)V=13.27+/-0.14,consistent with previous ground-based determinations and shorter thanthat found using Hipparcos subdwarfs. Coupling our distancedetermination with a new measure of the apparent magnitude of themain-sequence turnoff based on our HST data, we derive an age of13+/-2.5 Gyr. Based on observations with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by Association of Universities for Research in Astronomy, Inc.,under NASA contract NAS 5-26555.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| White Dwarfs in Common Proper Motion Binary Systems: Mass Distribution and Kinematics
We present the mass distribution, gravitational redshifts, radialvelocities, and space motions of white dwarf stars in common propermotion binary systems. The mass distribution we derive for the 41 DAwhite dwarfs in this study has a mean of 0.68+/-0.04 Msolar.This distribution has a slightly higher mean and larger dispersion thanmost previous white dwarf studies. We hypothesize that this is due to ahigher fraction of cool (average Teff~10,000 K), hence old,white dwarfs in our sample. Our results indicate that samples made up ofpredominantly cool, old white dwarf stars tend to have a bimodaldistribution with a second mass peak at ~1.0 Msolar, whichskews the mean toward a higher mass. Both the mean and individual whitedwarf masses we report here are in better agreement with thosedetermined from model atmosphere spectroscopic fits to line profilesthan with most previous gravitational redshift studies of cool whitedwarfs. Our results indicate that measurement biases and weak geocoronalemission lines in the observed spectra may have affected previousgravitational redshift measurements. These have been minimized in ourstudy. We present measurements for some previously unobserved whitedwarfs, as well as independent new measurements for some that have beenreported in the literature. A list of complete space motions for 50 widebinary white dwarfs is presented, derived from radial velocitymeasurements of their nondegenerate companions. We find that the UVWspace motions and dispersions of the common proper motion binaries thatcontain white dwarf components are consistent with those of old,metal-poor disk stars.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Virgo |
| Ascensió Recta: | 13h30m13.65s |
| Declinació: | -08°34'29.5" |
| Magnitud Aparent: | 11.721 |
| Distancia: | 18.018 parsecs |
| Moviment propi RA: | -1109.3 |
| Moviment propi Dec: | -473.1 |
| B-T magnitude: | 12.269 |
| V-T magnitude: | 11.767 |
Catàlegs i designacions:
|
