Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
A Spectroscopic Survey and Analysis of Bright, Hydrogen-rich White Dwarfs
We have conducted a spectroscopic survey of over 1300 bright (V <=17.5), hydrogen-rich white dwarfs based largely on the last publishedversion of the McCook & Sion catalog. The complete results from oursurvey, including the spectroscopic analysis of over 1100 DA whitedwarfs, are presented. High signal-to-noise ratio optical spectra wereobtained for each star and were subsequently analyzed using our standardspectroscopic technique where the observed Balmer line profiles arecompared to synthetic spectra computed from the latest generation ofmodel atmospheres appropriate for these stars. First, we present thespectroscopic content of our sample, which includes manymisclassifications as well as several DAB, DAZ, and magnetic whitedwarfs. Next, we look at how the new Stark broadening profiles affectthe determination of the atmospheric parameters. When necessary,specific models and analysis techniques are used to derive the mostaccurate atmospheric parameters possible. In particular, we employ Mdwarf templates to obtain better estimates of the atmospheric parametersfor those white dwarfs that are in DA+dM binary systems. Certain uniquewhite dwarfs and double-degenerate binary systems are also analyzed ingreater detail. We then examine the global properties of our sampleincluding the mass distribution and their distribution as a function oftemperature. We then proceed to test the accuracy and robustness of ourmethod by comparing our results to those of other surveys such as SPYand Sloan Digital Sky Survey. Finally, we revisit the ZZ Cetiinstability strip and examine how the determination of its empiricalboundaries is affected by the latest line profile calculations.Based on observations made with ESO Telescopes at the La Silla orParanal Observatories under program ID 078.D-0824(A).
| Catalogue of averaged stellar effective magnetic fields - II. Re-discussion of chemically peculiar A and B stars
This paper presents a catalogue and the method of determining averagedquadratic effective magnetic fields for 1212main-sequence and giant stars, and 11 white dwarf stars. The catalogueincludes stars that are members of several open clusters. We havecompiled measurements of the longitudinal magnetic field for thosestars, which were scattered in the existing literature. A new parameter,magnetization (MA), has been defined, and we present values of MA forstars of various spectral classes. Our sample includes a subset of 610chemically peculiar early-type stars. We confirm the conclusion of ourprevious study that the number distribution of all chemically peculiarstars versus the averaged magnetic field strength is described by adecreasing exponential function. Relations of this type also hold forstars of all the analysed subclasses of chemical peculiarity.Magnetization tends to correlate with the effective temperature only athigh MA, for He-weak and He-rich stars.
| 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.
| A Search for Kilogauss Magnetic Fields in White Dwarfs and Hot Subdwarf Stars
We present new results of a survey for weak magnetic fields among DAwhite dwarfs, including some brighter hot subdwarf stars. We havedetected variable circular polarization in the H? line of the hotsubdwarf star Feige 34 (spectroscopic type: sdO). From these data, weestimate that the longitudinal magnetic field of this star varies from-1.1+/-3.2 to +9.6+/-2.6 kG, with a mean of about +5 kG and a periodlonger than 2 hr. In this study, we also confirm the magnetic nature ofwhite dwarf WD 1105-048, found earlier in a study by Aznar Cuadrado andcoworkers, and present upper limits of kilogauss longitudinal magneticfields of the five brightest DA white dwarfs. Our data support thefinding of Aznar Cuadrado and coworkers that ~25% of white dwarfs havekilogauss magnetic fields. This frequency also confirms results of earlyestimates obtained using the magnetic field function of white dwarfs(Fabrika & Valyavin).
| 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.
| Magnetic Fields of White Dwarfs
We present results of Zeeman CCD measurements of DA white dwarfs carriedout with the 6-m telescope of the Special Astrophysical Observatory in1995 2000. The accuracy of the measurements ranges from 0.5 to 15 kG.Magnetic fields were detected in three of the 26 observed white dwarfs.We have confirmed the existence of magnetic fields on WD 1953-011 (Be=28±6 kG) and 40 Eri B (B e(min)=-2.8 kG, B e(max)=1.8 kG). Thedata are discussed in the context of a statistical study of theoccurrence of magnetic white dwarfs.
| Ionization of the Local Interstellar Medium
We have determined the distribution of the ionization of theinterstellar medium within the vicinity of the Sun due to theultraviolet radiation from nearby white dwarf stars. If one assumessteady state conditions, the ionization is irregular and does notconsist of one large continuous cloud, but instead consists of irregularregions of ionization with occasional mergers of the Strömgrenspheres. We also present a compilation of the known white dwarf starswithin 20 pc of the Sun.
| A Catalog of Spectroscopically Identified White Dwarfs
A catalog of 2249 white dwarfs which have been identifiedspectroscopically is presented complete through 1996 April. Thiscompilation is the fourth edition of the Villanova Catalog ofSpectroscopically Identified White Dwarfs. For each degenerate star, thefollowing data entries with references are provided: (1) a catalogcoordinate designation or WD number, in order of right ascension; (2)the right ascension and declination for epoch 1950.0; (3) the spectraltype based upon the new system; (4) a catalog symbol denoting binarymembership; (5) a list of most names known to exist for a given star;(6) proper motion and position angle; (7) broadband UBV photometry, V,B-V, U-B (8) multichannel spectrophotometry, v(MC), g-r (9)Strömgren narrowband photometry, y, b-y, u-b (10) an absolutevisual magnitude based upon the best available color-magnitudecalibration or trigonometric parallax; (11) the observed radial velocityuncorrected for gravitational redshift or solar motion; and (12) thetrigonometric parallax with mean error when available. Notes for unusualor peculiar stars and a coded Reference Key alphabetized by the firstauthor's last name are presented, as well as an expanded tablecross-referencing all names to the catalog WD number. An introductionand full descriptions of the entries are provided in the text.
| Close Binary White Dwarf Systems: Numerous New Detections and Their Interpretation
We describe radial velocity observations of a large sample of apparentlysingle white dwarfs (WDs), obtained in a long-term effort to discoverclose double-degenerate (DD) pairs, which might comprise viable Type Iasupernova (SN Ia) progenitors. We augment the WD sample with apreviously observed sample of apparently single subdwarf B (sdB) stars,which are believed to evolve directly to the WD cooling sequence afterthe cessation of core helium burning. We have identified 18 new radialvelocity variables, including five confirmed sdB + WD short-periodpairs. Our observations are in general agreement with the predictions ofthe theory of binary star evolution. We describe a numerical method toevaluate the detection efficiency of the survey and estimate the numberof binary systems not detected because of the effects of varying orbitalinclination, orbital phase at the epoch of the first observation, andthe actual temporal sampling of each object in the sample. Follow-upobservations are in progress to solve for the orbital parameters of thecandidate velocity variables.
| Magnetic field function of white dwarfs.
The frequency of magnetic white dwarfs is shown to decrease sharply withdistance. This is caused by two reasons: 1. the observational selection- we observe cool stars only at short distances; 2. evolution of whitedwarf magnetic fields - the white dwarfs' average magnetic fieldincreases with their cooling. The authors have estimated the realfrequencies of hot (T > 10000 K) and cool (T < 10000 K) magneticdegenerates. The frequency of hot stars is 3.5±0.5%, theirnumbers are selection-independent to distances 80±10 pc. Thefrequency of cool stars, as estimated for distances to 25 pc, is?20±5%. A magnetic field function (MEF) of white dwarfs wasstudied in the range of surface magnetic fields from 6 kG to 1 GG. Thisfunction is a power function with a spectral index ? =-1.5±0.1. The MEF of hot generates (initial) and the MEF of cooldegenerates (current) are discussed. The slopes of both functions arethe same in spite of the strong magnetic field evolution in whitedwarfs. This confirms the idea that the magnetic field evolution indegenerates does not depend in a first approximation on the initialfield strength. The authors have concluded from the MFEs analysis thatthe probable minimal large-scale magnetic field strength of hot whitedwarfs is about Bs ? 1 - 10 kG, and this value in coolwhite dwarfs is about Bs ? 10 - 50 kG.
| White dwarfs observed by the HIPPARCOS satellite.
The HIPPARCOS satellite has measured trigonometric parallaxes for 20white dwarfs. With the exception of one each of the spectral types DZ,DC, DB, DQ, the majority are of spectral type DA. We compare theparallaxes with the most recent ground-based determinations. From aspectroscopic analysis of new optical observations we determineatmospheric parameters T_eff_ and logg. From the angular diameters andthe parallaxes radii are obtained, and masses are calculated from theseradii and the spectroscopic logg. These data are used to test thetheoretical mass-radius relation. On the other hand, assuming thisrelation, including finite temperature evolutionary effects, we canderive radii and masses from the effective temperatures and parallaxes.From these we calculate surface gravities, which can be compared to theresults of the spectroscopic analysis.
| Measurements of magnetic fields on white dwarfs
Not Available
| A library of IUE white dwarf spectra for stellar population analyses.
We present high Signal to Noise ratio IUE spectra of different classesof white dwarfs, to be used as templates for stellar population analysesin the ultraviolet region. We present average stellar parametersassociated to each group. The library contains 6 groups for DA's, 2 forDO's and 5 for DB's. We also present equivalent widths of spectralfeatures, and continuum measurements. We call attention to the spectralcharacteristics which are promising indicators of the presence of whitedwarfs in the spectra of composite stellar populations.
| ICCD Speckle Observations of Binary Stars. XIV. A brief Survey for Duplicity Among White Dwarf Stars
Speckle observations were obtained in 1988 of the white dwarf stars GD294, GD 140, GD 319, HZ 43, and LHS 354 at the 3.6-mCanada-France-Hawaii Telescope. Faint, red companions to GD 319 and HZ43 were detected, and the companion to GD 319 confirmed at the 4-mtelescope on Kitt Peak in 1993. Additional observations havesubsequently been measured at the 100-in. telescope on Mt. Wilson andthe 4-m telescope on Cerro Tololo. In addition to the first precisemeasurements of the position angles and angular separations of thesecondaries for these two objects, we present values for ?V aswell. We demonstrate that the companion to GD 319 is optical while thatfor HZ 43 is physical in nature.
| A Search for Magnetic Fields among DA White Dwarfs
A survey for magnetism has been carried out for a magnitude-limitedsample of nearly 170 DA white dwarfs utilizing the technique of Zeemanspectropolarimetry. With a mean uncertainty for longitudinal fields = 8600 G, the results provide thefirst statistically useful survey of white dwarfs for magnetic fieldsweaker than a few million gauss. An additional, very weak field"minisurvey," with ˜ 2000G, is also reported. Because DA spectral types comprise nearly 80% ofall white dwarfs, the results are very likely representative of theentire class.Four new magnetic stars, with field strengths between˜105 and nearly ˜109 G, werediscovered, including the three weakest field white dwarfs yetidentified. The complete sample of 42 known magnetic examples isenumerated. The incidence of magnetism among white dwarfs is found to be4.0%±1.5% for fields between ˜3 × 104 and1O9 G. When broken down by field strength, the detection rateis consistent with being constant at a value of ˜1% per decadeinterval over the same range in B. There is no evidence thus far for therapid increase in number of magnetic stars with declining field strengthwhich must eventually occur. Implications for the origin of magneticwhite dwarfs and their relation to stars at other stages of evolutionare discussed.Several composite-spectrum binaries (DA + dM) were identified in thecourse of the survey, including one apparently noninteracting systemwith a photometric period of ˜4 hr.
| IUE temperatures for white dwarf stars in and around the ZZ Ceti instability strip
We have analyzed all the archival IUE images of the DA4 and DA5 whitedwarfs with model atmosphere fluxes incorporating the quasi-molecularopacities to obtain a consistent set of temperatures for both thevariables and nonvariables in and around the ZZ Ceti instability strip.We have found four nonvariable stars inside the instability strip. Takenat face value, this indicates that temperature is not the only parameterthat determines if a star pulsates or not, but the large uncertaintiesin the temperature determinations makes it impossible for us to offerthis as more than just a suggestion at the present time.
| A spectrophotometric atlas of white dwarfs compiled from the IUE archives
Ultraviolet spectrophotometric data are presented for 182 subluminousstars, mostly white dwarfs, that have been observed with thelow-resolution mode of the IUE satellite since 1978 and for whicharchived data became available on April 1, 1989. Tables of 20 A binneddata are given for all stars. Of these, 133 cover the entire wavelengthrange of the IUE, while the remainder have partial coverage. Plots ofthe original unbinned spectra are given. Many targets had multiplespectra combined, and the older spectra were reprocessed and correctedfor reduction procedure changes made over the past 10 years.
| Magnetic field measurements of white dwarfs
Zeeman observations of 11 bright DA white dwarfs on the 6-metertelescope are given. No magnetic fields are detected on them, and thecorresponding upper limits on mean magnetic fields are a few kGs. Themethod of observation using the hydrogen magnetometer and ways ofimproving its penetrating capacity are described.
| A Hydrogen Magnetometer of the 6-METER Telescope - Part One - Efficiency Estimation
Not Available
| Preliminary Version of the Third Catalogue of Nearby Stars
Not Available
| Spectrophotometry of white dwarfs as observed at high signal-to-noise ratio. II
CCD spectrophotometry is presented of 140 white dwarfs at high SNR andis analyzed in detail. Energy distributions at 14,000 A are given atbandpasses from 3571 to 8300 A, and equivalent widths of lines of H, HeI, metals, and atomic and molecular carbon are given as functions ofcolor for DB, DQ, DZ, and DA stars. New forbidden H I transitions at6068 A and 6632 A are found in at least the two hottest DB stars, newmetallic features are found in cooler DZ stars, and the presence of Ca Iin vMa 2 is confirmed. The spectrum of the hot DQAB star G227 - 5 andthe pressure-shifted carbon bands seen in 0038-226 are discussed indetail. Comparison of the optical energy distribution of the latter withpublished IR fluxes shows that the 1-2 micron region is stronglydepressed, with extensive blanketing. Equivalent widths, central depths,and width parameters are presented for H-alpha in 73 DA stars in thesample, and their dependences on color are studied.
| Kinematical tests of white dwarf formation channels and evolution
The authors have computed the space motions for all degenerate starshaving spectroscopic classification, proper motions, and measuredcolors, in order to meet three objectives: (1) identify kinematicallydistinct spectroscopic subgroups; (2) compare the results with earlieranalyses which used smaller, less well-observed samples and; (3) to testa number of theoretical scenarios and predictions of white dwarfformation channels. Some of the kinematical tests required thedetermination of effective temperatures, radii, masses, and coolingages, which are tabulated for the individual members of eachspectroscopic subgroup along with the individual space motions. For eachspectroscopic subgroup, average U, V, W, transverse velocity components,masses, radii, and their associated dispersions, are tabulated.
| A catalog of spectroscopically identified white dwarfs
A catalog of 1279 spectroscopically identified white dwarfs ispresented, complete to 1987 January. For each degenerate star, thecatalog lists a coordinate designation, in order of increasing rightascension; the full coordinates for 1950.0; the spectral type on the newwhite dwarf classification system; a symbol denoting binary membership;most known names; proper motion and position angle; broad-band,narrow-band, and multichannel colors; a best available absolutemagnitude; trigonometric parallax; and radial velocity. A Notes sectionand a coded Reference Key are presented, as well as a tablecross-referencing all names to catalog coordinate designation.
| An Upper Limit to the Space Density of Short-period Noninteracting Binary White Dwarfs
We have searched for short-period, detached binary stars in which bothstars are white dwarfs by looking for radial velocity variations inspectroscopically identified DA and DB white dwarfs. The search wassensitive to binaries with orbital periods between 30 s and 3 hr and,within that range, we would have detected roughly 90% of all binaries,depending on the distribution of their orbital periods, mass ratios, andspectral types. We observed 44 stars without finding any binaries. Thefraction of white dwarfs that are binaries is less than 1/20 with a 90%probability and less than 1/37 with a 70% probability. Other surveyssensitive to longer periods have also failed to find a large populationof binary white dwarfs. The space density of binary white dwarfs is toolow to account for the rate of Type I supernovae in the Galaxy and toolow to agree comfortably with recent estimates of their numbers fromtheoretical calculations of the evolution of close binaries. Binarywhite dwarfs are the dominant source of the background gravitationalwaves at periods between about 30 s and 1 hr. The background is likelyto be much lower than previously thought.
| Search for low-mass objects. II
The IR (K and L) magnitudes of 20 white dwarfs have been measured with aview to discovering brown dwarf companions, as previously discussed byKumar (1985). The white dwarf mass was taken to be 0.7 solar masses,with progenitor and brown dwarf masses being 2.1 and 0.05, respectively;a brown dwarf companion with this mass, an age of less than 5 Gyr, and adistance 1-20 AU from the white dwarf's progenitor would have beendetected. However, none was discovered.
| The frequency of hydrogen white dwarfs as observed at high signal to noise ratio
Despite their relatively high stability and slow evolution, white dwarfsare subject to important changes as they cool, since little material isrequired to contaminate an atmosphere that is only a few meters thick.With an eye to these complexities, about 140 white dwarfs were observedon eight nights in 1982, with about 10,000 to 30,000 photons/pixel beingdetected near the central wavelengths of the double CCD camera used.Steep Balmer decrements are noted to occur in the cooler degenerationdue to increasing He/H ratio. The predominance of non-DA stars as lateevolutionary stages prompts questions as to the fate of the accreted andresidual H in postasymptotic giant branch evolution. No definitivetheoretical argument is found to explain how most of the residualhydrogen is destroyed.
| Photometry of white dwarfs in the Stromgren system
Photometric observations of 182 faint stars, mostly white dwarfs aregiven. Primarily, the uby filters of the Strömgrenintermediate-band system have been employed, but measures of them1 index have been obtained for 44 of these objects. A totalof 43 stars, observed in common with Graham (1972), have been used todetermine final transformations for the present data set. Some aspectsof the two-color diagrams for the DA and non-DA stars are discussed.
| Catalogue of white dwarfs
Not Available
| Stromgren Photometry of Cool White Dwarfs
Strömgren photometric data are given for a selected group of 30white dwarfs. This sample consists mostly of nearby, cool, non-DA stars.In addition, colors for 16 more white dwarfs are compared to thoseobtained by other authors. By comparing the response of our localphotometric system to that of the standard Strömgren system, it isfound that different transformation coefficients must be used for DA andnon-DA stars. It is suggested that greater accuracy could be obtained ifthe spectral types of the stars are taken into account in futureobservations of white dwarfs.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Vasak |
| Sað Açýklýk: | 07h17m36.26s |
| Yükselim: | +58°24'20.4" |
| Görünürdeki Parlaklýk: | 12.021 |
| özdevim Sað Açýklýk: | 21.3 |
| özdevim Yükselim: | -29.3 |
| B-T magnitude: | 12.077 |
| V-T magnitude: | 12.026 |
Kataloglar ve belirtme:
|
INHABITED SKY
