The Temperature Scale and Mass Distribution of Hot DA White Dwarfs

Results are presented from a comprehensive spectroscopic survey of DA white dwarfs hotter than ~25,000 K. The observations consisted of CCD spectra with signal-to-noise ratios of ~100 in the blue, with ~5 A resolution. The majority of the spectra covered the wavelength range of 3500-7500 A, allowing the detection of a number of cool companions. To date, spectra have been obtained and analyzed for 174 DA stars. The spectra were analyzed using our model atmospheres, which are described in detail here, resulting in temperatures and gravities with average internal errors of 1% and 0.04 dex, respectively. Comparisons with previously published results showed that temperature determinations for Teff 1.1 M☉. The ultramassive DA's comprise a much larger fraction of the total than was the case for cooler samples. Calculations based on white dwarf evolutionary models showed that a higher proportion of massive white dwarfs is expected to be found in samples with Teff 30,000 K as a result of differential cooling effects. Within the range Teff > 40,000 K, the EUV-selected subsample did have proportionately more massive stars than the optically selected subsample. However, a detailed comparison showed that EUV and optical surveys were equally capable of detecting relatively nearby massive white dwarfs. On the other hand, interstellar EUV absorption eliminated from the EUV sample many of the more distant stars that were detectable optically. Therefore, the apparent excess of massive DA white dwarfs in the EUV sample is largely due to a relative deficit of stars with more typical masses. Results are presented for individual stars, including a number of subdwarf identifications and reports of detections of cool companions. Properties of some of the more interesting binaries are discussed. We also report on some stars in the sample that evidently have varying He abundances. WD 0612+177 (G104-27) had been observed, at one point, to have photospheric He I; we confirm that He has remained absent since then. WD 0718-316 (RE 0720-314) is in a post-common-envelope binary and has photospheric He II, the abundance of which appears to vary by more than an order of magnitude over timescales of months.

[1]  D. Finley The mass distribution of DA white dwarfs , 1995 .

[2]  M. Burleigh,et al.  An EUV-selected sample of DA white dwarfs from the ROSAT All-Sky Survey — II. EUV and soft X-ray properties , 1997 .

[3]  S. Bowyer,et al.  The temperature scale of hot DA white dwarfs - Temperatures from far-ultraviolet continuum fluxes , 1990 .

[4]  A. V. Filippenko,et al.  THE IMPORTANCE OF ATMOSPHERIC DIFFERENTIAL REFRACTION IN SPECTROPHOTOMETRY. , 1982 .

[5]  E. Sion,et al.  FAR-ULTRAVIOLET AND OPTICAL SPECTROSCOPY OF THE DETACHED WHITE DWARF-M DWARF BINARY RE J1629+780 , 1995 .

[6]  S. Vennes,et al.  Hot White Dwarfs in the Extreme Ultraviolet Explorer Survey. I. Properties of a Southern Hemisphere Sample , 1996 .

[7]  James Liebert,et al.  A spectroscopic analysis of DAO and hot DA white dwarfs: The implications of the prescence of helium and the nature of DAO stars , 1994 .

[8]  S. Bowyer,et al.  Discovery of Photospheric Helium in the Ultramassive DA White Dwarf GD 50 , 1996 .

[9]  I. Neill Reid,et al.  White Dwarf Masses-Gravitational Redshifts Revisited , 1996 .

[10]  H. Shipman,et al.  Masses and radii of white-dwarf stars. III. Results for 110 hydrogen-rich and 28 helium-rich stars. , 1979 .

[11]  H. Shipman,et al.  Masses and radii of white-dwarf stars. IV. The two-color diagram , 1980 .

[12]  David S. Finley,et al.  White Dwarf Standard Stars: G191-B2B, GD 71, GD 153, HZ 43 , 1995 .

[13]  J. Greenstein Multichannel spectrophotometry and the luminosities of white dwarfs. , 1976 .

[14]  Allan Sandage,et al.  OPTICAL IDENTIFICATION OF 3C 48, 3C 196, AND 3C 286 WITH STELLAR OBJECTS , 1963 .

[15]  Dimitri Mihalas,et al.  On the use of variable Eddington factors in non-LTE stellar atmospheres computations , 1970 .

[16]  Nicholas B. Suntzeff,et al.  SOUTHERN SPECTROPHOTOMETRIC STANDARDS. I. , 1992 .

[17]  K. Werner On the Balmer Line Problem , 1996 .

[18]  Paul S. Smith,et al.  A Search for Magnetic Fields among DA White Dwarfs , 1995 .

[19]  M. Wood,et al.  Constraints on the age and evolution of the Galaxy from the white dwarf luminosity function , 1992 .

[20]  B. Zuckerman,et al.  Orbital Separations of White Dwarf--Red Dwarf Binaries , 1996 .

[21]  D. W. Latham,et al.  A rediscussion of the atmospheric extinction and the absolute spectral-energy distribution of Vega. , 1975 .

[22]  S. Bowyer,et al.  The first Extreme Ultraviolet Explorer source catalog , 1994 .

[23]  James Liebert,et al.  A spectroscopic determination of the mass distribution of DA white dwarfs , 1992 .

[24]  E. R. Breeveld,et al.  Detection of a new white-dwarf binary system in the extreme ultraviolet using the Rosat Wide Field Camera , 1992, Nature.

[25]  John R. Thorstensen,et al.  Discovery of a New White Dwarf in a Binary System (EUVE 0720-317) in the Extreme Ultraviolet Explorer Survey and Implications for the Late Stages of Stellar Evolution , 1994 .

[26]  Dimitri Mihalas,et al.  The equation of state for stellar envelopes. I - An occupation probability formalism for the truncation of internal partition functions , 1988 .

[27]  F. Wesemael,et al.  Atmospheres for hot, high-gravity stars. I. Pure hydrogen models. , 1980 .

[28]  F. Paerels,et al.  A soft X-ray survey of hot white dwarfs with Exosat , 1989 .

[29]  S. Bowyer,et al.  Extreme ultraviolet explorer bright source list , 1994 .

[30]  R. Petre,et al.  An X-ray survey of hot white dwarf stars - Evidence for a m(He)/n(H) versus Teff correlation , 1986 .

[31]  Alan A. Wells,et al.  The ROSAT Wide Field Camera all-sky survey of extreme-ultraviolet sources – I. The Bright Source Catalogue , 1993 .

[32]  J. Thorstensen,et al.  The pre-cataclysmic binary EUVE 2013+400 , 1994 .

[33]  B. Zuckerman,et al.  Companions to white dwarfs : very low-mass stars and the brown dwarf candidate GD 165B , 1992 .

[34]  J. Liebert,et al.  Photospheric soft X-ray emission from hot DA white dwarfs , 1984 .

[35]  J. Liebert,et al.  The Palomar-Green Catalog of Ultraviolet-Excess Stellar Objects , 1986 .

[36]  J. Cooper,et al.  Hydrogen Stark-Broadening Tables , 1973 .

[37]  A. Renzini,et al.  The development of the red giant branch. II - Astrophysical properties , 1990 .

[38]  J. Liebert,et al.  The isolated massive DA white dwarf GD 50 , 1991 .

[39]  Patrick Jelinsky,et al.  The distribution of neutral hydrogen in the interstellar medium. 1: The data , 1994 .

[40]  G. Wegner Photometry of white dwarfs in the Stromgren system , 1983 .

[41]  Edwin E. Salpeter,et al.  MODELS FOR ZERO-TEMPERATURE STARS , 1961 .

[42]  A. Renzini,et al.  Temperatures, gravities, and masses for a sample of bright DA white dwarfs and the initial-to-final mass relation , 1995 .

[43]  P. Bergeron Stark Broadening in White Dwarf Atmospheres , 1993 .

[44]  D. Koester,et al.  Detection of Heavy Elements in the EUVE Spectrum of a Hot White Dwarf , 1996 .

[45]  G. Fontaine,et al.  Diffusion time scales in white dwarfs , 1979 .

[46]  G. Fontaine,et al.  Stromgren photometry of ZZ Ceti and other DA white dwarfs. , 1985 .

[47]  Matthew D. Lallo,et al.  Kinematical Tests of White Dwarf Formation Channels and Evolution , 1988 .

[48]  Martin A. Barstow,et al.  RE 0720-318: a pre-cataclysmic binary system with a DAO white dwarf primary , 1995 .

[49]  A. Sansom,et al.  ROSAT studies of the composition and structure of DA white dwarf atmospheres , 1993 .

[50]  M. Barstow,et al.  The coolest DA white dwarfs detected at soft X-ray wavelengths , 1992 .

[51]  D. Hayes,et al.  Stellar Absolute Fluxes and Energy Distributions from 0.32 to 4.0 μm , 1985 .

[52]  M. Wood,et al.  Theoretical white dwarf luminosity functions: DA models , 1995 .

[53]  J. Wilson,et al.  On the acoustic flux of Sirius A , 1981 .

[54]  Claudio Mendoza,et al.  The Opacity Project - Computation of Atomic Data , 1992 .

[55]  D. Hayes,et al.  Calibration of Fundamental Stellar Quantities , 1985 .

[56]  J. Holberg,et al.  UBV Photometry of Hot White Dwarf Stars , 1993 .

[57]  P. Green,et al.  CONSTRAINTS ON THE ORIGIN OF DWARF CARBON STARS , 1994 .

[58]  D. G. Hummer,et al.  The equation of state for stellar envelopes. IV - Thermodynamic quantities and selected ionization fractions for six elemental mixes , 1990 .

[59]  D. G. Hummer,et al.  Stellar Atmospheres: Beyond Classical Models , 1991 .

[60]  David A. H. Buckley,et al.  An EUV-selected sample of DA white dwarfs from the ROSAT All-Sky Survey — I. Optically derived stellar parameters , 1997 .

[61]  Gilles Fontaine,et al.  The Presence of Helium in Hot DA White Dwarfs: The Role of Radiative Levitation and the Case for Stratified Atmospheres , 1988 .

[62]  J. Liebert,et al.  The Luminosity Function of DA White Dwarfs , 1986 .

[63]  J. Liebert,et al.  Two ultramassive white dwarfs found among candidates for magnetic fields , 1992 .

[64]  G. Rybicki,et al.  A MODIFIED FEAUTRIER METHOD. , 1971 .

[65]  J. B. Holberg,et al.  DA white dwarf effective temperatures determined from IUE Lyman-alpha profiles , 1986 .