Spectroscopic Studies of DB White Dwarfs: The Instability Strip of the Pulsating DB (V777 Herculis) Stars

We have secured optical spectra for the eight currently known variable DB, or V777 Her, stars. With the help of a new generation of synthetic spectra, spectroscopic effective temperatures are derived for these objects, as well as for 15 other DB or DBA stars above 20,000 K. We find that the location of the boundaries of the instability strip is sensitive to the atmospheric hydrogen abundance assumed for DB stars: the strip covers the range 22,400-27,800 K if atmospheres of pure helium are used and the range 21,800-24,700 K if undetectable traces of hydrogen are allowed for in the DB models. These determinations provide independent constraints for current seismological analyses of the V777 Her stars. More sensitive searches for weak hydrogen features in hot DB stars should help decide between the two temperature scales.

[1]  A. Putney.  Surveying DC White Dwarfs for Magnetic Fields , 1997 .

[2]  P. Bergeron,et al.  Spectroscopic Studies of DB White Dwarfs: Improved Stark Profiles for Optical Transitions of Neutral Helium , 1997 .

[3]  S. Vennes,et al.  Ultraviolet Spectroscopy of the DBV White Dwarf Star GD 358 , 1996 .

[4]  R. Stobie,et al.  EC 20058 – 5234, a low-amplitude pulsating DB white dwarf , 1995 .

[5]  G. Fontaine,et al.  Optical and Ultraviolet Analyses of ZZ Ceti Stars and Study of the Atmospheric Convective Efficiency in DA White Dwarfs , 1995 .

[6]  Paul A. Bradley,et al.  An asteroseismological determination of the structure of the DBV white dwarf GD 358 , 1994 .

[7]  S. Vennes,et al.  A critical analysis of the ultraviolet temperature scale of the helium-dominated DB and DBV white dwarfs , 1991 .

[8]  J. Liebert,et al.  KUV 05134 + 2605 - A new pulsating DB white dwarf , 1989 .

[9]  H. Bond,et al.  PG 1456+103: A New Pulsating DB White Dwarf , 1988 .

[10]  D. Winget,et al.  Two Extreme Examples of Compact Pulsators: PG 1115+158 and PG 1351+489 , 1987 .

[11]  E. L. Robinson,et al.  PG 1654+160: a new pulsating DB white dwarf , 1984 .

[12]  NULL RESULTS FROM A SURVEY FOR PULSATING DB WHITE DWARFS. , 1983 .

[13]  C. Hansen,et al.  The pulsation properties of DB white dwarfs - A preliminary analysis , 1983 .

[14]  E. L. Robinson,et al.  Photometric observations of GD 358 : OB white dwarfs do pulsate. , 1982 .

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

[16]  G. Fontaine,et al.  Recent Advances in the Theoretical Determination of the ZZ Ceti Instability Strip , 1997 .

[17]  J. Liebert,et al.  A Continuing Spectroscopic Study of the Instability Strip of the Pulsating DB (V777 HER) White Dwarfs: The View in the Optical , 1997 .

[18]  D. Winget,et al.  Hot or not? Theoretical blue edges for DA and DB white dwarf models , 1994 .

[19]  G. Fontaine,et al.  The Equation of State in Astrophysics: Computations of static white dwarf models: A must for asteroseismological studies , 1994 .

[20]  J. Liebert,et al.  A Search for Trace Amounts of Hydrogen in DB Stars , 1991 .

[21]  G. Fontaine,et al.  Synthetic spectra and atmospheric properties of cool DA white dwarfs , 1991 .