The DBA white dwarf stars in the Palomar-Green sample

Astronomy and Astrophysics – Astrophysics

Scientific paper

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Abundance, Photosphere, Stellar Composition, White Dwarf Stars, Astronomical Spectroscopy, Balmer Series, Cool Stars, Helium, Hydrogen, Stellar Models

Scientific paper

A complete spectroscopic survey of the DB stars in the Palomar-Green sample has resulted in the discovery of five new DBA stars - that is, white dwarf stars with the usual He I lines found in ordinary DBs, but with reasonably strong (over 2 A equivalent width) Balmer lines - and the rediscovery of one (G 200-39). Model atmosphere analysis of these spectra indicate that these DBA stars have temperatures in the 14,000-19,000 K range, gravities near log g = 8, and H/He ratios which vary from 0.00003 to 0.0004. Since the PG sample is well characterized in a statistical sense, it is possible to determine that roughly 20 percent of the stars in the temperature range where an H abundance of a few times 0.00001 would be detectable are, in fact, DBAs. The hydrogen found in these stars could be primordial, left over from the AGB stage convectively mixed at some stage in the white dwarf cooling sequence, or it could be accreted from the interstellar medium. Neither of these hypotheses is fully consistent with current understanding of the photospheric chemical changes which might occur in cooling white dwarfs. This paper discusses these inconsistencies in some detail and provides suggestions for future theoretical and observational work.

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