Trace hydrogen in DB white dwarfs : GD 408 - the lowest H abundance case.

Abstract

Improved IUE observations revealed the presence of trace hydrogen in the helium atmosphere of GD408, a DB star at Teff of 13,500 K. The hydrogen abundance determined by spectroscopic analysis, log H/He = -6, is considerably smaller than the value determined for other DB stars, which are classified DBA due to the presence of visible hydrogen lines. A faint H-alpha absorption line, barely detected, is consistent with the abundances derived. The DBA stars thus probably do not constitute a separate subclass with a well-defined hydrogen trace abundance, but display a rather wide range of H abundances, limited only by the observational capabilities.