AAS Meeting #193 - Austin, Texas, January 1999
Session 37. White Dwarfs and Friends
Display, Thursday, January 7, 1999, 9:20am-6:30pm, Exhibits Hall 1

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[37.05] HST Observations of Carbon and Hydrogen in Hot DB White Dwarfs

J. L. Provencal, H. L. Shipman (University of Delaware)

We present GHRS observations centered at 1215 and 1335 \rm{Å} of the hot helium white dwarfs PG0112+104 and GD190. We detect hydrogen and carbon in both objects, raising the number of hot DBs with known traces of these elements to three (including GD 358). We calculate hydrogen and carbon surface abundances and corresponding mass fractions using the stellar atmospheres program Tlusty and the convection calculations of Althaus & Benvenuto (1996). The hydrogen mass fraction is \rm{MH\approx 10-17M*} in all three objects. This disturbing number is nearly two orders of magnitude below the predictions of chemical evolution. If the carbon originates from convective dredge-up of the underlying core, the process is initiated at temperatures much higher than expected.

The author(s) of this abstract have provided an email address for comments about the abstract: jlp@udel.edu

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