Carbon and hydrogen in hot DB white dwarfs

The roles played by carbon and hydrogen in helium atmosphere (DB) white dwarf evolution is not well understood. We present GHRS observations (centered at 1215 and 1335 Å) of the hot helium white dwarfs PG 0112+104 (≈30,000 K) and GD 190 (≈23,000 K) and EUVE observations of the well-studied DB GD 358 (≈27,000 K) and revisit pre-COSTAR observations of GD 358. We detect Lyα and carbon absorption features in all three of our hot DB white dwarfs. Using the non-LTE model atmosphere code TLUSTY, we calculate carbon and hydrogen surface abundances and mass fractions. We discuss the implications of our analysis for two aspects of DB evolution. First, we find hydrogen surface abundances corresponding to mass fractions over an order of magnitude too low to explain the observed lack of helium atmosphere white dwarfs between 45,000 and 30,000 K unless we allow extensive mixing in DB atmospheres. Second, the photospheric carbon in all three objects cannot be explained by interstellar accretion or conventional convective dredge-up. We present evidence for substantial horizontal motions in two of our objects that may explain both the presence of carbon and the lower than predicted hydrogen surface abundance.