Atmospheric Parameters and Carbon Abundance for Hot DB White Dwarfs

Accurate atmospheric parameters for DB (helium atmosphere) white dwarfs near 25000 K are difficult to determine, but are needed to help or confirm the asteroseismic analysis of these objects. Another important aspect is the new class of white dwarfs - the hot DQ - whose spectra are dominated by carbon lines. The origin of these stars is not yet understood, but they may have an evolutionary link with the hotter DBs as studied here. We use absolutely calibrated high-resolution UV spectra of five DBs in addition to available optical spectroscopy. We find photospheric C and no other heavy elements with extremely high limits on the C/Si ratio in two of the five objects. We compare various explanations for this unusual composition that have been proposed in the literature: accretion of interstellar or circumstellar matter, radiative levitation, carbon dredge-up from the deeper interior below the helium layer, and a residual stellar wind. None of these explanations is completely satisfactory, and the problem of the origin of the hot DQ remains an open question.