2014A&A...568A.118K

Atmospheric parameters for hot DB (helium atmosphere) white dwarfs near effective temperatures of 25000K are extremely difficult to determine from optical spectroscopy. The neutral He lines reach a maximum in this range and change very little with effective temperature and surface gravity. Moreover, an often unknown amount of hydrogen contamination can change the resulting parameters significantly. This is particularly unfortunate because this is the range of variable DBV or V777 Her stars. Accurate atmospheric parameters 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 analysis shows that their atmospheres are pure carbon. The origin of these stars is not yet understood, but they may have an evolutionary link with the hotter DBs, as studied here. Our aim is to determine accurate atmospheric parameters and element abundances and study the implications for the evolution of white dwarfs of spectral classes DB and hot DQ. High-resolution UV spectra of five DBs were studied with model atmospheres. We determined stellar parameters and abundances or upper limits of C and Si. These objects were compared with cooler DBs below 20000K. We find photospheric C and no other heavy elements - with extremely high limits on the C/Si ratio - in two of the five hot DBs. 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.