A Non-LTE Model Atmosphere Analysis of Hot, Hydrogen-Deficient White Dwarfs

White dwarf stars of spectral type DO populate the hot end of the hydrogen-deficient white dwarf cooling sequence. They are characterized by a helium-rich atmosphere, a high effective temperature (T_eff > 40,000 K) and a high surface gravity (log g > 7), and thus their spectrum is dominated by broad He II absorption features. Their hydrogen deficiency is thought to result from a so-called born-again evolution, during which a post-AGB star undergoes a late helium-shell flash, bringing the star back to the AGB and causing the remaining hydrogen to be burned or completely mixed in the envelope. In order to further our understanding of hydrogen-deficient white dwarf evolution, we perform a model atmosphere analysis of a large sample of DO stars. Atmospheric parameters are derived by comparing optical spectra to our new grid of non-LTE model atmospheres and synthetic spectra. We also make use of evolutionary sequences to obtain stellar masses and cooling ages. Based on these results, we discuss the physical properties of DO white dwarfs.