1996A&A...314..217D

We present a model atmosphere analysis of most known hot helium-rich white dwarfs of spectral type DO. The stars represent the non-DA white dwarf cooling sequence from the hot end (T_eff≃120000K) down to the DB gap (T_eff≃45000K). From medium resolution optical spectra, effective temperatures, surface gravities, and element abundances are determined by means of non-LTE model atmospheres. Compared to previous LTE analyses available for some of the program stars, higher effective temperatures are derived. The existence of the DB gap is confirmed. For the first time reliable surface gravities for a large sample of DO white dwarfs are determined. With the help of theoretical evolutionary tracks the DO masses are determined. We find a mean value of 0.59±0.08M_☉ which virtually coincides with the mean masses of the DA and DB white dwarfs. Hydrogen cannot be identified in any optical DO spectrum, which includes the former DOA prototype HZ21. Hence HD149499B remains the only DO white dwarf with a positive (FUV) detection of trace hydrogen in the photosphere. The number ratio of DA/non-DA white dwarfs significantly increases along the cooling sequence and thus corroborates the hydrogen float-up hypothesis as an explanation for the DB gap. From optical, IUE, and HST spectra metal abundances or upper limits could be derived for most DOs, allowing a comprehensive comparison with predictions from diffusion/radiative levitation calculations. A large scatter in metallicities is found, even among objects with similar parameters and no clear trend along the cooling sequence is detectable. This is severely at odds with theoretical predictions. The evolutionary link between DO white dwarfs, the PG1159 stars and DB white dwarfs is discussed, in particular considering the overlapping positions of DO and PG1159 stars in the HR diagram.