An EUV selected sample of DA white dwarfs from the ROSAT all sky survey.

A detailed study of the sample of hot DA white dwarfs detected in the EUV and soft X-ray bands of the ROSAT all-sky survey is presented. Interpretation of the ROSAT data requires a priori knowledge of the temperature, gravity and visual brightness of all the white dwarfs detected. This information was obtained by a series of optical photometric and spectroscopic follow-up observations which are presented in detail. A by-product of this work is the mass distribution of the EUV selected sample of white dwarfs which shows an unexpectedly large proportion of high mass objects when compared with the optically selected population. It is suggested that this bias arises from the fact that hot white dwarfs with high gravities have lower heavy element abundances and are, therefore, more luminous (and easier to detect) than the lower gravity stars. In keeping with earlier results, it is found that the majority of stars 40,000K have more or less pure H atmospheres, while those above this temperature contain significant quantities of heavy elements. However, the increase size of the sample yields some important new results. A number of stars in the 40,000K to 50,000K range have nearly pure H atmospheres, a result at odds with the predictions of radiative levitation calculations. Furthermore, the dispersion in the observed EUV/X-ray opacity at a given temperature is much larger than expected from theory. An additional mechanism is then needed, other than radiative levitation and gravity, that can modify the atmospheric abundances. Finally, the expected strong dependence of opacity with surface gravity is not seen, except for very high mass stars.