An EUV-selected sample of DA white dwarfs from the ROSAT All-Sky Survey - II. EUV and soft X-ray properties
We present here a detailed study of the complete sample of hot DA white dwarfs detected in the EUV by ROSAT during its All-Sky Survey. Earlier analyses of the white dwarf sample were limited to well-studied objects each with known temperature and surface gravity. After carrying out a spectroscopic survey of all the newly discovered DA stars, we can now extend this work to the complete sample, combining the EUV data with ROSAT X-ray fluxes and optically determined temperatures and gravities. In keeping with other studies, we find that most stars with temperatures below 40 000K have nearly pure-H atmospheres, while the majority of hotter stars contain significant quantities of heavier elements. However, with the enlarged sample of 89 objects we are able to fill in much more detail. In contrast to earlier work we find a significant number of objects with pure H atmospheres above 40 000K indicating that the EUV sample is biased to objects with lower opacity and higher mass, as expected. Nevertheless, all the stars detected with T_eff above ~55 000K contain significant quantities of heavy elements. A steep rise in opacity can be seen at 54 000K, which we interpret as arising from the appearance of significant quantities of radiatively supported Fe and other Fe- group elements.