AN EUV-SELECTED SAMPLE OF DA WHITE-DWARFS FROM THE ROSAT ALL-SKY SURVEY .2. EUV AND SOFT-X-RAY PROPERTIES

We present here a detailed study of the complete sample of hot DA whit e dwarfs detected in the EUV by ROSAT during its All-Sky Survey. Earli er analyses of the white dwarf sample were limited to well-studied obj ects each with known temperature and surface gravity. After carrying o ut a spectroscopic survey of all the newly discovered DA stars, we can now extend this work to the complete sample, combining the EUV data w ith ROSAT X-ray fluxes and optically determined temperatures and gravi ties. In keeping with other studies, we find that most stars with temp eratures below 40 000 K have nearly pure-H atmospheres, while the majo rity of hotter stars contain significant quantities of heavier element s. 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 significan t number of objects with pure H atmospheres above 40 000 K 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 ab ove approximate to 55 000 K contain significant quantities of heavy el ements. A steep rise in opacity can be seen at 54 000 K, which we inte rpret as arising from the appearance of significant quantities of radi atively supported Fe and other Fe-group elements.