An EUV-selected sample of DA white dwarfs from the ROSAT All-Sky Survey -- I. Optically derived stellar parameters

Abstract

One of the most important results of the ROSAT All-Sky X-ray and EUY Surveys has been the detection of a large number of white dwarfs, allowing a detailed study of the general properties of DA white dwarf atmospheres to be carried out. However, this work relies on a knowledge of the effective temperatures, surface gravities and visual magnitudes. We present analyses of optical data from our follow-up programme, including the values of T eff , logg, mv and stellar mass obtained. We also list the PSPC and WFC count rates for each star. The distribution of masses and surface gravities derived from the optical work also provides important information about the sample of white dwarfs and gives some indication of possible selection biases. Comparing the ROSAT sample of 89 stars with the cooler optically selected sample of Bergeron, Saffer & Liebert reveals a statistically significant excess (~3 times the number found by them) of hot, massive DAs, which may represent a population of coalesced binary white dwarfs. As the optical and EUY samples do not cover the same range of white dwarf temperatures, the highmass excess may partly arise from differences in the cooling rates of 'normal' (~O.6-M0) and massive (> 1.0-M0) stars. Consequently, this feature is not necessarily the result of selection on the basis of EUY flux, and it might also be present in an optically selected sample covering an appropriate temperature range.