A Comparative Study of the Mass Distribution of Extreme-Ultraviolet selected White Dwarfs

We present new determinations of e ective temperature, surface gravity, and masses for a sample of 46 hot DA white dwarfs selected from the Extreme Ultraviolet Explorer (EUV E) and ROSAT Wide Field Camera bright source lists in the course of a near-infrared survey for low-mass companions. Our analysis, based on hydrogen non-LTE model atmospheres, provides a map of LTE correction vectors, which allow a thorough comparison with previous LTE studies. We Ðnd that previous studies underestimate both the systematic errors and the observational scatter in the determination of white dwarf parameters obtained via Ðts to model atmospheres. The structure of very hot or low-mass white dwarfs depends sensitively on their history. To compute white dwarf masses, we thus use theoretical mass-radius relations that take into account the complete evolution from the main sequence. We Ðnd a peak mass of our white dwarf sample of 0.59 M in agreement with the results of previous analyses. However, we do _, not conÐrm a trend of peak mass with temperature reported in two previous analyses. Analogous to other EUV-selected samples, we note a lack of low-mass white dwarfs and a large fraction of massive white dwarfs. Only one white dwarf is likely to have a helium core. While the lack of helium white dwarfs in our sample can be easily understood from their high cooling rate, and therefore low detection probability in our temperature range, this is not enough to explain the large fraction of massive white dwarfs. This feature very likely results from a decreased relative sample volume for low-mass white dwarfs caused by interstellar absorption in EUV-selected samples.