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

Abstract

We present new determinations of effective temperature, surface gravity, and masses for a sample of 46 hot DA white dwarfs selected from the EUVE and ROSAT Wide Field Camera bright source lists in the course of a near-IR survey for low mass companions. Our analysis, based on hydrogen NLTE model atmospheres, provides a map of LTE correction vectors, which allow a thorough comparison with previous LTE studies. We find previous studies underestimate both the systematic errors and the observational scatter in the determination of white dwarf parameters obtained via fits 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 find a peak mass of our white dwarf sample of 0.59 solar masses, in agreement with the results of previous analyses. However, we do not confirm 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 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.