Atmospheric Parameters of a Soft X-Ray Selected Set of Hot DA White Dwarfs

Soft x-ray/EUV observations provide a sensitive means of probing the structure and composition of the atmospheres of hot hydrogen-rich (DA) white dwarf stars, this is due to the fact that soft x-rays originate from hotter, deeper layers of the photosphere than do longer wavelengths. A primary aim of this research is to expand the sample of DAs observed with soft x-ray instrumentation through a search for serendipitous observations of catalogued hot DA white dwarfs in existing soft x-ray databases. The positional coincidences of 31 catalogued DAs (McCook and Sion 1987) were examined in Einstein IPC and EXOSAT LE fields. As a result, three soft x-ray sources have been identified corresponding to the white dwarfs (WD0631 + 107, WD1113 + 413 and WD0425 + 168) and a fourth source which is probably not due to photospheric thermal emission from the coincident white dwarf. The three detected DAs have relatively low effective temperatures (27,200 +/ - 400 K, 26,200 +/- 1100 K and 24,000 +/- 500 K, respectively), as determined independently using complementary optical and UV spectroscopy. Applying these temperature constraints to the soft x-ray photometric data, the photospheres of these stars must be composed of effectively pure hydrogen (n(He)/n(H) < 10^{-5}), although the hottest star, WD0631 + 107, can have trace levels of homogeneously mixed helium less than 10^{-4.2} . The analysis of the soft x-ray observations are supplemented with independent determinations of temperature, gravity and V magnitude. The method of determination employed is to fit the well-sampled wings of the broad hydrogen absorption profiles with self-consistent model atmosphere predictions. Precise temperatures and gravities are obtained by fitting the observed profiles independently, then jointly determining a consistent solution for each object. In this effort, a ground-based observation program was established to obtain high quality spectra of the Balmer delta and gamma lines and in some cases the Balmer beta line. IUE spectrophotometry of the Lyman alpha line, when available, was also incorporated into the determination of the temperature and gravity. The model atmospheres used in these analyses require an independent flux normalization, which we chose as the V magnitude, therefore UBV photometry was obtained for those stars in our sample which had relatively poor determinations of the V magnitude. The temperatures and gravities of a total of 101 hot DA white dwarfs were spectroscopically analyzed in this manner. This expanded set of known hot DA white dwarfs represents a significant fraction of known DAs with temperatures in excess of 20,000 K. The simple mean and standard deviation of the log gravity distribution is 7.925 and 0.346, respectively. Assuming a zero-temperature C¹² Hamada-Salpeter mass radius for each star, we derive a mass distribution of M/M_{rm sun} = 0.539 +/- 0.196 from the adopted parameters.