Atmospheric parameters of a soft x-ray selected set of hot DA white dwarfs.

Abstract

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 δ and γ lines and in some cases the Balmer β line. IUE spectrophotometry of the Lyman α 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(sun) = 0.539 +/- 0.196 from the adopted parameters.