Soft X - ray Spectroscopy of the Hot DA White Dwarf LB 1919 and the PG 1159 Star PG 1520 + 525

We have performed soft X-ray spectroscopy of two hot white dwarfs with the Chandra observatory using the Low Energy Transmission Grating. The first target is the hot DA white dwarf LB1919 (T eff =69 000 K). This star is representative of a small group of hot DAs whose metallicities lie well below predictions from radiative levitation theory. The Chandra spectrum shows a rich absorption line spectrum which may allow to find the origin of the low-metallicity nature of these DAs. The second target is PG 1520+525, a very hot non-pulsating PG1159 star. We find that it is hotter (T eff =150 000 K) than the pulsating prototype PG 1159−035 (T eff =140 000 K) and conclude that both stars confine the blue edge of the GW Vir instability strip. 1. The hot DA white dwarf LB1919 The purity of WD atmospheres results from gravitational settling of heavy elements. The lightest element, either hydrogen or helium, is floating on top of the star, giving rise to the DA and non-DA spectral classes, respectively. Hydrogen in hot DA atmospheres is almost completely ionized so that the EUV/soft X-ray opacity is strongly reduced. As a consequence, DAs with T eff > 30 000 K can emit detectable amounts of thermal soft X-ray radiation, leaking out from deep, hot photospheric layers. The ROSAT all-sky survey, however, detected many fewer white dwarfs (< 200) than expected (> 5000) – X-ray emission turned out to be the exception rather than the rule. It was realized that additional absorbers must be present in the atmospheres. ROSAT and EUVE observations revealed that metals are the origin of this additional opacity, and that the EUV spectral flux distribution of hot DAs is most strongly determined by iron and nickel through their enormously large number of absorption lines. Radiative levitation can keep traces of these and other metals floating in the atmosphere against gravitational pull (e.g. Chayer et al. 1995): the metal abundances therefore generally increase with increasing T eff. As a consequence, only very few DAs with T eff > 60 000 K were detected in the EUVE and ROSAT all-sky surveys because the increasing EUV opacity is effectively blocking flux.