THE CONSTITUTION OF THE ATMOSPHERIC LAYERS AND THE EXTREME ULTRAVIOLET-SPECTRUM OF HOT HYDROGEN-RICH WHITE-DWARFS

We present an analysis of the atmospheric properties of hot, H-rich, DA white dwarfs. The analysis is based on optical, ultraviolet, and X-ray observations aimed at predicting detailed spectral properties of these stars in the range 80-800 angstrom, as covered by the Extreme Ultraviolet Explorer (EUVE). We begin by studying divergences between observations from a sample of 15 hot DA white dwarfs emitting in the EUV/soft X-ray range and pure H synthetic spectra calculated from a grid of model atmospheres characterized by T(eff) and g. EUV/soft X-ray synthetic spectra are compared to the count rates from the EXOSAT satellite and complementary synthetic spectra of the far ultraviolet range are compared with IUE data. The study of this wide spectral coverage reveals that seven out of 15 DA stars consistently show pure hydrogen atmospheres, that HZ 43 is not well understood, and that the remaining seven stars show inconsistency between FUV data and EUV/soft X-ray data that can be explained with the presence of trace EUV/soft X-ray absorbers. Consequently, we have computed synthetic spectra assuming two other possible chemical structures: photospheric traces of radiatively levitated heavy elements and a stratified hydrogen/helium distribution. With this new insight, we make predictions about forthcoming medium-resolution observations of the EUV spectrum of selected hot H-rich white dwarfs.