AN INTERPRETATION OF THE SPECTRAL PROPERTIES OF HOT HYDROGEN-RICH WHITE-DWARFS WITH STRATIFIED H/HE MODEL ATMOSPHERES

We present a grid of stratified H/He model atmospheres which are of particular interest for analyzing the EUV/soft X-ray observations of hot DA white dwarfs, and also for interpreting the optical spectra of DAO and DAB stars. The grid covers the range of effective temperature 25,000 K less-than-or-equal-to T(eff) less-than-or-equal-to 65,000 K and the range of hydrogen layer mass - 17 less-than-or-equal-to log q(H) = log (M(H)/M*) less-than-or-equal-to -13. The adopted gravity is log g = 8. The abundance profile in the H/He compositional transition zone is described in terms of pure diffusive equilibrium between gravitational settling and ordinary diffusion. We discuss the characteristic features of these models, including synthetic spectra covering the soft X-ray to the far-ultraviolet (FUV) as well as the optical domain for representative cases. With the help of these models, we next reanalyze critically the samples of hot DA white dwarfs which have been observed with EXOSAT and Einstein. Using conservative selection criteria, we find that six objects out of six with T(eff) less-than-or-equal-to 35,000 K do not show a EUV/soft X-ray flux deficiency and, therefore, can be understood solely in terms of pure hydrogen atmospheres. In contrast, a majority (six objects out of eight) of DA white dwarfs hotter than this value do show a flux deficiency and therefore require the presence of some absorbers in their atmospheres (currently believed to be provided by either the helium diffusive tail in a stratified atmosphere or by a complex of heavy elements levitating at low abundances in the radiation field). We show that the EXOSAT broad-band photometry of Feige 24 and G191 B2B cannot be explained in terms of stratified atmospheres. Absorption by heavy elements is most certainly responsible for the required EUV/soft X-ray opacity source in these cases. However, the EXOSAT data are consistent with the hypothesis of stratified atmospheres in the four remaining objects: GD 2, GD 153, GD 246, and GD 394. The inferred hydrogen layer masses for these stars are in the range q(H) almost-equal-to 10(-13.9)-10(-13.5). We cannot totally rule out absorption by heavy elements in the atmospheres of these four objects at this stage, however, and we suggest that EUV spectroscopy be carried out on them since atmospheric stratification and absorption by heavy elements leave quite distinct signatures in the EUV spectral range. Finally, we briefly discuss the cases of the optical spectra of DAO and DAB stars in terms of our stratified model atmospheres.