ROSAT STUDIES OF THE COMPOSITION AND STRUCTURE OF DA WHITE-DWARF ATMOSPHERES

We present here a detailed study of a sample of hot DA white dwarfs detected in the EUV and soft X-ray bands by ROSAT during its all-sky survey. The survey data are combined with additional ROSAT pointed-phase observations, earlier EXOSAT results and the available optically determined effective temperatures and gravities. We show that the spectra of those stars with temperatures below almost-equal-to 40 000 K are well described by nearly pure H atmospheres, with only very small traces of additional opacity due to He allowed by the data. Above almost-equal-to 40 000 K, however, neither homogeneous nor stratified H + He models can, in general, explain the observed EUV and X-ray fluxes. We conclude that additional sources of opacity in the form of trace metals must be present in the photospheres of these stars. The disappearance of this material when the stars cool below 40 000-50 000 K is in agreement with theoretical radiative levitation calculations. We propose that these results are direct observational evidence that the composition of most DA white dwarf atmospheres is dominated by the balance between gravitational and radiative forces. Limits on the H-layer mass obtained for the cooler stars indicate that He makes only a minimal, if any, contribution to the EUV/soft X-ray opacity in the phosphere.