EUV photometry of DA white dwarfs with EXOSAT.

Abstract

The authors present EXOSAT EUV observations for 9 white dwarfs of spectral type DA. These are analysed together with other observational data from the literature using theoretical model atmospheres. For 8 objects He/H abundance ratios and interstellar hydrogen column densities are obtained. The data for HZ 43 seem to contradict recent results of other authors; possible explanations for the discrepancies are discussed. The results show that higher He/H ratios as well as a larger range of observed values are found in the hotter objects with T_eff > 50,000K, whereas the cooler objects (T_eff < 45,000K) have remarkably similar He/H ratios.