SIMBAD references

Previous observations of the soft X-ray and EUV region with the Einstein, EXOSAT, and ROSAT satellites have indicated the presence of photospheric absorbers in most DA white dwarfs with T_eff>40000K. Several of these objects have now been observed with the Extreme Ultraviolet Explorer (EUVE). Since the detection of individual metal lines is rather difficult with EUVE spectra we chose the well studied DA G191-B2B as reference object for the analysis of other white dwarfs. In spectra obtained with the GHRS of the Hubble Space Telescope photospheric lines of nitrogen, silicon, iron, and nickel could be detected. With the abundances determined from these observations we were able to reproduce the EUVE spectrum of G191-B2B. The main source of EUV opacity turned out to be iron, followed by nickel. The analysis of both the EUV continuum and the hydrogen Balmer lines led to T_eff=56000±2000K. For the analysis of other DA white dwarfs we used the same relative abundances as for G191-B2B but introduced a free scaling factor (``metallicity'') for the total amount of metals. From EUVE spectra we determined relative metallicities for 20 objects with T_eff>40000K. The main result is that all DAs with T_eff>50000K contain additional photospheric absorbers, whereas at lower temperatures all observations, with the exception of GD394, are compatible with pure hydrogen atmospheres. As in the case of G191-B2B, iron determines also the EUV opacity in the objects at T_eff>50000K. This is in agreement with radiative levitation theory which predicts the support of iron above this temperature.