INTERSTELLAR AND PHOTOSPHERIC OPACITY FROM EUV SPECTROSCOPY OF DA WHITE-DWARFS

We present a derailed analysis of the extreme-ultraviolet (EUV) spectr a of 13 hydrogen-rich DA white dwarfs, observed by the Extreme Ultravi olet Explorer (EWE) satellite, paying attention to the possible source s of absorbing material along the lines of sight both in the local int erstellar medium (ISM) and in the photospheres of the stars themselves . The range of interstellar column densities seen are consistent with our previous understanding of the local distribution of material. Abso rption from interstellar He II is found in the direction of five stars , allowing us to measure directly the He ionization fraction and estim ate, indirectly, that of H. The weighted mean ionization fractions alo ng these fines of sight are 0.27 +/- 0.04 and 0.35 +/- 0.1 respectivel y. Where Hen is directly detected, the observed ionization fractions a re not correlated with direction or with the volume/column density of material along the line of sight. Furthermore, the limits on the amoun t of HeII established in all other directions completely encompass the range of observed values. Indeed, all. the data can be consistent wit h more or less constant He and H ionization fractions throughout the l ocal ISM. it is clear that there is little photospheric opacity, from either He or heavier elements, in the majority of the stars we have st udied. This poses further difficulties in explaining the observed divi sion of white dwarfs into H- and He-rich groups, the temperature gap i n the He-rich sequence and the detailed spectral evolution of tile H-r ich DA white dwarfs as they cool, A striking observational result is t hat our spectroscopic evidence indicates that radiative levitation eff ects are only important at temperatures above 50 000 K, rather than th e 40 000 K suggested by broad-band photometry. There is clearly an urg ent need for further theoretical work on the mechanisms that determine the photospheric composition of white dwarf stars.