M.A. Barstow, P.D. Dobbie, J.B. Holberg, I. Hubeny, and T. Lanz
We present a detailed analysis of the EUV spectra of 13 DA white dwarfs, observed by the EUVE satellite, paying attention to the possible sources of absorbing material along the lines-of-sight in both the local interstellar medium 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. Absorption from interstellar HeII is found in the direction of five stars, allowing us to measure directly the He ionization fraction and estimate, indirectly, that of H. The weighted mean ionization fractions along these lines-of-sight are 0.27 +/- 0.04 and 0.35 +/- 0.1 respectively. Where HeII is directly detected, the observed ionization fractions are not correlated with direction or with the volume/column density of material along the line-of-sight. Furthermore, the limits on the amount of HeII established in all other directions completely encompass the range of observed values. Indeed, all the data can be consistent with more or less constant He and H ionization fractions throughout the local ISM. It is clear that there is little photospheric opacity, from either He or heavier elements, in the majority of the stars we have studied. This poses further difficulties in explaining the observed division of white dwarfs into H- and He-rich groups, the temperature gap in the He-rich sequence and the detailed spectral evolution of the H-rich DA white dwarfs as they cool. A striking observational result is that our spectroscopic evidence indicates that radiative levitation effects are only important at temperatures above 50,000 K, rather than the 40,000 K suggested by broad band photometry. There is clearly an urgent need for further theoretical work on the mechanisms that determine the photospheric composition of white dwarf stars.
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