Insights into the ISM and the Spectral Evolution of White Dwarfs from EUVE Observations of GD 659 and EUVE J2009-604

We report on a new determination of the ionization state of the local interstellar medium based on extreme ultraviolet (EUV) photometry and spectroscopy of the hot white dwarfs EUVE J2009-604 and GD 659. Both stars exhibit EUV energy distributions consistent with a pure hydrogen photospheric composition, thus providing the most direct evidence for the interstellar origin of the detected He II absorption edges at 228 Angstroms. Direct spectroscopic measurements of the He I lambda 206 Angstroms autoionization feature and of the He II photoionization and line absorption features in these objects constrain the ionization fraction of helium in the ISM and allow us to infer the degree of hydrogen ionization assuming a cosmic abundance ratio. We describe a method that allows us to place limits on the kinetic temperature of the ionized helium in the interstellar medium using the EUV data. Our observations also place tight constraints on the photospheric metallicity of white dwarf stars and offer important clues concerning their spectral evolution. In particular we confirm a trend suggesting that early in the cooling sequence (T_eff ~ 40,000 K), white dwarf stars develop a pure hydrogen composition over their complete energy distribution. This work has been supported by NASA contract NAS5-30180 and NASA grant NAG5-245.