THE 1ST DETECTION OF IONIZED HELIUM IN THE LOCAL ISM - EUVE AND IUE SPECTROSCOPY OF THE HOT DA WHITE-DWARF GD-246

We report observations of the extreme ultraviolet spectrum of the hot degenerate star GD 246 (WD 2309 + 105) obtained with the Extreme Ultraviolet Explorer (EUVE). Our initial attempt at modeling the photospheric emission from the white dwarf reveals a relatively uncontaminated pure H spectrum in the range lambda greater-than-or-equal-to 200 angstrom, allowing a study of interstellar continuum absorption features in the line of sight of GD 246. Modeling of the He I autoionization transition (approximately 206 angstrom) discussed by Rumph, Bowyer, & Vennes (1993), and the EUV continuum (200 less-than-or-equal-to lambda less-than-or-equal-to 260 angstrom) using the white dwarf as a source of background radiation provides measurements of both neutral and, for the first time, singly ionized He column densities in the local interstellar medium (LISM). We estimate the He ionization fraction He II/(He I + He II) almost-equal-to 25% with a total He column of 1.40-1.65 x 10(18) cm-2. We have measured and compared H I column densities from the saturated Lyalpha ISM absorption in IUE high-dispersion spectroscopy and from EUV continuum absorption: the two measurements are in good agreement with a total H column of 1.2-1.6 x 10(19) cm-2. We discuss some implications for the nature of the LISM, particularly in the context of current models of the EUV radiation field.