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S. Vennes (1), J. Dupuis (1), T. Rumph (2), J. Drake (1), S. Bowyer (1,2), P. Chayer (3), and G. Fontaine (3)
1) Center for EUV Astrophysics, 2150 Kittredge Street,
University of California, Berkeley, CA 94720
2) Department of
Astronomy, University of California, Berkeley, Ca. 94720
3)
Departement de Physique, Universite de Montreal, CP 6128, Montreal, PQ,
Canada, H3C 3J7
We report observations of the extreme ultraviolet spectrum of the hot degenerate star GD 246 (WD2309+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 = ~200Å, allowing a study of interstellar continuum absorption features in the line of sight of GD 246. Modeling of the He iautoionization transition (206 Å) discussed by Rumph, Bowyer & Vennes (1993), and the EUV continuum (200 < lambda < 260 Å) 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 HeII/(HeI + HeII) at 25% with a total He column of 1.40--1.65E+18 cm-2 . We have measured and compared H icolumn densities from the saturated Lyman 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--1.6E+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.
Subject headings: ISM: abundances ISM: structure stars: white dwarfs ultraviolet: stars
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