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The first detection of ionized helium in the local ISM - EUVE and IUE spectroscopy of the hot DA white dwarf GD 246We report observations of the extreme ultraviolet spectrum of the hot degenerate star GD 246 obtained with the EUVE. Our initial attempt at modeling the photospheric emission from the white dwarf reveals a relatively uncontaminated pure H spectrum in the range above 200 A, allowing a study of interstellar continuum absorption features in the line of sight of GD 246. Modeling of the He I autoionization transition discussed by Rumph et al. (1993), and the EUV continuum 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 ISM (LISM). We estimate the He ionization fraction He II/(He I + He II) at roughly 25 percent with a total He column of 1.40-1.65 x 10 exp 18/sq cm. We have measured and compared H I column densities from the saturated Ly-alpha 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 exp 19/sq cm. We discuss some implications for the nature of the LISM, particularly in the context of current models of the EUV radiation field.
Document ID
19930057674
Document Type
Reprint (Version printed in journal)
External Source(s)
Authors
Vennes, Stephane (NASA Goddard Space Flight Center Greenbelt, MD, United States)
Dupuis, Jean (NASA Goddard Space Flight Center Greenbelt, MD, United States)
Rumph, Todd (NASA Goddard Space Flight Center Greenbelt, MD, United States)
Drake, Jeremy (NASA Goddard Space Flight Center Greenbelt, MD, United States)
Bowyer, Stuart (California Univ. Berkeley, United States)
Chayer, Pierre (NASA Goddard Space Flight Center Greenbelt, MD, United States)
Fontaine, Gilles (Montreal Univ. Canada)
Date Acquired
August 16, 2013
Publication Date
June 20, 1993
Publication Information
Publication: Astrophysical Journal, Part 2 - Letters
Volume: 410
Issue: 2
ISSN: 0004-637X
Subject Category
ASTROPHYSICS
Funding Number(s)
CONTRACT_GRANT: NAG5-2135
CONTRACT_GRANT: NAS5-30180
Distribution Limits
Public
Copyright
Other