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S. Vennes (1), J. Dupuis (1), S. Bowyer (2), G. Fontaine (3), A. Wiercigroch (1), P. Jelinsky (1), F. Wesemael (3), and R.F. Malina (1)
1) Center for EUV Astrophysics, 2150 Kittredge Street,
University of California, Berkeley, CA 94720
2) Astronomy
Department, University of California, Berkeley, CA 94720
3) Dept. de Physique, Universite de Montreal, P.O. Box 6128,
Station A Montreal, H3C - 3J7
The first comprehensive sky survey of the extreme ultraviolet (EUV) spectral range performed by the Extreme Ultraviolet Explorer (EUVE) has uncovered a handful of very bright sources at wavelengths longer than the HeI 504 Å photo-ionization edge. Among these objects are four white dwarfs with exceptionally low interstellar medium (ISM) column densities along the line of sight. Analysis of EUV photometry of the He-rich DO white dwarf MCT- 0501-2858 and the H-rich DA white dwarf MCT 0455-2812 along one line of sight and of the DA white dwarfs HZ 43 and GD 153 near the north Galactic pole indicates that the overall minimum column density of the neutral material centered on the Sun is n_HI= 0.5-1.0E+18 cm-2. In the case of MCT 0501-2858, EUV photometric measurements provide a clear constraint to the effective temperature (60,000- 70,000 K). Given these neutral hydrogen columns, the actual contribution to the density of neutral species from the immediate solar environment (the``local fluff'') would only cover a distance of ~2-3 pc (assuming an average density n__HI = 0.1 cm-3) leaving these lines of sight almost entirely within the hot phase of the ISM. A preliminary examination of the complete EUVE long wavelength survey indicates that these lines of sight are exceptional and set a minimum column density in the solar environment.
Subject Headings: ISM: abundances -- ISM: structure -- ultraviolet: stars -- white dwarfs
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