LOW-DENSITY OF NEUTRAL HYDROGEN AND HELIUM IN THE LOCAL INTERSTELLAR-MEDIUM - EXTREME-ULTRAVIOLET EXPLORER PHOTOMETRY OF THE LYMAN CONTINUUM OF THE HOT WHITE-DWARFS MCT 0501-2858, MCT 0455-2812, HZ 43, AND GD 153
S. Vennes et al., LOW-DENSITY OF NEUTRAL HYDROGEN AND HELIUM IN THE LOCAL INTERSTELLAR-MEDIUM - EXTREME-ULTRAVIOLET EXPLORER PHOTOMETRY OF THE LYMAN CONTINUUM OF THE HOT WHITE-DWARFS MCT 0501-2858, MCT 0455-2812, HZ 43, AND GD 153, The Astrophysical journal, 421(1), 1994, pp. 120000035-120000038
The first comprehensive sky survey of the extreme ultraviolet (EUV) sp
ectral range performed by the Extreme Ultraviolet Explorer (EUVE) has
uncovered a handful of very bright sources at wavelengths longer than
the He I 504 Angstrom photoionization edge. Among these objects are fo
ur white dwarfs with exceptionally low interstellar medium (ISM) colum
n 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 MC
T 0455-2812 along one line of sight and of the DA white dwarfs HZ 43 a
nd GD 153 near the north Galactic pole indicates that the overall mini
mum column density of the neutral material centered on the Sun is N(H
I) = 0.5-1.0 x 10(18) cm(-2) In the case of MCT 0501-2858, EUV photome
tric measurements provide a clear constraint to the effective temperat
ure (60,000-70,000 K). Given these neutral hydrogen columns, the actua
l contribution to the density of neutral species from the immediate so
lar environment (the ''local fluff'') would only cover a distance of a
pproximate to 2-3 pc (assuming an average density n(H I) = 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-wavelengt
h survey indicates that these lines of sight are exceptional and set a
minimum column density in the solar environment.