Knox S. Long
Space Telescope Science Institute, 3700 San Martin Drive,
Baltimore, MD 21218
long@stsci.edu

William P. Blair
Department of Physics & Astronomy,
The Johns Hopkins University, Baltimore, MD 21218
wpb@pha.jhu.edu

&

John C. Raymond
Smithsonian Astrophysical Observatory,
60 Garden Street,
Cambridge, MA 02138
raymond@cfa.harvard.edu

Abstract:

The Hopkins Ultraviolet Telescope was used during the Astro-2 space shuttle mission to obtain a far ultraviolet (820 - 1840 Å) spectrum of the dwarf nova U Geminorum in its low state. At the time of the observation, U Gem had been at optical minimum for 185 days. The HUT spectrum is dominated by the white dwarf star, and shows (among other things) very strong, broad hydrogen Lyman absorption lines. A comparison with solar abundance white dwarf models indicates a temperature near 30,000 K. Comparison to a similar spectrum obtained during the Astro-1 mission in 1990, only 10 days after U Gem had returned to the low state following a normal outburst, confirms that the average temperature of the white dwarf in U Gem drops substantially between outbursts. Furthermore, differences between the models and the data at the shortest wavelengths in both spectra support the hypothesis that the surface temperature is non-uniform, and that a decrease in the relative importance of a hot region, possibly a belt, on the surface of the white dwarf causes the observed 30% decline in flux at 1450 Å far from outburst.

Subject headings: Stars: Novae, Cataclysmic Variables -- Stars: Individual (U Geminorum) -- Stars: Binaries: Close -- Ultraviolet: Stars

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