Hopkins Ultraviolet Telescope Observations of U Geminorum Far from Outburst

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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 nonuniform 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.