Observations of the white dwarf in the U Geminorum system with the Hopkins Ultraviolet Telescope

We have used the Hopkins Ultraviolet Telescope to obtain a far-ultraviolet (830-1860 Å) spectrum of the dwarf nova U Geminorum. The spectrum, which was obtained 10 days after U Gem had returned to the low state following a normal outburst, shows prominent absorption features due to the Lyman series of hydrogen and the Balmer lines of He II, as well as weaker absorption lines due to metals, including C III λ977, C III λ1176, N v λλ1239, 1243, Si IV λλ1394, 1403, C IV λλ1548, 1551, and possibly O VI λλ1032, 1038. The continuum, which extends to the Lyman limit but not beyond, is dominated by emission from the white dwarf. The average surface temperature appears to have been ∼38,000 K if all the UV light comes from the white dwarf. This temperature is higher than the temperature derived from previous measurements with IUE obtained further from outburst. There is evidence for a hot (∼57,000 K) component in the continuum which may be due to the boundary-layer region of the white dwarf photosphere. There is no obvious evidence of UV emission from the accretion disk. A preliminary analysis of the strength of the absorption lines suggests that most of the lines, but not those from ions with the highest ionization potential, arise in the photosphere of the white dwarf, which has nearly solar surface composition due to accretion from the secondary companion.