The Cooling of the White Dwarf in U Geminorum Following Hearing By Two Different Outbursts

Abstract

We present the analysis of 13 orbital phase-resolved IUE (large and small aperture) SWP spectra, sampling the quiescent intervals of the prototype dwarf nova U Geminorum following two individual outbursts, the outburst of 1992, 29 August-14 September, and the outburst of 1993, 19 March-5 April. The quiescent interval following the outburst 29 August 1992 is the longest ever recorded. During quiescence, the photospheric radiation of the exposed white dwarf dominates the far ultrviolet. The variations in absorption line strengths and continum flux levels are analyzed as a function of both orbital phase and elapsed time, since the return to optical quiescence, by fitting the IUE spectra with a grid of high gravity, LTE, model atmospheres with solar composition constructed with TLUSTY and SYNSPEC (Hubeny, I. Tlusty and Synnspec: A Users Guide (1994)). We present evidence from both absorption line variations and continuum variations, as a function of time since the outbursts, that the white dwarf photosphere has cooled by several thousand degrees. Within the signal-to-noise limitations, we find no evidence of a difference in heating and cooling between the two outbursts.