A STUDY OF THE ULTRAVIOLET EVOLUTION OF U-GEMINORUM BETWEEN OUTBURSTS

This paper examines the ultraviolet evolution of the dwarf nova U Gem from one outburst to the next. We present a series of seven sets of IUE observations that monitor the quiescent period of 22 days-103 days after outbursts (the seventh set is 2 days prior to the next outburst). An eighth set of observations obtained 75 days after the second outburst is included for comparison. In agreement with earlier studies, analyses of Ly-alpha profiles and 1200-2000 A continuum spectra during 103 days of quiescence are consistent with the presence of a optical quiescence. These changes in flux appear consistent with the added presence of a hot component (perhaps the inner part of the disk or boundary layer) in addition to the white dwarf. Analyses of the absorption line spectra are also inconsistent with a single photospheric origin. Abundances derived from the observed metal absorption lines are consistent with accretion of solar composition material onto the white dwarf. However the presence of N V and He II demands the existence of a line-formation region that is much hotter T(eff) > 60,000 K) than the 30,000 K global photosphere.