A Study of the Ultraviolet Evolution of U Geminorum between Outbursts

Abstract

This paper examines the ultraviolet evolution of the dwarf nova U Gem from one outburst to the next. A series of seven sets of IUE observations that monitor the quiescent period of 22-103 days after outbursts is presented. 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 30,000 K white dwarf. Direct evidence is provided that the UV flux levels continue to decrease throughout 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.