Ultraviolet spectroscopy of the dwarf nova U Geminorum.

Abstract

Spectrophotometry at 6 A resolution is presented for the dwarf nova U Gem over the range 1150-3200 A, with simultaneous visual broad-band photometry, from observations with the International Ultraviolet Explorer. The observations were made at 1, 5, 11, 26, 71, and 108 days after the start of an outburst. Near the outburst maximum the spectra show weak Lyman-alpha and strong N V absorptions. In the Balmer continuum, the spectrum of U Gem rises more steeply toward shorter wavelength than other dwarf novae in outburst. During the decline, the absorption lines weaken, and emission appears at Mg II. Model accretion disks are found to reproduce the ultraviolet flux distribution at maximum light, with a mass flux of 5 x 10 to the 17th g/s for a distance of 90 pc. The interpretation of the broad Lyman-alpha absorption and the flux distribution at quiescence indicates that the white dwarf primary produces most of the ultraviolet light at that time.