Hubble Space Telescope GHRS Spectroscopy of U Geminorum during Two Outbursts

We obtained Hubble Space Telescope Goddard High-Resolution Spectrograph medium-resolution (G160M grating) phase-resolved spectroscopic observations of the prototype dwarf nova U Geminorum during different stages of two different outbursts. The spectral wavelength ranges were centered on three different line regions: N V (1238 Å, 1242 Å), Si III (1300 Å), and He II (1640 Å). The spectrum corresponding to the early decline phase of outburst 1 is essentially featureless except for weak N V absorption and narrow interstellar lines, while the spectrum at the peak of outburst 2 reveals broad emission peaks separated by narrow central absorption. The double-peaked emission-line profile structure with low-velocity central absorption seen in the second outburst suggests a disk origin, but the emission velocity widths appear narrower than the widths of the optical disk emission features. We interpret the high-excitation emission lines, with central absorption below the continuum, to be due to photoionized material (coronal?) above the disk plane with the thickened outer disk absorbing the boundary layer or inner disk radiation. The possibility of a wind origin for the profiles is also discussed, as well as the possibility of an ejected optically thin shell. The N V absorption velocity versus orbital phase traces the motion of the white dwarf, but the He II absorption velocity appears to deviate from the white dwarf motion. We present the results of synthetic accretion disk spectral fitting to the data of both outbursts and derive accretion rates for the two outbursts of 6 × 10-10 M☉ yr-1 and 2 × 10-9 M☉ yr-1. Implications are discussed.