FUSE Observations of U Geminorum During Outburst and Decline

Abstract

We have obtained FUV (904 -- 1187 Å) spectra of U Gem in outburst with FUSE. Three of the observations were acquired during the plateau phase of the outburst, while the fourth was obtained during late outburst decline. The plateau spectra have continuum shapes and fluxes that are approximated by steady-state accretion disk model spectra with $\dot{m} \simeq$ 7e-09 M_sun/yr. The spectra also show numerous absorption lines of H I, He II, and two- to five-times ionized transitions of C, N, O, P, S and Si. There are no emission features in the spectra, with the possible exception of a weak feature on the red wing of the O VI doublet. The absorption lines are narrow (FWHM ~ 500 km/s), too narrow to arise from the disk photosphere, and at low velocities (<= 700 km/s). The S VI and O VI doublets are optically thick. The absorption lines in the plateau spectra show orbital variability: in spectra obtained at orbital phases 0.53 -- 0.79, low-ionization absorption lines appear and the central depths of the pre-existing lines increase. The increase in line absorption occurs at the same orbital phases as previously observed EUV and X-ray light curve dips. If the absorbing material is in (near-) Keplerian rotation around the disk, it must be located at large disk radii. The final observation occurred when U Gem was about two magnitudes from optical quiescence. The spectra are dominated by emission from an $\simeq$43,000 K, metal-enriched white dwarf (WD). The inferred radius of the WD is 4.95e+08 cm, close to that observed in quiescence. Allowing for a hot heated region on the surface of the WD improves the fit to the spectrum at short (< 960 Å) wavelengths.