HUBBLE-SPACE-TELESCOPE FOS SPECTROSCOPY OF THE ULTRASHORT-PERIOD DWARF NOVA WZ SAGITTAE - THE UNDERLYING DEGENERATE

Two consecutive Hubble Space Telescope Faint Object Spectrograph (FOS) spectra of the exposed white dwarf in the ultrashort-period, high-amp litude, dwarf nova WZ Sge, reveal a rich absorption line spectrum of n eutral carbon and ionized metals, the Stark-broadened Ly alpha absorpt ion wing, the H-2 quasi-molecular Ly alpha ''satellite'' absorption li ne, and a double-peaked C IV emission line which is variable with orbi tal phase. A synthetic spectral analysis of the white dwarf yields T-e ff = 14,900 K +/- 250 K, log g = 8.0. In order to fit the strongest C I absorption lines and account for the weakness of the silicon absorpt ion lines, the abundance of carbon in the photosphere must be similar to 0.5 solar, silicon abundance is 5 x 10(-3) solar, with all other me tal species appearing to be 0.1-0.001 times solar. The H-2 quasi-molec ular absorption is fitted very successfully. The photospheric metals h ave diffusion timescales of fractions of a year, and thus they must ha ve been accreted long after the 1978 December outburst. The source of the most abundant metal, carbon, is considered. If the time-averaged a ccretion rate during quiescence is low enough for diffusive equilibriu m to prevail, then the equilibrium accretion rate of neutral carbon is 7 x 10(-16) M. yr(-1). A convective dredge-up origin for the concentr ation of carbon is extremely unlikely, given that the white dwarf atmo sphere is H-rich while in single degenerates showing carbon and hydrog en, the C and H are trace elements in a helium background. Additional implications are explored.