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-amplitude, dwarf nova WZ Sge, reveal a rich absorption line spectrum of neutral carbon and ionized metals, the Stark-broadened Lyα absorption wing, the H₂ quasi-molecular Lyα "satellite" absorption line, and a double-peaked C IV emission line which is variable with orbital phase. A synthetic spectral analysis of the white dwarf yields T_eff = 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 absorption lines, the abundance of carbon in the photosphere must be ∼0.5 solar, silicon abundance is 5 × 10^-3 solar, with all other metal species appearing to be 0.1-0.001 times solar. The H₂ quasi-molecular absorption is fitted very successfully. The photospheric metals have diffusion timescales of fractions of a year, and thus they must have been accreted long after the 1978 December outburst. The source of the most abundant metal, carbon, is considered. If the time-averaged accretion rate during quiescence is low enough for diffusive equilibrium to prevail, then the equilibrium accretion rate of neutral carbon is 7 × 10^-16 M_⊙ yr^-1. A convective dredge-up origin for the concentration of carbon is extremely unlikely, given that the white dwarf atmosphere is H-rich while in single degenerates showing carbon and hydrogen, the C and H are trace elements in a helium background. Additional implications are explored.