Hubble space telescope fos spectroscopy of the ultrashort-period dwarf nova WZ Sagittae: The underlying degenerate

Edward M. Sion, F. H. Cheng, Knox S. Long, Paula Szkody, Ron L. Gilliland, Min Huang, Ivan Hubeny

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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 Teff = 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.

Original languageEnglish (US)
Pages (from-to)957-962
Number of pages6
JournalAstrophysical Journal
Issue number2
StatePublished - Feb 1 1995

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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