The Accretion Disk and White Dwarf in the U Geminorum type Dwarf Novae UZ Serpentis and SS Aurigae during Quiescence

Abstract

We have carried out high-gravity model atmosphere and accretion disk synthetic spectral fits for the U Geminorum type dwarf novae during quiescence. For UZ Ser, we find that the far-UV spectrum is best fitted with a log g = 8, Teff = 27,000 K photosphere with abundances near solar. The best-fit accretion disk model for UZ Ser has Mwd = 1.0 M⊙, disk inclination i = 75°, and M = 10-9 M⊙ yr-1. For SS Aur, the best-fit model photosphere has Teff = 30,000K, log g = 8, and solar composition abundances, while the best-fit accretion disk model has Mwd = 1.0 M⊙, i = 41°, and Ṁ = 10-10 M⊙ yr-1. These accretion rates are discussed in connection with the critical accretion rates predicted by the disk instability theory. Whether a white dwarf or a disk accounts for the far-UV spectra, they must be quite hot. If indeed the white dwarf dominates the far-UV light, then UZ Ser and SS Aur add two important temperatures in support of the conclusion that the accreting white dwarfs above the period gap are a factor of 1.5-2 times hotter than the accreting cataclysmic variable degenerates in systems below the period gap.