The Accretion Disk and White Dwarf during the Quiescence of the Dwarf Novae VW Vulpeculae and χ Leonis

Abstract

We have carried out an International Ultraviolet Explorer archival comparative study of the two U Geminorum type dwarf novae VW Vulpeculae and χ Leonis. The spectrum of χ Leo does not show any C IV, a feature normally seen in the vast majority of other dwarf novae. For VW Vul, a single-temperature high-gravity photosphere does not produce a consistent fit. The best-fit accretion disk model corresponds to M_wd=0.55 M_solar, i=41^deg, and M=10^-9 M_solar yr^-1. For χ Leo, we find that a white dwarf model with T_eff=33,000 K gives the best fit to the far-UV spectrum. However, a model accretion disk with parameters M=0.80 M_solar, i=60^deg, and M=10^-10 M_solar yr^-1 also gives a satisfactory fit to the far-UV continuum. However, the accretion rate derived for VW Vul is within a factor of 3 of the critical value, while the accretion rate of χ Leo is within a factor of 60 of the critical rate. If we have identified the accreting white dwarf in χ Leo, then it strengthens the overall conclusion that the white dwarfs in cataclysmic variables above the period gap appear to be a factor of 1.5-2 times hotter than the accreting white dwarfs in dwarf novae below the period gap.