The Extraordinary Cataclysmic Binary RU Pegasi: The Hottest White Dwarf in a Dwarf Nova?

Abstract

We present the results of the first multicomponent synthetic spectral analysis of International Ultraviolet Explorer (IUE) archival spectra of the long-period dwarf nova RU Peg during quiescence. The best-fit, high-gravity, solar composition photosphere models yield T_eff=50,000-53,000 K with scale factor distances of 250 pc. Optically thick accretion disk models imply accretion rates between 1×10^-9 and 1×10^-10M_solar yr^-1 in order to match the steeply sloping far-UV continuum. However, the best-fit accretion disk models yield distances from 600 to 1300 pc, well beyond the estimated distance range of 130-300 pc. Using rough theoretical flux arguments and the distance estimates, we find better agreement between the observed far-UV luminosity and the predicted far-UV luminosity of a hot, massive, white dwarf model than with an accretion disk model. RU Peg appears to contain the hottest white dwarf yet found in a dwarf nova. We cannot rule out that the far-UV energy distribution is due to a multitemperature white dwarf with cooler, more slowly rotating higher latitudes and a rapidly spinning, hotter equatorial belt. We discuss implications of our analysis for theoretical predictions of the disk instability theory of dwarf nova outbursts. We discuss a comparison between RU Peg's white dwarf and the observed properties of other analyzed white dwarfs in dwarf novae.