The Underlying White Dwarf Accretor in the Dwarf Nova UU Aquilae

Abstract

We present a high-gravity, solar composition model atmosphere and model accretion disk study of the U Gem type dwarf nova UU Aql. We have identified the far-UV signature of the underlying white dwarf for the first time. Our best-fit model atmosphere to the observed far-UV spectrum in quiescence is 27,000+/-1000 K. The solar abundance high-gravity photosphere provides a consistent explanation for the sharp absorption lines due to metals in the white dwarf's atmosphere. This interpretation is also consistent with the predicted 20,000-30,000 K small, hot, far-UV source (smaller than an accretion disk) proposed by Patterson & Raymond. Model accretion disk fits do not account for the sharp absorption lines and continuum slope. The best-fit accretion disk corresponds to M_wd=0.8 M_solar, i=60^deg, and M=10^-9.5 M_solar yr^-1. Optically thick disk models at accretion rates lower than 1×10^-9.5 M_solar yr^-1 are ruled out because of marked flux deficiency shortward of 1400 Å. Theoretical arguments are presented that rule out accretion rates as high as 1×10^-9.5 M_solar yr^-1 during dwarf nova quiescence. While we can rule out the quiescent disk as being a significant UV flux contributor, we believe the heated white dwarf accounts for the majority of the far-UV flux. The white dwarf is only the fifth accretor among U Gem systems with a known surface temperature. The temperatures of all five accretors lie between 25,000 and 35,000 K.