The surface temperatures of white dwarf accretors in dwarf novae: The active dwarf nova CN Orionis during quiescence

We have carried out a synthetic spectral analysis of five IUE NEWSIPS archi val spectra of the peculiar, very active dwarf nova CN Orionis taken at the system's lowest flux levels during quiescence. AAVSO data indicate the vis ual magnitude V similar to 14 for CN Ori at this time. On the assumption th at the white dwarf contributes significantly to the far-UV light of the sys tem, we have computed a two-parameter grid of synthetic, high-gravity spect ra in LTE with solar composition using TLUSTY195 and SYNSPEC42 and carried out fits of these pure photospheric models to the far-UV continuum and narr ow absorption line spectra. We find that the far-UV spectrum in quiescence is well represented by a hot (T-eff = 30,000 K log g = 7) white dwarf with probable subsolar silicon abundance and all other observed metal transition s at essentially their solar values. For comparison, we have fitted optical ly thick accretion disk models to the same spectra for and disk inclination angle i = 60 degrees for accretion rates (M) over dot = 10(-10.5) and 10(- 9.5) M. yr(-1). We compare the T-eff of CN Ori, one of a handful of dwarf n ovae above the period gap with a model photospheric analysis, to all other dwarf novae with white dwarfs of known T-eff derived using white dwarf mode l atmospheres. We find evidence that the white dwarfs in dwarf novae above the period gap are hotter than the ones below the gap. This trend suggests more heated, younger degenerates above the gap and is consistent with the o verall empirical evidence that mass transfer rates in dwarf novae above the period gap are higher than mass transfer rates in systems below the gap.