The Surface Temperatures of White Dwarf Accretors in Dwarf Novae: The Active Dwarf Nova CN Orionis during Quiescence

Abstract

We have carried out a synthetic spectral analysis of five IUE NEWSIPS archival spectra of the peculiar, very active dwarf nova CN Orionis taken at the system's lowest flux levels during quiescence. AAVSO data indicate the visual magnitude V~14 for CN Ori at this time. On the assumption that the white dwarf contributes significantly to the far-UV light of the system, we have computed a two-parameter grid of synthetic, high-gravity spectra in LTE with solar composition using TLUSTY195 and SYNSPEC42 and carried out fits of these pure photospheric models to the far-UV continuum and narrow absorption line spectra. We find that the far-UV spectrum in quiescence is well represented by a hot (T_eff=30,000 K, logg=7) white dwarf with probable subsolar silicon abundance and all other observed metal transitions at essentially their solar values. For comparison, we have fitted optically thick accretion disk models to the same spectra for M_wd=0.80 M_solar and disk inclination angle i=60^deg for accretion rates M=10^-10.5 and 10^-9.5 M_solar yr^-1. We compare the T_eff of CN Ori, one of a handful of dwarf novae 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 model 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 overall empirical evidence that mass transfer rates in dwarf novae above the period gap are higher than mass transfer rates in systems below the gap.