Composite Accretion Disk and White Dwarf Model Analyses of the Quiescence of Dwarf Novae: EM Cygni, CZ Orionis, and WW Ceti

Abstract

We explore the origin of the far-UV (FUV) spectra of three dwarf novae using combined high-gravity photosphere and accretion disk models. We have carried out an IUE archival comparative study of the three U Gem-type dwarf novae EM Cyg, CZ Ori, and WW Cet. For EM Cyg, the FUV spectrum during quiescence is dominated by an accretion disk with M=5×10^-11 M_solar yr^-1 contributing 92% of the FUV light and a white dwarf with upper limit T_eff<24,000 K contributing <~8% of the light. For CZ Ori, the accretion disk with M=3.5×10^-10 M_solar yr^-1 contributes 99% of the FUV light while the white dwarf has an upper limit T_eff<21,000 K and contributes <~1% of the light. For WW Cet, we find that best-fitting disk models and disk plus white dwarf models yield a distance that appears far too large. A single-temperature white dwarf fit with T_eff=22,000 K implies a distance of 150 pc. CZ Ori and EM Cyg are dominated by the accretion disk during quiescence but with accretion rates that differ by over a factor of 10. In the case of EM Cyg, which has a higher inclination, it is possible that the nearly edge-on aspect of the disk may hide a much hotter white dwarf. There are now 17 analyzed dwarf nova systems with P_orb<120 mintues and 11 systems with P_orb>180 m. The average T_eff below the lower boundary of the period gap is T_eff=15,547 K, while the average T_eff above the upper boundary of the period gap is T_eff=31,182 K. The T_eff of the white dwarfs strengthens the overall conclusion that the white dwarfs in cataclysmic variables above the period gap appear to be a factor of 2 times hotter than the accreting white dwarfs in dwarf novae below the period gap.