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 Ṁ = 5 × 10 -11 M⊙ yr-1 contributing 92% of the FUV light and a white dwarf with upper limit Teff < 24, 000 K contributing ≲ 8% of the light. For CZ Ori, the accretion disk with Ṁ = 3.5 × 10-10 Ṁ yr-1 contributes 99% of the FUV light while the white dwarf has an upper limit Teff < 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 Porb < 120 mintues and 11 systems with Porb > 180 m. The average Teff below the lower boundary of the period gap is Teff = 15, 547 K, while the average Tiff above the upper boundary of the period gap is Teff = 31, 182 K. The Teff 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.