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 = 5 × 10-11 M☉ yr-1 contributing 92% of the FUV light and a white dwarf with upper limit Teff 180 m. The average Teff below the lower boundary of the period gap is Teff = 15,547 K, while the average Teff 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.