Composite Accretion Disk and White Dwarf Photosphere Analyses of the FUSE and Hubble Space Telescope Observations of EY Cygni

Abstract

We explore the origin of FUSE and Hubble Space Telescope STIS far-UV spectra of the dwarf nova EY Cygni during its quiescence using combined high-gravity photosphere and accretion disk models, as well as model accretion belts. The best-fitting single-temperature white dwarf model to the FUSE plus STIS spectrum of EY Cyg has T_eff=24,000 K and logg=9.0 with an Si abundance of 0.1 times solar and C abundance of 0.2 times solar, but the distance is only 301 pc. The best-fitting composite model consists of a white dwarf with T_eff=22,000 K and logg=9, plus an accretion belt with T_belt=36,000 K covering 27% of the white dwarf surface with V_beltsini=2000 km s^-1. The accretion belt contributes 63% of the far-UV light and the cooler white dwarf latitudes contribute 37%. This fit yields a distance of 351 pc, which is within 100 pc of our adopted distance of 450 pc. EY Cyg has very weak C IV emission and very strong N V emission, which is atypical of the majority of dwarf novae in quiescence. We also conducted a morphological study of the surroundings of EY Cyg using direct imaging in narrow nebular filters from ground-based telescopes. We report the possible detection of nebular material associated with EY Cyg. Possible origins of the apparently large N V/C IV emission ratio are discussed in the context of nova explosions, contamination of the secondary star, and accretion of nova abundance-enriched matter back to the white dwarf via the accretion disk or as a descendant of a precursor binary that survived thermal-timescale mass transfer. The scenario involving pollution of the secondary by past novae may be supported by the possible presence of a nova remnant-like nebula around EY Cyg.