Modeling Emission Lines from Dwarf Novae in Quiescence

Abstract

For cataclysmic variables with disk accretion, it is plausible that X-rays/extreme-ultraviolet (EUV) radiation as a result of accretion can back-illuminate the accretion disk. Such illumination will photoionize and heat up the disk to form a temperature-inverted layer in which emission lines can form. We make self-consistent, non-LTE calculations of the vertical structure, ionization, ion level populations, and the emitted spectrum for such photoionized layers. We compare the emerging emission-line spectra with the observations of dwarf novae in quiescence from lUE. We show that a strong soft X-ray/EUV component is necessary to explain the observed UV line strengths. We also explore the possibility of UV line production from disks heated only by viscous dissipation.