The Accreting White Dwarf in SS Cygni Revealed

TL;DR

This study combines HST and FUSE spectra to analyze the white dwarf in SS Cygni during quiescence, revealing its temperature, accretion rate, and implications for long-term heating models.

Contribution

It provides the first detailed spectral analysis using combined HST and FUSE data, with updated white dwarf mass and distance, refining temperature and accretion rate estimates.

Findings

White dwarf temperature is 45-55,000K in quiescence.

Accretion disk models alone do not fit the FUSE spectrum well.

The white dwarf's temperature suggests specific long-term accretion and heating scenarios.

Abstract

We have carried out a combined Hubble Space Telescope (HST/GHRS) and Far Ultraviolet Spectroscopic Explorer FUSE) analysis of the prototype dwarf nova SS Cygni during quiescence. The FUSE and HST spectra were obtained at comparable times after outburst and have matching flux levels where the two spectra overlap. In our synthetic spectral analysis, we have used SS Cygni's accurate HST FGS parallax giving d = 166pc, a newly determined mass for the accreting white dwarf (Bitner et al. 2007) of Mwd=0.81Msun (lower than the previous, widely used 1.2 Msun) and the reddening E_{B-V} values 0.04 (Verbunt 1987; La Dous 1991) and 0.07 (Bruch and Engel 1994) derived from the 2175A absorption feature in the IUE LWP spectra. From the best-fit model solutions to the combined HST + FUSE spectral energy distribution, we find that the white dwarf is reaching a temperature Teff of 45-55,000K in quiescence, assuming Log(g)= 8.3 with a solar composition accreted atmosphere. The exact temperature of the WD depends on the reddening assumed and on the inclusion of a quiescent low mass accretion rate accretion disk. Accretion disk models alone fit badly in the FUSE range while, and if we take the distance to be a free parameter, the only accretion disk model which fits well is for a discordant distance of at least several hundred pc and an accretion rate (1.E-8 Msun/yr which is unacceptably high for a dwarf nova in quiescence. We discuss the implications of the white dwarf's temperature on the time-averaged accretion rate and long term compressional heating models.