ECLIPSE STUDIES OF THE DWARF NOVA HT CASSIOPEIAE .2. WHITE-DWARF AND ACCRETION DISK

We use the simultaneous UBVR light curves presented by Horne, Wood, & Stiening (Paper I) to investigate the white dwarf and accretion disk in the dwarf nova HT Cas. For the white dwarf we use log g = 8, pure hydrogen model atmospheres to fit the UBVR fluxes, subject to the constraint that the models are consistent with the observed UV flux. We find a temperature T(W) = 14,000 +/- 1000 K and a distance D = 125 +/- 8 pc for E(B-V) = 0.0. A blackbody fit to the UBVR fluxes gives T(W) = 13,600 +/- 1000 K and D = 140 +/- 12 pc. These estimates are consistent with the distance calculated from the near-infrared spectrum of the M5.4 companion star (Marsh 1990). The IUE spectra provide a limit on the reddening of E(B-V) < 0.2. If E(B-V) = 0.2, then T(W) = 17,000 +/- 1000 K and D = 107 +/- 6 pc. Eclipses are used to map the quiescent accretion disk. We derive maps of the disk at UBVR based on the mean of 12 light curves obtained in 1983 November-December (Paper I). These maps show that the quiescent disk in HT Cas has a flat brightness temperature profile (5000-7000 K), similar to our white-light eclipse maps of Z Cha and OY Car. The multicolor data available for HT Cas show that the disk emission is characteristic of a partially ionized, optically thin gas. Simple LTE, isothermal slab models fitted to the UBVR disk maps give the gas temperature (6500-9500 K) and hydrogen number density (approximately 10(14) H cm-3) as functions of radius. The observed radiative cooling can be balanced with viscous heating in this model only if the viscosity parameter, alpha, is approximately 10-200. Such a high value of alpha is unlikely to be possible and would be inconsistent with HT Cas's long quiescent interval. The fitted temperatures show that in quiescence T(e) approximately 3300-5400 K, and therefore the critical effective temperature, at which an outburst occurs, cannot be less than approximately 5400 K.