Application of realistic model atmospheres to eclipse maps of accretion disks: The effective temperature and flare of the disk in the dwarf nova Z Chamaeleontis

In a previous paper we presented ultraviolet (lambda(eff) = 1550 Angstrom) high-speed photometry of the eclipsing dwarf nova Z Cha obtained near the p eak of a normal eruption, and we derived a maximum-entropy disk map from th e eclipse light curve. In the present paper we report a new calculation of the disk map and temperature profile. The calculation differs from the prev ious one-and all other calculations of disk maps-in two ways: first, we hav e used realistic model atmospheres to convert the surface brightness of the disk to effective temperature, and second, we have included the effects of limb darkening, which is large at ultraviolet wavelengths and strongly aff ects the inferred temperature and geometry of the disk. Inclusion of limb d arkening in the models made the distribution of surface brightness with rad ius much steeper than the distribution derived without limb darkening, but the use of realistic model spectra instead of blackbody spectra to convert the surface brightness to temperature flattened the distribution again so t hat the net effect was to leave the temperature distribution nearly unchang ed. The distribution remains much flatter than the r(-3/4) law predicted fo r optically thick, steady-state disks. In agreement with our earlier result s, we find that the disk in Z Cha becomes flared during eruptions. The deri ved flare angle is large, 6 degrees, but not as large as the flare needed f or a disk with zero limb darkening. For comparison, model atmospheres for d isks similar to the disk in Z Cha give photosphere heights corresponding to hare angles between 3 degrees and 4 degrees.