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

Abstract

In a previous paper we presented ultraviolet (λeff=1550 A) high-speed photometry of the eclipsing dwarf nova Z Cha obtained near the peak of a normal eruption, and we derived a maximum-entropy disk map from the eclipse light curve. In the present paper we report a new calculation of the disk map and temperature profile. The calculation differs from the previous one—and all other calculations of disk maps—in two ways: first, we have 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 affects the inferred temperature and geometry of the disk. Inclusion of limb darkening in the models made the distribution of surface brightness with radius 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 that the net effect was to leave the temperature distribution nearly unchanged. The distribution remains much flatter than the r−3/4 law predicted for optically thick, steady-state disks. In agreement with our earlier results, we find that the disk in Z Cha becomes flared during eruptions. The derived flare angle is large, 6°, but not as large as the flare needed for a disk with zero limb darkening. For comparison, model atmospheres for disks similar to the disk in Z Cha give photosphere heights corresponding to flare angles between 3° and 4°.