Ultraviolet limb darkening and spectra for accretion disks in cataclysmic variables

Limb darkening is an important effect, more so for flat objects such as accretion disks (ADs) than for stars, since no averaging over emergent angles occurs for disks. As with stars, limb darkening in ADs is especially important in the ultraviolet, and at a given wavelength it is more important for low-temperature atmospheres. We study the size of the limb-darkening effect in the ultraviolet (925 A to 1750 A) spectra of ADs in cataclysmic variables, using a grid of models and spectra of disk atmospheres. Radial emissivity (surface brightness) distributions in the ultraviolet are presented, along with a brief comparison of limb darkening in stars and disks. The changes in the absorption-line profiles in the model disk spectra as the disk inclination is varied, because of both limb darkening and kinematic broadening, are also discussed. Limb-darkening correction factors for the integrated disk brightness at 1448 Å are derived for a representative set of disk parameters and orbital inclinations.