Impact-driven eccentricity in accretion disks

Enhanced mass-transfer rates have been claimed to sometimes occur shortly before superoutbursts in SU UMa binaries. Superoutbursts are accompanied by superhumps, which have been interpreted as arising from eccentric precessing accretion disks. We consider the effects of a stream-disk impact as a means of generating eccentricity in an accretion disk. We adopt a simplified model of the stream-disk interaction in which the postimpact stream adjusts to the disk edge velocity within a small fraction of the disk circumference. Two major contributions to eccentricity generation need to be considered. The first is present even for a disk of zero eccentricity; the second occurs because the disk has some nonzero eccentricity. We show the first contribution can cause eccentricity growth only for a time-varying mass-transfer rate and is fairly inefficient. The second contribution causes eccentricity damping. The result is that disk eccentricity grows during periods of changing mass-transfer rate. If the mass-transfer rate changes linearly in time, then the eccentricity changes by amount of order beta delta M/Md, where beta less than 1 is the efficiency by which m = 1 waves are generated by the stream impact, delta M is the mass added to the disk in a single binary orbit period, and Md is the disk mass. Once the mass-binary rate becomes constant (but possibly enhanced) the eccentricity damps on a time scale of order Md/M dot, where M dot is the mass-transfer rate carried by the stream. We conclude that an observationally significant eccentricity cannot be generated by this mechanism applied to superoutbursts. However, the effects of eccentricity damping may be important in the postsuperoutburst evolution.