Irradiation-induced mass-overflow instability as a possible cause of superoutbursts in SU UMa stars.

The heating of the secondary's atmosphere in cataclysmic variables by strong far UV and soft X-ray radiation due to accretion and the resulting increase in mass-overflow rate are shown to lead to a positive feed-back instability between accretion and mass-overflow in certain circumstances. It is suggested that this irradiation-induced mass-overflow instability is a possible cause of superoutbursts in SU UMa stars. Examination of irradiation effects and of mass-overflow rates indicates that this instability operates ouly in those cataclysmic variables with very short periods (i.e., very cool secondary) and with low mass-transfer rates, a condition which SU UMa stars seem to satisfy. A new superhump model is proposed along this line. It is suggested that a slowly precessing elliptic disk develops during superoutbursts and that irradiating radiation on the secondary's atmosphere varies periodically with the synodic period between the precessing elliptic disk and the secondary because of varying disk shadow around the elliptic disk. The superhump phenomenon can be explained by a periodic variation in mass-transfer rate with this period. It is argued that the elliptic disk may be self-sustaining during superoutbursts because of the built-in feed-back loop between the non-axisymmetric disk and the mass-transfer rate.