THE ACCRETION DISK LIMIT-CYCLE MODEL - TOWARD AN UNDERSTANDING OF THELONG-TERM BEHAVIOR OF SS-CYGNI

We present detailed computations of the limit cycle model for dwarf no va outbursts with application to Cygni the best studied dwarf nova. We examine how secular changes in the input parameters of the model affe ct the properties associated with the outbursts. For input parameters which reproduce the observed outburst recurrence times and outburst du rations for SS Cyg, our time-dependent, accretion disk instability cod e generates light curves in which one or more short outbursts tend to be sandwiched between two long outbursts. We find that the relative fr equency of long versus short outbursts can be influenced by changes in the accretion disk viscosity and mass transfer rate. By forming movin g averages taken from the long-term AAVSO light curve of SS Cyg, we fi nd a strong correlation between the recurrence time for outbursts t(c) and the ratio of the number of long outbursts N(L) to the number of s hort outbursts N(S) occurring in a given time interval. This can be ac counted for in the model in several ways. Changes in the radius of the inner edge of the disk produce correlated changes in t(c) and N(L)/N( S), as do slow variations in either alpha(cold) or alpha(hot) - the ac cretion disk viscosity parameters in quiescence and outburst. In this latter scenario, there must be a fundamental asymmetry between the two alphas in the sense that both quantities cannot be varying in step wi th each other-i.e., one must remain fixed while the other varies. Fina lly, variations in the mass transfer rate M(T) cannot directly account for the observed long-term changes in the light curve of SS Cyg as wa s postulated by Hempelmann & Kurths and by Cannizzo & Mattei.