Evolutionary Effects of Irradiation in Cataclysmic Variables

The orbital evolution of cataclysmic variables in which the companion isilluminated by a fraction of the accretion luminosity consists ofirradiation-driven limit cycles on thermal timescales, superimposed on asecular evolution toward shorter periods due to systemic angular momentumlosses. We show that positive orbital period derivatives during bright phasesare a natural consequence of the expansion of the companion during high masstransfer phases in the limit cycle. The irradiation instability may be enhancedby consequential angular momentum losses, CAML, accompanying the limit cycle.We investigate the secular evolution of cataclysmic binaries under the combinedeffects of irradiation and CAML and show that faster than secular transferfluctuations that occur during these cycles can account for the observeddispersion in disk luminosities or estimated accretion rates at a given orbitalperiod. If indeed irradiation-driven and CAML--assisted mass transferfluctuations on timescales faster than secular occur, as discussed in thispaper, then we may be able to predict the relative abundances of the differenttypes of cataclysmic variable at a given orbital period. For example thismechanism may explain the relative paucity of dwarf novae with respect tonova-like variables between 3 and 4 hours.Comment: 35 pages, AAS LATEX macros v4.0, 16 postscript figures, Accepted for publication in the Astrophysical Journal; cormick@rouge.phys.lsu.edu, frank@rouge.phys.lsu.ed