Evolutionary Effects of Irradiation in Cataclysmic Variables

The orbital evolution of cataclysmic variables in which the companion is illuminated by a fraction of the accretion luminosity consists of irradiation–driven limit cycles on thermal timescales, superimposed on a secular evolution toward shorter periods due to systemic angular momentum losses. We show that positive orbital period derivatives during bright phases are a natural consequence of the expansion of the companion during high mass transfer phases in the limit cycle. The irradiation instability may be enhanced by consequential angular momentum losses J̇CAML accompanying the limit cycle. We investigate the secular evolution of cataclysmic binaries under the combined effects of irradiation and J̇CAML and show that faster than secular orbital period excursions of either sign may occur. We discuss whether the mass transfer fluctuations that occur during these cycles can account for the observed dispersion in disk luminosities or estimated accretion rates at a given orbital period. If indeed irradiation–driven and CAML–assisted mass transfer fluctuations on timescales faster than secular occur, as discussed in this paper, then we may be able to predict the relative abundances of the different types of cataclysmic variable at a given orbital period. For example this mechanism may explain the relative paucity of dwarf novae with respect to nova–like variables between 3 and 4 hours. Subject headings: accretion, accretion disks — binaries: close — cataclysmic variables – instabilities