Evolutionary Effects of Irradiation in Cataclysmic Variables.

Abstract

We developed a simple stellar computational model consisting of two concentric polytropic shells in order to investigate compact binary evolution. In this thesis we focus the investigation on the effects of irradiation on the orbital evolution of cataclysmic variables (CVs). In these systems, when the companion is illuminated by a fraction of the accretion luminosity, the orbital evolution consists of irradiation-driven limit cycles on thermal timescales. These cycles are superimposed on the secular evolution toward shorter periods due to systemic angular momentum losses. When the irradiation instability is enhanced by consequential angular momentum losses $j\sb{\rm CAML}$ the net effect is that mass transfer rates vary by orders of magnitude at any given period. This result agrees with observations which show a large dispersion in disk luminosities, associated with mass transfer rates, for all CV periods. The standard theoretical model without irradiation (i.e. the secular evolution) predicts an approximately constant mass transfer rate throughout the binary evolution. In addition, we show that large amplitude 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. We investigate the secular evolution of cataclysmic binaries under the combined effects of irradiation and $j\sb{\rm CAML}$ and show that faster than secular orbital period excursions of either sign may occur. If indeed irradiation-driven mass transfer fluctuations on timescales faster than secular as discussed in this thesis occur, then we may predict the relative abundances of the different types of cataclysmic variables at a given orbital period. For example this mechanism may explain the relative paucity of dwarf novae with respect to nova-like variables with orbital periods between 3 and 4 hours.