The Thermal Evolution of Mass Losing Donors in AM CVn Binary Systems

Abstract. The thermal evolution of arbitrarily degenerate stellar models forthe mass losing component in AM CVn binary systems formed through the whitedwarf evolutionary channel is investigated for a range of initial orbital periodsand initial system masses. Based on these models, the evolutionary phasesand properties of AM CVn binary systems are described and the observationalsignature of their donors is discussed. 1. IntroductionInteracting binary systems with compact stellar components characterized byshort orbital periods are intrinsically fascinating as they may be sites for adiverse range of high energy transient phenomena. Among such systems are theclass of objects known as AM CVn binaries. These systems consist of a pair ofinteracting white dwarfs with orbital periods possibly as low as 5 minutes (Israelet al. 2002; Ramsay et al. 2002) to as long as 65 minutes. Of particular interestare the short period population (less than 20 minutes) since they are expected tobe detectable in the Galaxy by space based gravitational wave interferometers(Nelemans et al. 2001; Deloye et al. 2005).The formation of these systems must have involved a phase of common en-velope evolution where the progenitor system of long orbital period was trans-formed into one of short orbital period (Iben & Livio 1993, Taam & Sandquist2000; see also Sandquist et al. 2000). For AM CVn systems, three binary evo-lutionary channels have been proposed involving this phase. In particular, onechannel involves a cataclysmic variable-like evolution in which one common en-velope phase had occurred whereas the other two channels involve a series of twocommon envelope stages. In the latter case, systems involving the progenitors ofdonors less massive than about 2.3 M