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M. Politano (Ariz. St. Univ.)
Population synthesis calculations of cataclysmic variables (CVs) have relied on a simple energy prescription to relate the orbital parameters of the binary before and after the common envelope (CE) phase. Essentially this prescription approximates the CE phase by saying that when some fraction of the orbital energy released in the spiral-in process equals the initial binding energy of the primary’s envelope, the CE becomes unbound and is presumably (somehow) ejected. This prescription is parameterized by an efficiency parameter, \alphaCE, which measures the efficiency at which orbital energy is transferred to the CE during the spiral-in process. All population synthesis calculations of CVs that we are aware of assume that \alphaCE is a constant for the entire population of binaries involved in the synthesis. However, detailed hydrodynamical calculations of the CE phase do not appear to support this assumption. Rather, it is more likely that \alphaCE is a function of the mass and evolutionary state of the giant, and the mass of the secondary. In this paper, we present the first step toward a more sophisticated prescription for the CE phase in population synthesis calculations of CVs. We explore the extent to which using an \alphaCE that varies with system parameters affects the resulting model population of CVs. In particular, we examine the consequences of choosing a different value of \alphaCE for progenitor systems containing an RGB primary at contact than for those containing an AGB primary at contact.
This work was funded in part by NASA grant NAG 5-8481 and NSF grant AST-0098742 to Arizona State University.