AAS 207th Meeting, 8-12 January 2006
Session 70 Cataclysmic Variables and Novae
Poster, Tuesday, 9:20am-6:30pm, January 10, 2006, Exhibit Hall

## [70.09] Population Synthesis Studies of Close Binary Systems using a Common Envelope Efficiency Parameter that Depends on the Mass of the Secondary

K.P. Weiler, M. Politano (Marquette University)

A phase of common envelope (CE) evolution is believed to occur in a significant fraction of close binary stars. During this phase, sizeable amounts of mass and angular momentum may be shed from the binary if the amount of energy transferred from the orbit into the CE is sufficient to unbind the envelope. The efficiency of this transfer is very uncertain and is parameterized by a quantity, \alphaCE. Thus far, population synthesis calculations of close binary systems have assumed that \alphaCE is a global constant for the entire population of binaries. In the calculations presented in this poster, we have relaxed this assumption and have allowed \alphaCE to be a function of secondary mass, Ms. We examined two types of dependencies: (1) a power law dependence, \alphaCE = Msn, where n = 0.5, 1.0, 1.5 and 2.0 and (2) a dependence in which \alphaCE approaches 1 for large values of Ms and equals 0 if Ms is less than some assumed cutoff mass, \alphaCE = 1 – Ms/Ms,cut, where Ms,cut = 0.0375 M\odot, 0.075 M\odot and 0.15 M\odot. We used a Monte Carlo population synthesis code to compute theoretical populations of zero-age cataclysmic variables (ZACVs) and post-CE binaries (PCEBs) at the present epoch. An important finding is that the ZACV population is quite sensitive to the value of \alphaCE if \alphaCE \la 0.3. Previous investigations of the dependence of the CV population on the assumed value for \alphaCE focused only on the regime \alphaCE \ge 0.3. In both of the dependencies investigated, \alphaCE approaches 0 as Ms approaches 0. Thus, if \alphaCE indeed scales with Ms in this way, then understanding CE evolution involving secondaries with very small masses (Ms \la 0.3 M\odot) would be very important for correctly modeling populations of CVs and possibly other semi-detached close binary systems with short orbital periods. Other findings and a comparison with observed populations will be presented.

This work was funded in part by NSF grant AST-0328484 to Marquette University.