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[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, \alpha_CE. Thus far, population synthesis calculations of close binary systems have assumed that \alpha_CE 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 \alpha_CE to be a function of secondary mass, M_s. We examined two types of dependencies: (1) a power law dependence, \alpha_CE = M_sⁿ, where n = 0.5, 1.0, 1.5 and 2.0 and (2) a dependence in which \alpha_CE approaches 1 for large values of M_s and equals 0 if M_s is less than some assumed cutoff mass, \alpha_CE = 1 – M_s/M_s,cut, where M_s,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 \alpha_CE if \alpha_CE \la 0.3. Previous investigations of the dependence of the CV population on the assumed value for \alpha_CE focused only on the regime \alpha_CE \ge 0.3. In both of the dependencies investigated, \alpha_CE approaches 0 as M_s approaches 0. Thus, if \alpha_CE indeed scales with M_s in this way, then understanding CE evolution involving secondaries with very small masses (M_s \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.

The author(s) of this abstract have provided an email address for comments about the abstract: Kevin.Weiler@mu.edu

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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.