Efficiency of mass transfer in massive close binaries, Tests from double-lined eclipsing binaries in the SMC

摘要

One of the major uncertainties in close binary evolution is the efficiency ofmass transfer beta: the fraction of transferred mass that is accreted by asecondary star. We attempt to constrain the mass-transfer efficiency forshort-period massive binaries undergoing case A mass transfer. We present a grid of about 20,000 detailed binary evolution tracks withprimary masses 3.5-35 Msun, orbital periods 1-5 days at a metallicity Z=0.004,assuming both conservative and non-conservative mass transfer. We perform asystematic comparison, using least-squares fitting, of the computed models witha sample of 50 double-lined eclipsing binaries in the Small Magellanic Cloud,for which fundamental stellar parameters have been determined. About 60% of thesystems are currently undergoing slow mass transfer. In general we find good agreement between our models and the observeddetached systems. However, for many of the semi-detached systems the observedtemperature ratio is more extreme than our models predict. For the 17semi-detached systems that we are able to match, we find a large spread in thebest fitting mass-transfer efficiency; no single value of beta can explain allsystems. We find a hint that initially wider systems tend to fit better to lessconservative models. We show the need for more accurate temperaturedeterminations and we find that determinations of surface abundances ofnitrogen and carbon can potentially constrain the mass-transfer efficiencyfurther.