WIND-DRIVEN MASS-TRANSFER IN INTERACTING BINARY-SYSTEMS

Stars in close binary systems can suffer two kinds of mass change: (1) mass transfer between the stars, and (2) mass loss completely from the system. The former process occurs when one of the stars fills its Roche lobe while the latter may be in the form of a stellar wind. When both of these exist together, observational estimates indicate that the mass-transfer rate and the mass-loss rate are of the same order. A simple explanation for this similarity can be found if the mass loss, by stellar wind, from the Roche filling star is the driving mechanism behind mass transfer. This is the case if the time-scale for radius changes following mass loss is much shorter than that for radius changes owing to nuclear evolution and if the change in the stellar and Roche lobe radii are such that the Roche lobe is forced to be smaller than the star. We find quantitative estimates for these conditions and find that the mass-transfer rate and the mass-loss rate are indeed similar. If in addition angular momentum is lost from the system on an even shorter time-scale, this can be the driving force behind mass transfer. We present some numerical models of the expected evolution and examine the observational predictions. We find that the radii of evolved semi-detached systems are more consistent with wind-driven evolution than the traditional nuclear-driven Roche-lobe overflow.