Evolutionaty Mass Exchange in Close Binary Systems.

Abstract

Observations have found many close binary systems with one component near the main sequence and the other a subgiant filling its lobe of the Roche limit, but it is always the secondary and never the primary that fills its limit. This situation can be explained in terms of the primary evolving, first, to the Roche limit and then transferring most of its mass to the other star, so that the roles of primary and secondary are interchanged, leaving the new secondary filling its limiting lobe. It is shown that the transition is fast enough that very few systems will be caught making it. Plotting Roche limits of typical systems on an HR diagram shows that the primary component in most systems can reach its limit only in the Hertzsprung gap, where even normal evolution is comparatively fast. For the case in which the primary expands to the Roche limit during the early evolutionary phase, calculations are made for the effect of mass loss on the radii of various stellar models and for the effect of mass transfer on the size of the limiting surface. For all models in thermal equilibrium, though not for the one in only hydrostatic equilibrium, the primary lobe of the limit shrinks faster than the star, demonstrating that the process must be unstable on a Kelvin time scale.