Evolution of Close Binaries VIII. Mass Exchange on the Dynamical Time Scale

The process of mass exchange in a close binary system is studied in the case where the more massive component has a deep convective envelope while filling up its Roche lobe. It is shown that a considerable fraction of mass is transferred on a dynamical time scale, provided the surface boundary condition for the mass losing component (i.e. the mixing length in its convective envelope) is not affected by the mass loss. However, if the mixing length is increased by the mass outflow, the latter may be strongly suppresed. Following Jedrzejec (1969) the relation is derived between the rate of mass transfer in a close binary and the extent to which the mass losing component overflows its Roche lobe. This relation was used in the evolutionary computations presented in this paper.