The effects of tide and rotation on the mass exchange process in binary systems

The criterion for the onset of mass exchange when the radius of a component star reaches its Roche lobe is only approximate, introduced to make the calculation one-dimensional. We propose a more strict criterion which still leaves the problem one-dimensional. We examine the Case A evolution of a 9 M-circle dot + 6M(circle dot) binary system according to both the strict and approximate criteria. The results show that, with the strict criterion, the mass exchange process starts earlier, the fast mass exchange process becomes shorter with a larger average mass exchange rate, and the slow mass exchange process stops later, leading to different component masses, different orbital period and different positions on the HR diagram.