On synchronization in early-type binaries
Jean‐Louis Tassoul · The Astrophysical Journal · 1987
A hydrodynamical mechanism is described that provides a simple, natural explanation to the general trend toward synchronism in the components of early-type binaries. Because the tidal interaction with a companion destoys the axial symmetry with respect to the rotation axis, concomitant azimuthal variations in the rotational velocity produce an Ekman-type boundary layer as long as the component does not rotate in synchronism with its orbital motion. This surface boundary layer acts as a sink or source of matter and, hence, exchanges angular momentum with the largely inviscid interior of the radiative envelope. Consequently, large-scale, mechanically driven currents come into existence, and an inexorable spin-down of the tidally distorted star ensues. These transient currents, which are much faster than the steady, thermally driven Eddington-Vogt currents, cease to exist as soon as synchronization has been achieved in the star.