Tidal friction in early-type stars

Theoretical and observational results related to tides in early-type stars are summarized. The results suggest that the tidal torque on an early-type star is concentrated near the boundary between the convective core and the radiative envelope, and that a train of gravity waves which is excited at this boundary outwardly transports the angular momentum removed from the fluid by the torque. Tidal despinning to synchronous rotation is shown to proceed from the outside to the inside of the star. The present model can account for the previous finding that Zahn's (1975, 1977) theory for tidal evolution in early-type close binaries is compatible with the observed rates of orbit circularization, while significantly underestimating the observed rates of spin synchronization.