Close-binary evolution - I. Tidally induced shear mixing in rotating binaries

Context. Tides are known to play an important role in binary evolution, leading in particular to synchronization of axial and orbital rotations and to binary mass transfer. Aims. We study how tides in a binary system induce some specific internal shear mixing that can substantially modify the evolution of close binaries prior to mass transfer. Methods. We constructed numerical models accounting for tidal interactions, meridional circulation, transport of angular momentum, shears and horizontal turbulence. Furthermore, we considered a variety of orbital periods and initial rotation velocities. Results. Depending on orbital periods and rotation velocities, tidal effects may spin down (spin-down case) or spin up (spin-up case) the axial rotation. In both cases, tides may induce a high internal differential rotation. The resulting tidally induced shear mixing is so efficient that the internal distributions of angular velocity and chemical elements are highly influenced. The evolutionary tracks are modified, and in for spin down as well as for spin up, large amounts of nitrogen can be transported to the stellar surfaces before any binary mass transfer. Meridional circulation, when properly treated as a advection, always tends to counteract the tidal interaction, tending to spin up the surface when it is braked down and vice versa. As a consequence, the times needed for the axial angular velocity to become equal to the orbital angular velocity may be longer than given by typical synchronization timescales. Moreover, because of the meridional circulation some differential rotation remains in tidally locked binary systems.