Close binary evolution I. The tidally induced shear mixing in rotating binaries

TL;DR

This paper investigates how tidal forces in close binary systems induce shear mixing that significantly alters stellar evolution, angular momentum distribution, and surface chemical composition, with implications for pre-mass transfer evolution.

Contribution

It introduces detailed numerical models of tidal shear mixing in rotating binaries, highlighting its impact on internal rotation, chemical mixing, and evolutionary tracks, which was not thoroughly explored before.

Findings

Tidal effects can spin down or spin up stellar rotation depending on orbital parameters.

Tidal shear mixing can transport large amounts of nitrogen to stellar surfaces before mass transfer.

Meridional circulation counteracts tidal effects, affecting synchronization timescales.

Abstract

We study how tides in a binary system induce some specific internal shear mixing, able to substantially modify the evolution of close binaries prior to mass transfer. We construct numerical models accounting for tidal interactions, meridional circulation, transport of angular momentum, shears and horizontal turbulence and consider a variety of orbital periods and initial rotation velocities. 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 large internal differential rotation. The resulting tidally induced shear mixing (TISM) is so efficient that the internal distributions of angular velocity and chemical elements are greatly influenced. The evolutionary tracks are modified, and in both cases of spin down and spin up, large amounts of nitrogen can be transported to the stellar surfaces before any binary mass transfer. Meridional circulation, when properly treated as an 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 larger than given by typical synchronization timescales. Also, due to meridional circulation some differential rotation remains in tidally locked binary systems.