On equilibrium tides in fully convective planets and stars

Abstract

We consider the tidal interaction of a fully convective primary star and a point mass. Using a normal mode decomposition we calculate the evolution of the primary angular velocity and orbit for arbitrary eccentricity e. The dissipation acting on the tidal perturbation is associated with convective turbulence. A novel feature of the Paper is that, to take into account of the fact that there is a relaxation time t_{c}, being the turn-over time of convective eddies, associated with the process, this is allowed to act non locally in time, producing a dependence of the dissipation on tidal forcing frequency. Results are expressed in terms of the Fourier coefficients of the tidal potential. We find analytical approximations for these valid for $e>0.2$. When the tidal response is frequency independent, our results are equivalent to those obtained in the standard constant time lag approximation. When there is the frequency dependence of the dissipative response, the evolution can differ drastically. In that case the system can evolve through a sequence of spin-orbit corotation resonances with Omega_{r}/Omega_{orb}=n/2, where Omega_{r} and Omega_{orb} are the rotation and orbital frequencies and n is an integer. We study this case analytically and numerically.