Abstract
The role played by tidal forces in shaping the mass-loss history of binary stars is discussed. Tidal forces drive oscillations on the surface of stars whose rotation and orbital periods are not synchronized and/or whose orbits are eccentric. Using a simple, two-dimensional numerical model, we show how the oscillation amplitudes depend on the star's radius and its rotational angular velocity in a given system. Except for synchronous systems, our models for circular orbits result in oscillating stellar surfaces, with periods depending on the ratio of the spin to the orbital angular velocites. In the case of eccentric systems, as the star spins-down (or spins-up) due to tidal torques, large amplitude oscillations are predicted when a resonance between the spin and orbital angular velocities is encountered, suggesting the occurrence of numerous episodes of enhanced mass-loss in its circularization timescale. Because the amplitudes of the oscillations increase rapidly with an increasing stellar radius, tidal oscillations will most likely drive a phase of rapid and probably non-spherically symmetric mass-loss as the star leaves the main sequence and expands. Furthermore, enhanced mass-loss rates might be expected even from MS stars in non-equilibrium binary systems, when compared with analogous single stars. Thus, we suggest that the mass-loss history and the post-MS evolutionary tracks followed by stars in non-equilibrium binary systems may differ significantly from those of single stars. Systems such as η Carinae and the Wolf-Rayet/Luminous Blue Variable HD 5980 may be displaying the consequences of tidally driven pulsations.
