The response of tidally heated stars

We systematically investigate the response of a main-sequence star that is internally heated as a result of a tidal-capture and subsequent circularization process. The response depends critically on the time-scale on which the tidal energy is thermalized within the star and the region of the star in which the energy is deposited. If most of the energy is deposited in a thin layer near the surface, the star will expand rapidly, but only by a small factor, and efficiently lose the deposited energy by radiation or, if the tidal energy deposition rate is of the order of or larger than the Eddington luminosity, in an evaporative wind. If the energy is deposited deep inside the star, the star will try to expand to giant dimensions. However, for a typical turn-off star in a globular cluster capturing a neutron star, the expansion time-scale is generally much longer than the dissipation time-scale. Thus, in both limiting cases, the expansion of the star is relatively modest during the circularization phase, and the system is likely to survive the tidal-capture process as a detached binary. In the intermediate case, where the energy is deposited throughout most of the star or in a large fraction of the envelope, the star may overfill its tidal lobe during the circularization phase. This will lead to dynamical mass transfer and the merger of the two stars rather than a detached binary system. Whether the tidal-capture process leads to a merger of the two stars or to a detached binary depends in this case on the periastron distance in the initial encounter and the mass of the main-sequence star.