Tidal Interactions of Red Giants with Environment Stars in Globular Clusters

We investigate the tidal interactions of a red giant with a main-sequence star in the dense stellar core of globular clusters by the smoothed particle hydrodynamics method. Two models of [image] red giants with surface radii 20 and [image] are used with a 0.6 or [image] main-sequence star treated as a point mass. We demonstrate that even for the wide encounters in which the two stars fly apart, the angular momentum of orbital motion can be deposited into the red giant envelope to such an extent as to trigger rotational mixing and to explain the fast rotation observed for the horizontal branch stars, and also that sufficient mass can be accreted on the main-sequence stars to disguise their surface convective zone with the matter from the red giant envelope. On the basis of the present results, we discuss the parameter dependence of these transfer characteristics with nonlinear effects taken into account and derive fitting formulae to give the amounts of energy and angular momentum deposited into the red giant and of mass accreted onto the perturber as functions of the stellar parameters and the impact parameter of the encounter. These formulae are applicable to the encounters not only of the red giants but also of the main-sequence stars and are useful in the study of the evolution of stellar systems with the star-star interactions taken into account.