Tidal Interactions of Red Giants with Environment Stars in Globular Clusters

TL;DR

This study uses hydrodynamic simulations to analyze how tidal encounters between red giants and main sequence stars in globular clusters transfer angular momentum and mass, affecting stellar rotation and surface composition.

Contribution

It provides new quantitative models and fitting formulae for energy, angular momentum transfer, and mass accretion during star-star tidal interactions in dense stellar environments.

Findings

Angular momentum transfer can induce rapid rotation in red giants.

Main sequence stars can accrete enough mass to alter their surface composition.

Derived formulae applicable to various stellar encounter scenarios.

Abstract

We investigate the tidal interactions of a red giant with a main sequence in the dense stellar core of globular clusters by Smoothed Particle Hydrodynamics method. Two models of $0.8 \msun$ red giant with the surface radii 20 and $85 R_\sun$ are used with 0.6 or $0.8M_\sun$ main sequence star treated as a point mass. We demonstrate that even for the wide encounters that 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 non-linear 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 stellar parameters and the impact parameter of encounter. These formulae are applicable to the encounters not only of the red giants but also of the main sequence stars, and useful in the study of the evolution of stellar systems with the star-star interactions taken into account.