Collisions of Main-Sequence Stars and the Formation of Blue Stragglers in Globular Clusters

Abstract

We report the results of new SPH calculations of parabolic collisions between two main-sequence stars in a globular cluster. Such collisions are directly relevant to the formation of blue stragglers. In particular, we consider parent stars of mass $M/M_{TO}=0.2,0.5,0.75$, and $1$, where $M_{TO}$ is the cluster turnoff mass (typically about $0.8\,M_\odot$). Low-mass stars (with $M=0.2 M_{TO}$ or $0.5 M_{TO}$) are assumed to be fully convective and are therefore modeled as $n=1.5$ polytropes. Stars at the turnoff (with $M= M_{TO}$) are assumed to be mostly radiative and are modeled as $n=3$ polytropes. Intermediate-mass stars (with $M=0.75 M_{TO}$) are modeled as composite polytropes consisting of a radiative core with polytropic index $n=3$ and a convective envelope with $n=1.5$. We focus our study on the question of hydrodynamic mixing of helium and hydrogen, which plays a crucial role in determining the observable characteristics of blue stragglers. In all cases we find that there is negligible hydrodynamic mixing of helium into the outer envelope of the merger remnant. The amount of hydrogen mixed into the core of the merger depends strongly on the entropy profiles of the two colliding stars. In contrast to what has been done in previous studies, blue stragglers formed by direct stellar collisions should not necessarily be assumed to have initially homogeneous composition profiles.