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Session 98 - Globular Clusters.
Oral session, Friday, January 09

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

J. C. Lombardi Jr. (Cornell University), F. A. Rasio (MIT), S. L. Shapiro (UIUC)

We discuss the results of smoothed particle hydrodynamic (SPH) simulations 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_ødot). 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.

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