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

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