Evolution of Stellar Collision Products in Globular Clusters. II. Off-Axis Collisions

We continue our exploration of collisionally merged stars in the blue straggler region of the color-magnitude diagram. We report the results of new smoothed particle hydrodynamics (SPH) calculations of parabolic collisions between two main-sequence stars, with the initial structure and composition profiles of the parent stars having been determined from stellar evolution calculations. Parallelization of the SPH code has permitted much higher numerical resolution of the hydrodynamics. We also present evolutionary tracks for the resulting collision products, which emerge as rapidly rotating blue stragglers. The rotating collision products are brighter, bluer, and remain on the main sequence longer than their nonrotating counterparts. In addition, they retain their rapid rotation rates throughout their main-sequence lifetime. Rotationally induced mixing strongly affects the evolution of the collision products, although it is not sufficient to mix the entire star. We discuss the implications of these results for studies of blue straggler populations in clusters. This work shows that off-axis collision products cannot become blue stragglers unless they lose a large fraction of their initial angular momentum. The mechanism for this loss is not apparent, although some possibilities are discussed.