The Evolution of Blue Stragglers Formed via Stellar Collisions

We have used the results of recent smoothed particle hydrodynamic simulations of colliding stars to create models appropriate for input into a stellar evolution code. In evolving these models, we find that little or no surface convection occurs, precluding angular momentum loss via a magnetically driven stellar wind as a viable mechanism for slowing rapidly rotating blue stragglers that have been formed by collisions. Angular momentum transfer to either a circumstellar disk (possibly collisional ejecta) or a nearby companion are plausible mechanisms for explaining the observed low rotation velocities of blue stragglers. Under the assumption that the blue stragglers seen in NGC 6397 and 47 Tuc have been created solely by collisions, we find that the majority of blue stragglers cannot have been highly mixed by convection or meridional circulation currents at any time during their evolution. Also, on the basis of the agreement between the predictions of our nonrotating models and the observed blue straggler distribution, the evolution of blue stragglers is apparently not dominated by the effects of rotation.