COLLISIONS OF MAIN-SEQUENCE STARS AND THE FORMATION OF BLUE STRAGGLERS IN GLOBULAR-CLUSTERS

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 particu lar, we consider parent stars of mass M/M(TO)=0.2, 0.5, 0.75, and 1, w here M(TO) is the cluster turnoff mass (typically about 0.8 M.). The p arent star models are more realistic, and the numerical resolution of the hydrodynamics more detailed, than in previous studies. We focus on the hydrodynamic mixing of helium and hydrogen, which plays a crucial role in establishing the color, luminosity, and lifetime of collision al blue stragglers. In all cases we find negligible hydrodynamic mixin g of helium into the outer envelope of the merger remnant. The amount of hydrogen carried into the core of the merger remnant depends strong ly on the entropy profiles of the colliding stars. For stars with near ly equal masses (and hence entropy profiles), the composition profile of the remnant closely resembles that of the parents. If the parent st ars were close to turnoff, very little hydrogen is present at the cent er of the merger remnant and the main-sequence lifetime of the blue st raggler could be short. In contrast, during a collision between stars with sufficiently different masses (mass ratio q less than or similar to 0.5), the hydrogen-rich material originally in the smaller star mai ntains, on average, a lower specific entropy than that of the more mas sive star and therefore settles preferentially in the core of the merg er remnant. Through this process, moderately massive blue stragglers ( with masses M(TO)less than or similar to M(BS)less than or similar to 1.5M(TO)) can obtain a significant supply of fresh hydrogen fuel, ther eby extending their main-sequence lifetime. We conclude, in contrast t o what has often been assumed, that blue stragglers formed by direct s tellar collisions do not necessarily have initially homogeneous compos ition profiles. However, we also demonstrate that the final merged con figurations, although close to hydrostatic equilibrium, are typically far from thermal equilibrium. Therefore, it is possible that convectiv e, semiconvective, or rotationally induced mixing could occur on a the rmal timescale, as the merger remnant recontracts to the main sequence .