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 particular, we consider parent stars of mass M=MTO = 0:2; 0:5; 0:75, and 1, where MTO is the cluster turno mass (typically about 0:8M ). Low-mass stars (with M = 0:2MTO or 0:5MTO) are assumed to be fully convective and are therefore modeled as n = 1:5 polytropes. Stars at the turno (with M = MTO) are assumed to be mostly radiative and are modeled as n = 3 polytropes. Intermediate-mass stars (with M = 0:75MTO) 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 nd 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 pro les of the two colliding stars. For two stars with nearly equal masses (and hence entropy pro les) very little hydrodynamic mixing occurs at all, especially if they are close to the turno point. This is because the hydrogen-rich material from both stars maintains, on average, a higher speci c entropy than the helium-rich material. If the two parent stars are close to turno , very little hydrogen is present at the center of the merger remnant and the main-sequence lifetime of the blue straggler could be very short. In contrast, during a collision between two stars of su ciently di erent masses (mass ratio q < 0:5), the hydrogen-rich material Center for Radiophysics and Space Research, Cornell University, Ithaca, NY 14853. Email: [email protected]. Department of Astronomy, Cornell University. Department of Physics, MIT 6-201, Cambridge, MA 02139. Email: [email protected]. Center for Astrophysics & Relativity, 326 Siena Drive, Ithaca, NY 14850