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

Abstract

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 turnoff 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 turnoff (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 find 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 profiles of the two colliding stars. For two stars with nearly equal masses (and hence entropy profiles) very little hydrodynamic mixing occurs at all, especially if they are close to the turnoff point. This is because the hydrogen-rich material from both stars maintains, on average, a higher specific entropy than the helium-rich material. If the two parent stars are close to turnoff, 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 sufficiently different masses (mass ratio q ∼< 0.5), the hydrogen-rich material Center for Radiophysics and Space Research, Cornell University, Ithaca, NY 14853. Email: lombardi@spacenet.tn.cornell.edu. Department of Astronomy, Cornell University. Department of Physics, MIT 6-201, Cambridge, MA 02139. Email: rasio@mit.edu. Center for Astrophysics & Relativity, 326 Siena Drive, Ithaca, NY 14850