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/M_TO = 0.2, 0.5, 0.75, and 1, where M_TO is the cluster turnoff mass (typically about 0.8 M_⊙). The parent 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 collisional blue stragglers. In all cases we find negligible hydrodynamic mixing of helium into the outer envelope of the merger remnant. The amount of hydrogen carried into the core of the merger remnant depends strongly on the entropy profiles of the colliding stars. For stars with nearly equal masses (and hence entropy profiles), the composition profile of the remnant closely resembles that of the parents. If the parent stars were 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 short. In contrast, during a collision between stars with sufficiently different masses (mass ratio q ≲ 0.5), the hydrogen-rich material originally in the smaller star maintains, on average, a lower specific entropy than that of the more massive star and therefore settles preferentially in the core of the merger remnant. Through this process, moderately massive blue stragglers (with masses M_TO ≲ M_BS ≲ 1.5M_TO) can obtain a significant supply of fresh hydrogen fuel, thereby extending their main-sequence lifetime. We conclude, in contrast to what has often been assumed, that blue stragglers formed by direct stellar collisions do not necessarily have initially homogeneous composition profiles. However, we also demonstrate that the final merged configurations, although close to hydrostatic equilibrium, are typically far from thermal equilibrium. Therefore, it is possible that convective, semiconvective, or rotationally induced mixing could occur on a thermal timescale, as the merger remnant recontracts to the main sequence.