Stellar collisions during binary–binary and binary–single star interactions

Physical collisions between stars occur frequently in dense star clusters, either via close encounters between two single stars, or during strong dynamical interactions involving binary stars. Here we study stellar collisions that occur during binary–single and binary–binary interactions, by performing numerical scattering experiments. Our results include cross sections, branching ratios, and sample distributions of parameters for various outcomes. For interactions of hard binaries containing main-sequence stars, we find that the normalized cross section for at least one collision to occur (between any two of the four stars involved) is essentially unity, and that the probability of collisions involving more than two stars is significant. Hydrodynamic calculations have shown that the effective radius of a collision product can be 2–30 times larger than the normal main-sequence radius for a star of the same total mass. We study the effect of this expansion, and find that it increases the probability of further collisions considerably. We discuss these results in the context of recent observations of blue stragglers in globular clusters with masses exceeding twice the main-sequence turnoff mass. We also present Fewbody, a new, freely available† numerical toolkit for simulating small-N gravitational dynamics that is particularly suited to performing scattering experiments.