Hydrodynamics of collisions between binary stars

Hydrodynamical processes in collisions between two binary stars, presumed to have formed by tidal capture, are investigated using a smoothed particle hydrodynamics code. Stellar mergers occur frequently in such events, when the binaries approach one another sufficiently closely that they could, in principle, convert substantial internal binary binding energy to external translational energy. Relative to binary-binary collisions involving point masses, hydrodynamical effects reduce the average translational energy produced by a factor ∼3. This may be sufficient to eliminate tidal-capture binaries as a viable direct energy source in star clusters, although they may still contribute indirectly through formation and stellar evolution of mergers. Multiple mergers are common, giving rise to remnants consisting of three or even all four stars. The relevance of these results to the dynamics of star clusters is discussed, along with possible implications for the hypothesis that blue stragglers form by stellar mergers.