CLOSE APPROACH DURING HARD BINARY-BINARY SCATTERING

It is now clear that there is a substantial population of primordial b inaries in Galactic globular clusters and that binary interactions are a major influence on globular cluster evolution. Collisional interact ions involving stars in binaries may provide a significant channel for the formation of various stellar exotica, such as blue stragglers, X- ray binaries and millisecond pulsars. We report on an extensive series of numerical experiments of binary-binary scattering, analysing the c ross-section for close approach during interactions for a range of har d binary parameters of interest in globular cluster cores. We consider the implied rate for tidal interactions for different globular cluste rs and compare our results with previous, complementary estimates of s tellar collision rates in globular clusters. We find that the collisio n rate for binary-binary encounters dominates in low-density clusters if the binary fraction in the cluster is larger than 0.2 for wide main -sequence binaries. In dense clusters, binary-single interactions domi nate the collision rate and the core binary fraction must be less than or similar to 0.1 per decade in semimajor axis, or too many collision s take place compared with observations. The rates are consistent if b inaries with semimajor axes similar to 100 au are overabundant in low- density clusters, or if breakup and ejection substantially lower the b inary fraction in denser clusters. Given reasonable assumptions about fractions of binaries in the cores of low-density clusters such as NGC 5053, we cannot account for all the observed blue stragglers by stell ar collisions during binary encounters, suggesting that a substantial fraction may be due to coalescence of tight primordial binaries.