Stellar collisions in globular clusters and the blue straggler problem

The hypothesis that the blue stragglers in globular clusters are produced by physical stellar collisions is critically examined. The most serious objection to the collisional hypothesis is that direct stellar collisions between single stars are not expected to occur frequently enough in low-density globular clusters to account for the observed number of blue stragglers in such clusters. However, if a small fraction of the main-sequence stars in those systems are primordial binaries, then physical stellar collisions can occur at the required rate of one every about 10 to the 8th yr. This is because: (1) mass segregation will result in a high binary frequency in the cluster core; (2) the cross section for a binary-binary collision is about 100 times that for a single-single collision; and (3) physical stellar collisions are likely to occur during strong binary-binary interactions. Since a small fraction of binaries cannot be ruled out in the low-density globular clusters, then neither can the collisional hypothesis for the blue stragglers. Several observations that could be used to test the collisional hypothesis are listed.