The origins of blue stragglers and binarity in globular clusters

Two basic formation channels have been proposed for blue straggler stars in globular clusters: binary star evolution and stellar collisions. We recently showed that the number of blue stragglers found in the core of a globular cluster is strongly correlated with the total stellar mass of the core, but not with the collision rate in the core. This result strongly favoured binary evolution as the dominant channel for blue straggler formation. Here, we use newly available empirical binary fractions for globular clusters to carry out a more direct test of the binary evolution hypothesis, and also of collisional channels that involve binary stars. More specifically, using the correlation between blue straggler numbers and core mass as a benchmark, we test for correlations with the number of binary stars, as well as with the rates of single–single, single–binary and binary–binary encounters. We also consider joint models, in which blue straggler numbers are allowed to depend on star/binary numbers and collision rates simultaneously. Surprisingly, we find that the simple correlation with core mass remains by far the strongest predictor of blue straggler population size, even in our joint models. This is despite the fact that the binary fractions themselves strongly anticorrelate with core mass, just as expected in the binary evolution model. At first sight, these results do not fit neatly with either binary evolution or collisional models in their simplest forms. Arguably, the simplest and most intriguing possibility to explain this unexpected result is that observational errors on the core binary fractions are larger than the true intrinsic dispersion associated with their dependence on core mass. In the context of the binary evolution model, this would explain why the combination of binary fraction and core mass is a poorer predictor of blue straggler numbers than core mass alone. It would also imply that core mass is a remarkably clean predictor of core binary fractions. This would be of considerable importance for the dynamical evolution of globular clusters and provides an important benchmark for models attempting to understand their present-day properties.