Chapter 13 Blue Stragglers in Globular Clusters: Observations, Statistics and Physics

In order to gain some intuition, let us start by considering the two simplest distinct formation channels for blue stragglers in globular clusters. First, blue stragglers may form in the same way in clusters as they do in the Galactic field, i.e. via mass transfer or coalescence in binary systems. In this case, we may expect the number of blue stragglers in any given cluster (NBSS) to scale with the number of binary stars in the cluster (Nbin), NBSS ∝ Nbin ∝ fbinMtot, (13.1) where fbin is the fraction of binaries among the cluster members and Mtot is the total mass of the cluster. In reality, fbin should really be the fraction of close binaries (since only these can be the progenitors of blue stragglers), but let us assume for the moment that these two quantities track each other, so that we can ignore this subtlety. The second possibility is that blue stragglers in globular clusters form primarily via dynamical encounters. Here, the simplest possibility is that the most important encounters are direct collisions between two single stars. In this case, the number of blue stragglers should scale with the 1+1 collision rate (Γcoll,1+1), which is deter-mined by the conditions in the dense cluster core vi