Blue straggler populations beyond the Milky Way

Richard de Grijs, Weijia Sun, Chengyuan Li, Licai Deng

Published 2018-11-22Version 1

Although the formation of blue straggler stars (BSSs) is routinely attributed to stellar interactions in binary systems, the relative importance of the direct collision and slow(er) stellar coalescence formation channels is still poorly understood. We selected a sample of 24 Magellanic Cloud star clusters for which multi-passband Hubble Space Telescope images are available to address this outstanding question. We compiled a BSS database, containing both traditional and evolved BSSs. We found a robust correlation between the number of BSSs in a cluster's core and its core mass, $N_{\rm BSS, core} \propto M_{\rm core}^{0.51 \pm 0.07}$, which supports the notion that BSS formation is linked to a population's binary fraction. At low stellar collision rates, the mass-normalised number of BSSs does not appear to depend on the collision rate, which implies that the coalescence-driven BSS formation channel dominates. Comparison with simulations suggests that stellar collisions contribute less than 20% to the total number of BSSs formed.