Blue Straggler Stars beyond the Milky Way. II. A Binary Origin for Blue Straggler Stars in Magellani

Authors: Sun, Weijia; Li, Chengyuan; de Grijs, Richard; Deng, Licai Bibcode: 2018ApJ...862..133S arXiv: 1806.04805

We have analyzed populations of blue straggler stars (BSSs) in 24 Magellanic Cloud (MC) star clusters using multi-passband Hubble Space Telescope images. We compiled a homogeneous BSS database, containing both traditional and evolved BSSs.

Number of core BSS populations including evolved BSSs vs. the core masses of our clusters.

We uncovered a sublinear correlation between the number of BSSs in the cluster cores and the clusters’ core masses, characterized by a power-law index of 0.51 ± 0.07. For low stellar collision rates, the mass-normalized number of BSSs depends only weakly (or perhaps not at all) on the collision rate, implying that the binary-driven BSS formation channel dominates. Comparison with simulations suggests that stellar collisions contribute less than 20% to the total number of BSSs formed. Further tests, including the analysis of the BSS-speci!c frequencies and their population numbers at larger cluster radii, suggest that binary interactions may be their main formation channel, hinting at an anti-correlation between a cluster’s binary fraction and its core mass.

mass-normalized number of BSSs in the core vs. the mass-normalized collision parameter. BSSs formed through the binary and collision channels in the simulation are presented, respectively, in the left- and right-hand panels as colorful squares, with different initial binary fractions indicated by different colors.