# 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](https://ui.adsabs.harvard.edu/abs/2018ApJ...862..133S/abstract)\
> arXiv:  [1806.04805](https://arxiv.org/abs/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.](https://2570543513-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MEAw1OGLXwuhdLZSNYH%2F-MEBGeKBOzYKvqhDHT6A%2F-MEBJNwQBp9ESO5K9QbV%2FCleanShot%202020-08-08%20at%2014.25.50%402x.png?alt=media\&token=1deb3e55-e1da-416e-8938-a94b50f5d8ff)

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.](https://2570543513-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-MEAw1OGLXwuhdLZSNYH%2F-MEBGeKBOzYKvqhDHT6A%2F-MEBJanIVBwvAxJq-X0S%2FCleanShot%202020-08-08%20at%2014.26.52%402x.png?alt=media\&token=c1e4c862-908b-4188-b92b-695c5f112857)