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A binary origin for ‘blue stragglers’ in globular clusters

Abstract

Blue stragglers in globular clusters are abnormally massive stars that should have evolved off the stellar main sequence long ago. There are two known processes that can create these objects: direct stellar collisions1 and binary evolution2. However, the relative importance of these processes has remained unclear. In particular, the total number of blue stragglers found in a given cluster does not seem to correlate with the predicted collision rate3,4, providing indirect support for the binary-evolution model. Yet the radial distributions of blue stragglers in many clusters are bimodal, with a dominant central peak5,6,7: this has been interpreted as an indication that collisions do dominate blue straggler production, at least in the high-density cluster cores7,8. Here we report that there is a clear, but sublinear, correlation between the number of blue stragglers found in a cluster core and the total stellar mass contained within it. From this we conclude that most blue stragglers, even those found in cluster cores, come from binary systems. The parent binaries, however, may themselves have been affected by dynamical encounters. This may be the key to reconciling all of the seemingly conflicting results found to date.

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Figure 1: The observed number of core blue stragglers versus the number expected from single–single collisions.
Figure 2: The observed number of core blue stragglers versus the estimated core mass.
Figure 3: Results of fitting the generalized model to the observed blue straggler number.

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Acknowledgements

Research support for C.K. was provided by the UK Science and Technology Facilities Council. A.S. and N.L. are supported by the Natural Sciences and Engineering Research Council of Canada. C.K. would like to thank T. Maccarone for discussions.

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Knigge, C., Leigh, N. & Sills, A. A binary origin for ‘blue stragglers’ in globular clusters. Nature 457, 288–290 (2009). https://doi.org/10.1038/nature07635

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