Binary origin of blue straggler stars in Galactic star clusters

Abstract

Building on the recent release of a new Gaia-based blue straggler star catalog in Galactic open star clusters (OCs), we explored theproperties of these stars in a cluster sample spanning a wide range in fundamental parameters. We employed Gaia EDR3 to assessthe membership of any individual blue or yellow straggler to their parent cluster. We then made use of the ASteCA code to estimatethe fundamental parameters of the selected clusters, in particular, the binary fraction. With all this at hand, we critically revisitedthe relation of the blue straggler population and the latter. For the first time, we found a correlation between the number of bluestragglers and the host cluster binary fraction and binaries. This supports the hypothesis that binary evolution is the most viablescenario of straggler formation in Galactic star clusters. The distribution of blue stragglers in the Gaia color-magnitude diagram wasthen compared with a suite of composite evolutionary sequences derived from binary evolutionary models that were run by exploringa range of binary parameters: age, mass ratio, period, and so forth. The excellent comparison between the bulk distribution of bluestragglers and the composite evolutionary sequences loci further supports the binary origin of most stragglers in OCs and paves theway for a detailed study of individual blue stragglers.