A Study of the Blue Straggler Population of the Old Open Cluster Collinder 261

Blue stragglers (BSs) are stars located in an unexpected region of the color-magnitude diagram (CMD) of a stellar population, as they appear bluer and more luminous than the stars in the turn-off region. They are ubiquitous, since they have been found among Milky Way field stars, in open and globular clusters, and also in other galaxies of the Local Group. Here we present a study on the BS population of the old and metal-rich open cluster Collinder 261, based on Gaia DR2 data and on a multi-epoch radial velocity survey conducted with Fibre Large Array Multi Element Spectrograph (FLAMES) at the Very Large Telescope (VLT). We also analyze the radial distribution of the BS population to probe the dynamical status of the cluster. BS candidates were identified first with Gaia DR2, according to their position on the CMD, proper motions, and parallaxes. Their radial distribution was compared with those of main sequence, red giant, and red clump stars, to evaluate mass segregation. Additionally, their radial velocities (and the associated uncertainties) were compared with the mean radial velocity and velocity dispersion of the cluster. When possible, close binaries and long-period binaries were also identified, based on the radial velocity variations for the different epochs. We also looked for yellow stragglers, i.e., possible evolved BSs. We found 53 BS members of Collinder 261, six of them were already identified in previous catalogs. Among the BS candidates with radial velocity measurements, we found one long-period binary, five close-binary systems, three nonvariable stars; we also identified one yellow straggler.