A study of the blue straggler population of the old open cluster Collinder 261

Abstract

Blue Stragglers are stars located in an unexpected region of the color-magnitude diagram of a stellar population, as they appear bluer and more luminous than the stars in the turnoff 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 blue straggler 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 FLAMES@VLT. We also analyze the radial distribution of the blue straggler population to probe the dynamical status of the cluster. Blue straggler 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 the 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 the 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 blue stragglers. We found 53 blue stragglers members of Collinder 261, six of them already identified in previous catalogs. Among the blue straggler candidates with radial velocity measurements, we found one long-period binary, five close-binary systems, three non-variable stars; we also identified one yellow straggler.