Binary Fraction in Galactic Star Clusters: FSR 866, NGC 1960, and STOCK 2

Abstract The study of binary stars in different astronomical environments offers insights into the dynamical state of the hosting stellar systems. The binary fraction in fact plays a crucial role in the dynamical evolution of stellar systems, regulating processes like mass segregation and dynamical heating, and in some cases leading to the formation of exotic objects, like, for instance, blue straggler stars. We used two methodologies to estimate the binary fraction in three different-age open star clusters: FSR 866, NGC 1960 (M36), and Stock 2. The first is a photometric approach based on color–magnitude diagram analysis, and the second is a spectroscopic technique that employs radial velocity measurements. We used Gaia DR3 data in tandem with new spectroscopic observations, and employed the Density-Based Spatial Clustering of Applications with Noise algorithm to identify probable cluster members based on proper motion and parallax in 3D space. The new sample of cluster members allows us to provide new estimates of the cluster fundamental parameters. As a by-product, we found two previously undetected, small physical groups of stars in the background of NGC 1960. The resulting binary fractions lie in the range 0.3–0.5 and are in good agreement with those expected theoretically for open clusters.