Spatial structure and dynamical state of the old open cluster Collinder 261 based on a clustering method.

This article focuses on the investigation of the memberships, spatial structure, and dynamical state of the old open cluster Collinder 261 using Gaia Data Release 3 (Gaia-DR3) astrometric data. Employing a robust clustering method based on the DBSCAN algorithm, the study identifies 2,619 likely cluster members, including 54 blue stragglers, within 20 arcminutes of the cluster center and estimates a cluster distance of 2,909 ± 90 parsecs. The analysis reveals that Collinder 261 has a larger detectable radius (at least 14′ or ~11.8 pc), a core radius of approximately 2.9 pc, and a tidal radius of about 16.8 pc, indicating a clear but relatively loose core–halo structure compared to other old open clusters. Additionally, the cluster is found to orbit the Galaxy on a near-circular path well inside the solar circle, with low eccentricity (~0.025) and a maximum height of ~533 pc above the Galactic plane. The membership probabilities derived are also suitable for studying mass segregation within the cluster.