2023PASJ...75...82G

In this paper, we investigate the memberships, spatial structure, and dynamical state of the old open cluster Collinder 261 (8 Gyr) based on Gaia-DR3 data. We develop a robust clustering method to calculate membership probabilities for 24858 sample stars within 20' of the cluster center. We can identify 2619 likely cluster members (including 54 blue stragglers) down to G ∼ 20 mag. We use a Monte Carlo simulation method to estimate a distance of 2909 ± 90 pc for the cluster based on 281 high-probability members (P > 0.9). We find that the detectable cluster radius (R_cl) is at least 14' (∼11.8 pc), which is much larger than previously thought. Based on the obtained cluster members, the core and tidal radii of the cluster are determined to be $R_{\rm c}={3{^{\prime }_{.}}4}\pm {0.1}$ (∼2.9 ± 0.1 pc) and $R_{\rm t}={19{^{\prime }_{.}}8}\pm {1{^{\prime }_{.}}9}$ (∼16.8 ± 1.6 pc), respectively. A concentration parameter of log (R_t/R_c) ∼0.76 is determined, indicating that Collinder 261 has formed a clear core-halo structure, but its spatial structure is much looser than the old open clusters M 67 (4 Gyr) and NGC 188 (7 Gyr). We also find that the membership probabilities are suitable for analyzing the mass segregation effect in the cluster. The orbit parameters show that Collinder 261 is moving along a near-circular orbit (e ∼ 0.025) well inside the solar circle (R_p ∼ 6.928 kpc, R_a ∼ 7.285 kpc), and it can reach a maximum height of Z_max ∼ 533 pc from the Galactic plane.