2024PASJ...76.1253G

This paper investigates the spatial structure and tidal tails of the nearby ($\sim$1100 pc) old open cluster NGC 2112 ($\sim$1.2 Gyr) based on reliable cluster members from Gaia-DR3. A robust clustering method is used to calculate membership probabilities for sample stars within $2.^{\!\!\!\circ }5$ ($\sim$48 pc) of the cluster center. More than 1200 cluster members are identified based on the membership probabilities. We find that the cluster members can be naturally segregated into two distinct components (core and halo) using the Gaussian mixture model. The major and minor axes of the core and halo are determined using principal component analysis (PCA). The lengths of the semi-major axes of the core and halo are estimated to be $\sim\! 22^{\prime }$ ($\sim$7 pc) and $\sim 86^{\prime }$ ($\sim$28 pc), respectively. We find that the core and halo have minor to major axis ratios of $e\sim 0.92$ and 0.75 respectively, indicating that the halo is strongly influenced by the external tidal field. Moreover, we find two $\sim$36 pc long tidal tails around the cluster, nearly aligned with the orbit of the cluster. The tidal tails contain various types of stars, including main-sequence stars, main-sequence turn-off (MSTO) stars, blue straggler (BS) stars, and red giant branch stars. The high-mass tail stars (e.g., MSTO stars, BS stars) could not have formed only via star evaporation driven by two-body relaxation. We estimate that the observed tidal tails could have formed within the last $\sim$30 Myr.