The spatial structure and dynamical state of the open cluster NGC 2112

Abstract

Abstract This paper investigates the spatial structure and dynamical state of the old open cluster NGC 2112 based on likely cluster members from Gaia Early Data Release 3. Using the Density-Based Spatial Clustering of Applications with Noise (DBSCAN) algorithm, we find 1193 likely cluster members down to G ∼ 21 mag within a radius of 1.°5 from the cluster center. These likely cluster members can be divided into 865 core members and 328 border members by DBSCAN. We find that the core members are, on average, significantly brighter and more centrally concentrated than the border members. This suggests the existence of clear mass segregation within the cluster. We find that the outer regions of the cluster exhibit a slightly elongated shape, which may be caused by external tidal perturbations. We estimate a distance of D = 1108 ± 3 pc for the cluster based on bright core members. We find that NGC 2112 has a cluster radius of Rcl ∼ 40′ (∼12.9 pc) and a core radius of $R_{\rm c} \sim {4{^{\prime }_{.}}8} \pm {0{^{\prime }_{.}}2}$ (∼1.5 pc). This indicates that NGC 2112 has a central concentration parameter of C = log (Rcl/Rc) ∼ 0.92, which is significantly larger than previously thought. In addition, we estimate a total mass of Mcl = 858 ± 12 M⊙ and an initial mass of Mini = (2.2 ± 0.5) × 104 M⊙ for the cluster. This implies that NGC 2112 may have lost more than $90\%$ of its initial mass. Based on the obtained distance and kinematical data, we also calculate the Galactic orbit of the cluster.