A clustering study of the old open cluster Trumpler 19

Abstract

In this paper, we investigate the spatial structure and dynamical state of the poorly studied old (∼4 Gyr) open cluster Trumpler 19 based on reliable cluster members from Gaia-DR3. The DBSCAN clustering algorithm is used to estimate membership probabilities and select likely cluster members in a normalized 5D parametric space. We identify 859 likely cluster members down to G∼20 mag in the field of the cluster. We estimate a cluster radius of Rcl∼18 arcmin (13.1 pc) based on the radial distribution of the cluster members. We find that the cluster is deficient in faint and low-mass stars (G>18.5 mag) and strong mass segregation has taken place within the cluster. We estimate a core radius of Rc=3.0±0.2 arcmin (2.2±0.1 pc) and a tidal radius of Rt=30.7±9.0 arcmin (22.3±6.5 pc) for the cluster. We estimate a concentration parameter of log(Rt/Rc)∼1.0, indicating that Trumpler 19 has formed a clear core-halo structure due to dynamical evolution. We investigate the dynamical state of the cluster using 26 blue stragglers (BSs) identified in the cluster. We find that the BSs extend to a radius of ∼10.5 arcmin (3.5Rc), and 16 of them (∼62%) are located in the core radius of the cluster. We also find that the BSs are significantly more concentrated than the red giant branch (RGB) and main-sequence (MS) stars, indicating that Trumpler 19 is a dynamically old cluster.