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 R cl ∼ 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 > 19 mag), possibly resulting from star evaporation and external tidal effects. Strong mass segregation effect can be detected in the cluster based on the obtained membership probabilities. We estimate a core radius of R c = 3.0 ± 0.2 arcmin (2.2 ± 0.1 pc) and a tidal radius of R t = 30.7 ± 9.0 arcmin (22.3 ± 6.5 pc) for the cluster. We estimate a concentration parameter of log(R t /R c) ∼ 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 (3.5R c), 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) stars, indicating that Trumpler 19 is a dynamically old cluster.