Astrophysical parameters and dynamical evolution of open clusters: NGC 2587, Collinder 268 and Melotte 72, and Pismis 7

Abstract

We determined astrophysical and dynamical parameters of the open clusters (OCs) NGC 2587, Collinder 268 (Col 268), Melotte 72 (Mel 72), and Pismis 7 from Gaia DR2 photometric/astrometric data and a new technique, fitCMD. fitCMD provides (Z, Age(Gyr)) as (0.025, 0.45) for NGC 2587, (0.0025, 0.5) for Col 268, (0.011, 1.25) for Mel 72, and (0.008, 1.00) for Pismis 7, respectively. As compared to Gaia DR2 distances, the obtained photometric distances from fitCMD provide somewhat close distances. For NGC 2587 and Mel 72, both distances are in good concordance. Except for NGC 2587, the ages of the remaining OCs are higher than their relaxation times, which suggests that they are dynamically relaxed. NGC 2587 did not undergo dynamical evolution. Mel 72 and Pismis 7 with relatively flat MF slopes indicate signs of a somewhat advanced dynamical evolution, in the sense that they appear to have lost a significant fraction of their low-mass stars to the field. Pismis 7's negative/flat mass function slope indicates that its high mass stars slightly outnumber its low mass ones. Given its mild dynamical evolution, the high mass stars move toward the central region, while low-mass stars are continually being lost to the field. Col 268 presents small dimensions, which suggest a primordial origin. The outer parts of Mel 72 and Pismis 7—with large cluster radii expand with time, while Mel 72's core contracts because of dynamical relaxation.