ASTROPHYSICAL AND STRUCTURAL PARAMETERS, AND DYNAMICAL EVOLUTION OF THE OPEN CLUSTERS NGC 1245 AND NGC 2099

Abstract

We derive astrophysical and structural parameters of open clusters NGC 1245 and NGC 2099 from 2MASS JHKs and Gaia DR2 photometric / astrometric data bases. Their likely members have been determined from Gaia DR2 proper motion data. Our E(B – V) values (2MASS) are slightly smaller than the literature values, whereas our E(B – V) values (Gaia DR2) agree with the literature within the uncertainties. Their distance moduli/distances and ages are in a good agreement with the early studies. NGC 1245 has steep negative core mass function slope (MFs) (𝜒𝑐𝑜𝑟𝑒=−1.24). Its halo (𝜒ℎ𝑎𝑙𝑜=+0.78) and overall (𝜒𝑜𝑣𝑒𝑟𝑎𝑙𝑙=−0.95) MFs indicate small-scale mass segregation from its core to the outer regions, due to its [𝑡𝑟𝑙𝑥(𝑜𝑣𝑒𝑟𝑎𝑙𝑙),𝜏𝑜𝑣𝑒𝑟𝑎𝑙𝑙]=[302 𝑀𝑦𝑟,5]. The MFs of NGC 2099 is very negative steep (𝜒𝑐𝑜𝑟𝑒=−2.67) in the core, and quite positive steep (𝜒ℎ𝑎𝑙𝑜=+1.41) in the halo. This kind of MF slope steeping from the core to the outskirts indicates that low-mass stars in the core are transferred to the cluster's outskirts, while massive stars sink in the core, because of mass segregation. NGC 2099's flat overall MFs (𝜒𝑜𝑣𝑒𝑟𝑎𝑙𝑙=+0.91) and its 𝜏𝑜𝑣𝑒𝑟𝑎𝑙𝑙=8 indicate mass segregation. These OCs with the relatively large masses 8700 M (NGC 1245) and 5660 M (NGC 2099), which locate at RGC > 9 kpc, expose to external perturbations such as tidal effects and shock waves, and they lose their stars in low-proportions.