Membership and fundamental parameters of the intermediate-age open clusters NGC 2281 and NGC 2539 based on Gaia-DR2

Abstract

Abstract This paper presents an investigation on the two intermediate-age open clusters NGC 2281 and NGC 2539 based on Gaia Data Release 2 (Gaia-DR2). A combined machine-learning method is used to identify likely cluster members of the two clusters, taking into account the astrometric and photometric data of Gaia-DR2. It is found that NGC 2281 and NGC 2539 each have more than 600 likely cluster members, which are highly suitable for further investigation of the fundamental parameters of the two clusters. Robust estimates of the distances, proper motions, and radial velocities for the two clusters are obtained via a Monte Carlo simulation technique. NGC 2281 is found to have a distance of 522$\, \pm \,$12 pc, a proper motion of (〈μαcos δ〉, 〈μδ〉) = ($-2.947\, \pm \,$0.014, $-8.344\, \pm \,$0.014) mas yr−1, and a radial velocity of $+20.0\, \pm \,$0.7 km s−1. The distance, proper motion, and radial velocity of NGC 2539 are determined to be 1271$\, \pm \,$70 pc, (〈μαcos δ〉, 〈μδ〉) = ($-2.335\, \pm \,$0.007, $-0.583\, \pm \,$0.006) mas yr−1, and $+29.6\, \pm \,$0.4 km s−1, respectively. We find that NGC 2281 has a core radius of ${8{^{\prime}_{.}}89}$$\, \pm \,$${0{^{\prime}_{.}}27}$ (1.3 pc) and a limiting radius of ${79{^{\prime}_{.}}26}$$\, \pm \,$${2{^{\prime}_{.}}86}$ (12.0 pc). The core and limiting radius of NGC 2539 are determined to be ${5{^{\prime}_{.}}62}$$\, \pm \,$${0{^{\prime}_{.}}28}$ (2.1 pc) and ${33{^{\prime}_{.}}65}$$\, \pm \,$${1{^{\prime}_{.}}96}$ (12.4 pc), respectively. In addition, we find strong evidence for the existence of mass segregation in the two clusters.