A Comprehensive Analysis of NGC 2158 in the Gaia Era: Photometric Parameters, Apex, and Orbit

Abstract

We present an investigation of NGC 2158 using Gaia DR2 data. We identified 800 most likely cluster members with membership probability higher than 90%. The mean proper motions of this object are determined as ( μ x = −0.203 ± 0.003, μ y = −1.99 ± 0.004) mas yr −1 . The limiting radius, log(age), and distance of the cluster are obtained as 23.5 arcmin, 9.38 ± 0.04 Gyr, and 4.69 ± 0.22 kpc, respectively. The overall mass function slope (0.93 ± 0.14) is flatter than the Salpeter value (1.35) within the mass range of 1.17–1.44 M ⊙ . This cluster also shows the mass-segregation effect and our study demonstrates that NGC 2158 is a dynamically relaxed open cluster. Using the apex diagram (AD), the apex coordinates of the cluster are obtained in different ways and are examined using the ( μ U , μ T ) diagram. The best value of the apex coordinates is determined as A = 8724 ± 160, D = −3661 ± 530. We also determined the orbit of the cluster and found that NGC 2158 moves almost in the solar antapex direction. The resulting spatial velocity of NGC 2158 is 51 km s −1 . A significant oscillation along the Z -coordinate up to 529 pc is detected. Various scenarios regarding the origin of this cluster are also discussed.