PPMXL and Gaia morphological analysis of Melotte 22 (Pleiades) and Melotte 25 (Hyades)

Abstract

In this article, we present a morphological study of two extended open clusters in the sky, Melotte 22 and Melotte 25 using J, H, and K_s photometry with Gaia DR2 and PPMXL proper motions. Various photometric and kinematic astrophysical parameters have been determined.

Based on proper motions of these clusters, we extract our member candidates and hence fit isochrones with Solar metallicity Z = 0.019, log (age) = 8.2 ± 0.05 for Melotte 22, and metallicity Z = 0.024, log (age) = 8.9 ± 0.1 for Melotte 25. Some photometric parameters are estimated, e.g. cluster heliocentric distances are 135 ± 4 and 48 ± 2 pc respectively, luminosity and mass functions with mass-luminosity relation using a second order polynomial, and we have estimated masses, i.e. 662 ± 25 and 513 ± 22 M_⊙. The relaxation time of these clusters are smaller than the estimated cluster ages, which indicates that these clusters are dynamically relaxed.

The second part is devoted to some kinematics of these clusters using the internal and/or external motion of members. We find the apex 96° ± 0°.1, -48° ± 0°.1, and 97° ± 1°, 6° ± 0°.1 for Melotte 22 and Melotte 25 respectively using the AD-chart method, also we have determined the components of space velocity and the cluster centers in pc as well as elements of the Solar motion. The results are in good agreement with those obtained previously.