A comprehensive study of the old open cluster NGC 6791 using <i>Gaia</i> DR3 data and <i>BV</i> photometry

ABSTRACT NGC 6791 is a remarkable stellar system that stands out among Galactic open clusters due to its dual distinction as one of the oldest and the most metal-rich clusters. This cluster is among the most thoroughly investigated open clusters, boasting a mass of approximately 4000 ${\rm M}_{\odot }$, an age of about 8 billion years, and a high metallicity of [Fe/H] $\approx 0.4$, making it the most metal-rich cluster identified in the Milky Way. This paper aims to revisit its kinematic, orbital, photometric properties, and stellar populations by using Gaia Data Release 3 (DR3) and BV photometry. To assess membership, we employed the pyupmaskpython package with the HDBSCAN algorithm. The cluster’s proper motions ($\mu _{\alpha }\cos \delta$, $\mu _{\delta }$) and parallax ($\varpi$) are −0.419 $\pm$ 0.12 mas yr$^{-1}$, −2.284 $\pm$ 0.21 mas yr$^{-1}$, and 0.192 $\pm$ 0.042 mas, respectively. Utilizing the PARSEC stellar isochrone fit, we derived the physical parameters of the cluster. The average cluster age and relaxation time are estimated to be 8.2 $\pm$ 0.5 Gyr and 153.9$\pm$ 12.54 Myr, respectively. For the Gaia data, the colour excess $E(G_{BP}-G_{RP})$ is determined to be 0.22 $\pm$ 0.04 mag, and the distance modulus is 13.1 $\pm$ 0.08 mag, corresponding to a distance of 4170 $\pm$ 262 pc. But for BV data, the distance modulus and the colour excess E(B − V) are 12.98 $\pm$ .08 and 0.17 $\pm$ 0.04 mag. The orbit of the cluster has been established. Finally, the mass function is well matched by the two Gaussian functions, which suggest that there are two stellar mass populations in NGC 6791 cluster.