5D memberships and fundamental properties of the old open cluster NGC 6791 based on Gaia-DR2

This paper investigates reliable memberships and fundamental astrophysical properties of the old (similar to 8 Gyr) open cluster NGC 6791 using the astrometric and photometric data of the Gaia data release 2 (Gaia-DR2). We identify 2536 likely cluster members (>= 0.6) in a five-dimensional (5D) parameter space using a Gaussian mixture model (GMM) clustering method. Based on these reliable 5D cluster members, we determine the core and tidal radii of the cluster to be R-c = 3.69' +/- 0.23' and R-t = 16.96' +/- 2.29', respectively. We find evidence for the presence of mass segregation in the cluster. Among the likely cluster members, we identify six blue subdwarfs, 43 blue stragglers, and 27 red clump giants. We find that 34 blue stragglers (similar to 79%) lie within the core radius (<3.69') of the cluster, indicating that these stars may be formed by star collisions or by the evolution of primordial binaries within the dense core. The 27 red clump giants are used to accurately estimate the distance of the cluster. We determine a distance of 4118 +/- 23 pc for the cluster using a Monte-Carlo simulation method. We confirm the presence of a significant zero-point offset (similar to-0.055 +/- 0.004 mas) in the Gaia parallaxes of the cluster members. Using the same simulation method, the mean proper motion and radial velocity of the cluster are found to be (<mu(alpha) cos delta >, <mu(delta)>) = (-0.434 +/- 0.008,-2.266 +/- 0.010) mas/yr and < V-r > =-46.7 +/- 0.6 km/s, respectively.