The old Galactic open clusters FSR1716 and Czernik23

TL;DR

This study characterizes two old open clusters, FSR 1716 and Czernik 23, revealing their ages, metallicities, structural parameters, and survival despite dynamical dissolution processes, thus adding to the limited known population of such ancient clusters.

Contribution

It provides detailed analysis and confirmation of the ages, metallicities, and structural properties of two old open clusters, expanding the known sample of ancient clusters in the Galaxy.

Findings

FSR 1716 is likely an old (~7 Gyr) open cluster, possibly a low-mass globular cluster.

Czernik 23 is an approximately 5 Gyr old open cluster with low extinction.

Both clusters have low mass (~200 solar masses) and small radii, indicating significant dynamical evolution.

Abstract

Open clusters older than $\sim4$ Gyr are rare in the Galaxy. Affected by a series of mass-decreasing processes, the stellar content of most open clusters dissolves into the field in a time-scale shorter than $\sim1$ Gyr. In this sense, improving the statistics of old objects may provide constraints for a better understanding of the dynamical dissolution of open clusters. Isochrone fits indicate that FSR 1716 is more probably an old ($\sim7$ Gyr) and absorbed ($\aV=6.3\pm0.2$) open cluster, located $\approx0.6$ kpc inside the Solar circle in a contaminated central field. However, we cannot rule out the possibility of a low-mass, loose globular cluster. Czernik 23 is shown to be an almost absorption-free open cluster, $\sim5$ Gyr old, located about 2.5 kpc towards the anti-centre. In both cases, Solar and sub-Solar ($[Fe/H]\sim-0.5$) metallicity isochrones represent equally well the stellar sequences. Both star clusters have a low mass content ($\la200 \ms$) presently stored in stars. Their relatively small core and cluster radii are comparable to those of other open clusters of similar age. These structural parameters are probably consequence of the several Gyrs of mass loss due to stellar evolution, tidal interactions with the disk (and bulge in the case of FSR 1716), and possibly giant molecular clouds. Czernik 23, and especially FSR 1716, are rare examples of extreme dynamical survivors. The identification of both as such represents an increase of $\approx10%$ to the known population of open clusters older than $\sim4$ Gyr in the Galaxy.