New insights in the origin and evolution of the old, metal-rich open cluster NGC 6791

TL;DR

This study investigates the origin and evolution of the old, metal-rich open cluster NGC 6791, suggesting it formed in the inner Galaxy and migrated outward, with evidence from orbital dynamics, survivability simulations, and stellar mass function analysis.

Contribution

It provides new insights into NGC 6791's formation, migration history, and initial mass, combining orbital analysis, N-body simulations, and chemical and stellar population data.

Findings

NGC 6791 likely formed in the inner thin disc or bulge.

The cluster's initial mass was at least 5×10^4 solar masses.

Its flat stellar mass function indicates a history of strong tidal interactions.

Abstract

NGC 6791 is one of the most studied open clusters, it is massive ($\sim5000\,M_{\odot}$), located at the solar circle, old ($~8\,$Gyr) and yet the most metal-rich cluster (${\rm [Fe/H]}\simeq0.4$) known in the Milky Way. By performing an orbital analysis within a Galactic model including spiral arms and a bar, we found that it is plausible that NGC 6791 formed in the inner thin disc or in the bulge, and later displaced by radial migration to its current orbit. We apply different tools to simulate NGC 6791, including direct $N$-body summation in time-varying potentials, to test its survivability when going through different Galactic environments. In order to survive the 8 Gyr journey moving on a migrating orbit, NGC 6791 must have been more massive, $M_0 \geq 5\times10^4 M_{\odot}$, when formed. We find independent confirmation of this initial mass in the stellar mass function, which is observed to be flat; this can only be explained if the average tidal field strength experienced by the cluster is stronger than what it is at its current orbit. Therefore, the birth place and journeys of NGC 6791 are imprinted in its chemical composition, in its mass loss, and in its flat stellar mass function, supporting its origin in the inner thin disc or in the bulge.