Evidences of tidal distortion and mass loss from the old open cluster NGC 6791

TL;DR

This study provides the first clear evidence of tidal distortions and ongoing mass loss in the old open cluster NGC 6791, indicating interactions with the Milky Way's tidal field affecting its structure and evolution.

Contribution

It presents observational evidence of tidal distortions in NGC 6791 and estimates its initial mass, highlighting the effects of Galactic tidal interactions on open clusters.

Findings

Detection of elongation and irregular density distribution in NGC 6791

Presence of two tidal tails aligned with proper motion and Galactic center

Estimated initial mass of the cluster as 1.5-4.0 x 10^5 solar masses

Abstract

We present the first evidence of clear signatures of tidal distortions in the density distribution of the fascinating open cluster NGC~6791. We find that the 2D density map shows a clear elongation and an irregular distribution starting from $\sim 300^{\prime\prime}$ from the cluster center and two tails extending in opposite directions beyond the tidal radius. These features are aligned to both the absolute proper motion and to the Galactic centre directions. Accordingly we find that both the surface brightness and star count density profiles reveal a departure from a King model starting from $\sim600^{\prime\prime}$. These observational evidences suggest that NGC~6791 is currently undergoing mass-loss likely due to gravitational shocking and interactions with the tidal field of the Milky Way. We derive the expected mass-loss due to stellar evolution and tidal interactions and we estimate the initial cluster mass to be $M_{ini} = (1.5-4.0 ) \times 10^5 M_{\odot}$.