A New Look at the Old Star Cluster NGC 6791

TL;DR

This study provides a detailed analysis of NGC 6791, including its membership, photometry, and structural properties, revealing an extended horizontal branch and emphasizing the importance of differential reddening.

Contribution

It offers the first comprehensive proper-motion catalog for NGC 6791, confirming membership and revealing new features like an extended horizontal branch.

Findings

Confirmed cluster membership for over 4800 stars

Discovered an extended horizontal branch

Identified the importance of differential reddening

Abstract

We present comprehensive cluster membership and g'r' photometry of the prototypical old, metal-rich Galactic star cluster NGC 6791. The proper-motion catalog contains 58,901 objects down to g'=24, limited to a circular area of radius 30 arcmin. The highest precision of the proper motions is 0.08 mas/yr. Our proper motions confirm cluster membership of all main and also some rare constituents of NGC 6791. The total number of probable cluster members down to g'=22 (M_V=+8) is 4800, corresponding to M_tot=5000 M_solar. New findings include an extended horizontal branch in this cluster. The angular radius of NGC 6791 is at least 15 arcmin (the effective radius is R_h=4.4 arcmin while the tidal radius is r_t=23 arcmin). The luminosity function of the cluster peaks at M_g'=+4.5 and then steadily declines toward fainter magnitudes. Our data provide evidence that differential reddening may not be ignored in NGC 6791.