Evidence for Extended Star Formation in the Old, Metal-Rich Open Cluster, NGC 6791?

TL;DR

This study investigates NGC 6791, an old, metal-rich open cluster, providing evidence that suggests an extended period of star formation leading to an age spread within the cluster.

Contribution

It presents the first evidence of an age spread in NGC 6791, challenging the view of it as a simple, single-age stellar population.

Findings

Color gradient indicates possible age variation within the cluster.

Inner stars are approximately 1 Gyr older than outer stars.

No significant metallicity spread detected.

Abstract

NGC 6791 is an old, metal-rich star cluster normally considered to be a disk open cluster. Its red giant branch is broad in color yet, to date, there is no evidence for a metallicity spread among its stars. The turnoff region of the main sequence is also wider than expected from broad-band photometric errors. Analysis of the color-magnitude diagram reveals a color gradient between the core of the cluster and its periphery; we evaluate the potential explanations for this trend. While binarity and photometric errors appear unlikely, reddening variations across the face of the cluster cannot be excluded. We argue that a viable alternative explanation for this color trend is an age spread resulting from a protracted formation time for the cluster; the stars of the inner region of NGC 6791 appear to be older by ~1 Gyr on average than those of the outer region.