The Unique Na:O Abundance Distribution in NGC 6791: The First Open(?) Cluster with Multiple Populations

TL;DR

This study reveals that the massive open cluster NGC 6791 uniquely exhibits multiple stellar populations with distinct Na:O abundance patterns, challenging the traditional view that open clusters are chemically homogeneous.

Contribution

It provides the first evidence of multiple populations within an open cluster based on detailed chemical abundance analysis, bridging characteristics of open and globular clusters.

Findings

NGC 6791 has [Fe/H]=+0.42, consistent with previous studies.

The Na:O distribution shows intrinsic variations and a possible anticorrelation.

NGC 6791 hosts two subpopulations with different chemical homogeneity levels.

Abstract

Almost all globular clusters investigated exhibit a spread in their light element abundances, the most studied being a Na:O anticorrelation. In contrast, open clusters show a homogeneous composition and are still regarded as Simple Stellar Populations. The most probable reason for this difference is that globulars had an initial mass high enough to retain primordial gas and ejecta from the first stellar generation and thus formed a second generation with a distinct composition, an initial mass exceeding that of open clusters. NGC 6791 is a massive open cluster, and warrants a detailed search for chemical inhomogeneities. We collected high resolution, high S/N spectra of 21 members covering a wide range of evolutionary status and measured their Na, O and Fe content. We found [Fe/H]=+0.42$\pm 0.01$, in good agreement with previous values, and no evidence for a spread. However, the Na:O distribution is completely unprecedented. It becomes the first open cluster to show intrinsic abundance variations that cannot be explained by mixing, and thus the first discovered to host multiple populations. It is also the first star cluster to exhibit two subpopulations in the Na:O diagram with one being chemically homogeneous while the second has an intrinsic spread that follows the anticorrelation so far displayed only by globular clusters. NGC 6791 is unique in many aspects, displaying certain characteristics typical of open clusters, others more reminiscent of globulars, and yet others, in particular its Na:O behavior investigated here, that are totally unprecedented. It clearly had a complex and fascinating history.