Multiple stellar populations in the Globular Clusters NGC1851 and NGC6656 (M22)

TL;DR

This paper reviews the complex multiple stellar populations in the globular clusters NGC 1851 and NGC 6656, highlighting their distinct chemical and evolutionary features revealed through photometric and spectroscopic studies.

Contribution

It provides a detailed summary of the properties and chemical compositions of multiple populations in NGC 1851 and NGC 6656, emphasizing their unique star formation histories.

Findings

NGC 1851 and NGC 6656 host multiple stellar populations with distinct chemical signatures.

Both clusters show Na-O and C-N anticorrelations within their sub-populations.

NGC 6656's s-rich stars are also enriched in iron and calcium.

Abstract

In the last years, photometric and spectroscopic evidence has demonstrated that many, maybe all the Globular Clusters (GC) host multiple stellar populations. High-resolution spectroscopy has established that, while most GCs are mono-metallic with no significant abundance spread in $s$-elements, in all the globulars studied to date the presence of different stellar generation is inferred by the Na-O and the C-N anticorrelations. In this context, NGC 1851 and NGC 6656 are among the most intriguing clusters. Contrary to the majority of GCs, they host two groups of stars with different s-elements abundance that are clearly associated to the two distinct sub-giant and red-giant branches detected in their color-magnitude diagrams (CMD). In the case of NGC 6656 s-rich stars are also enriched in iron and calcium. Each $s$-element group exhibits its own Na-O and C-N anticorrelations thus indicating the presence of sub-populations and suggesting that the parent clusters have experienced a very complex star-formation history. In this paper we summarize the properties of multiple populations in NGC 1851 and NGC 6656.