Spectroscopy and photometry of the least-massive Type-II globular clusters: NGC1261 AND NGC6934

TL;DR

This study investigates the chemical composition of the least-massive Type-II globular clusters, NGC1261 and NGC6934, revealing their metallicity enhancements and the relationship between cluster mass and iron enrichment, with implications for their formation history.

Contribution

It provides the first detailed chemical analysis of the least-massive Type-II GCs, highlighting the correlation between cluster mass and iron enrichment, and the lack of s-element variations in these clusters.

Findings

Red stars in NGC1261 are slightly enhanced in [Fe/H] by ~0.1 dex.

Red stars in NGC6934 are enhanced in [Fe/H] by ~0.2 dex.

Most Type II GCs show minimal s-element variation, suggesting a mass threshold for s-process enrichment.

Abstract

Recent work has revealed two classes of Globular Clusters (GCs), dubbed Type-I and Type-II. Type-II GCs are characterized by a blue- and a red- red giant branch composed of stars with different metallicities, often coupled with distinct abundances in the slow-neutron capture elements (s-elements). Here we continue the chemical tagging of Type-II GCs by adding the two least-massive clusters of this class, NGC1261 and NGC6934. Based on both spectroscopy and photometry, we find that red stars in NGC1261 are slightly enhanced in [Fe/H] by ~0.1 dex and confirm that red stars of NGC 6934 are enhanced in iron by ~0.2 dex. Neither NGC1261 nor NGC6934 show internal variations in the s-elements, which suggests a GC mass threshold for the occurrence of s-process enrichment. We found a significant correlation between the additional Fe locked in the red stars of Type-II GCs and the present-day mass of the cluster. Nevertheless, most Type II GCs retained a small fraction of Fe produced by SNe II, lower than the 2%; NGC6273, M54 and omega Centauri are remarkable exceptions. In the appendix, we infer for the first time chemical abundances of Lanthanum, assumed as representative of the s-elements, in M54, the GC located in the nucleus of the Sagittarius dwarf galaxy. Red-sequence stars are marginally enhanced in [La/Fe] by 0.10\pm0.06 dex, in contrast with the large [La/Fe] spread of most Type II GCs. We suggest that different processes are responsible for the enrichment in iron and s-elements in Type-II GCs.