NGC6362: the least massive globular cluster with chemically distinct multiple populations

TL;DR

This study measures chemical abundances in NGC 6362, a low-mass globular cluster, revealing fully mixed stellar populations with distinct Na enrichment but uniform Fe levels, marking it as the least massive cluster with such features.

Contribution

First spectroscopic measurement of Fe and Na in NGC 6362, demonstrating multiple populations in a very low-mass globular cluster.

Findings

NGC 6362 has [Fe/H] = -1.09 with no intrinsic dispersion.

Na distribution is broad and bimodal, indicating multiple populations.

Red giant branch includes both first and second generation stars.

Abstract

We present the first measure of Fe and Na abundances in NGC 6362, a low-mass globular cluster where first and second generation stars are fully spatially mixed. A total of 160 member stars (along the red giant branch and the red horizontal branch) have been observed with the multi-object spectrograph FLAMES at the Very Large Telescope. We find that the cluster has an iron abundance of [Fe/H]=--1.09$\pm$0.01 dex, without evidence of intrinsic dispersion. On the other hand, the [Na/Fe] distribution turns out to be intrinsically broad and bimodal. The Na-poor and Na-rich stars populate, respectively, the bluest and the reddest red giant branches detected in the color-magnitude diagrams including the U filter. The red giant branch is composed of a mixture of first and second generation stars in a similar proportion, while almost all the red horizontal branch stars belong to the first cluster generation. Until now, NGC 6362 is the least massive globular cluster where both the photometric and spectroscopic signatures of multiple populations have been detected.