Helium variation in four Small Magellanic Cloud GCs

TL;DR

This study investigates multiple stellar populations in four Small Magellanic Cloud globular clusters, revealing slight helium enhancements and chemical abundance patterns similar to those in Galactic globular clusters, using Hubble Space Telescope data.

Contribution

It provides the first detailed chemical composition analysis of extragalactic GCs in the SMC, highlighting similarities with Galactic GCs and measuring helium variations.

Findings

Second-population stars are helium-enhanced in three clusters.

Chemical abundance patterns resemble those in Galactic GCs.

Lindsay 1 shows no significant helium variation.

Abstract

The multiple stellar populations (MPs) of $\sim$11-13 Gyr-old Globular Clusters (GCs) in our Galaxy are characterized by different content of several light elements. These elements describe well-defined patterns like the C-N and the Na-O anticorrelations and the He-N and Na-N correlations. The discovery of the MPs in Magellanic Cloud GCs opened up new paths for the investigation of chemical anomalies in clusters with different age and physical properties. In this context, we used Hubble Space Telescope photometry to investigate the MPs and constrain their chemical composition of four $\sim$6-11 Gyr extragalactic GCs, namely NGC 121,NGC 339, NGC 416 and Lindsay 1 in the Small Magellanic Cloud. The comparison of the stellar colors with synthetic spectra suggests that second-population stars of NGC 121, NGC 339, NGC 416 are slightly enhanced in helium by $\delta$Y = 0.009$\pm$0.006, 0.007$\pm$0.004 and 0.010$\pm$0.003, respectively, with respect to the first population, while we find no significant helium variation in Lindsay 1 ($\delta$Y = 0.000$\pm$0.004). Moreover, second-population stars of all the clusters are, on average, enhanced in nitrogen and depleted in carbon and oxygen, in close analogy with what we observe in Galactic GCs.