The origin of the spurious iron spread in the globular cluster NGC 3201

TL;DR

This study re-analyzed iron abundance measurements in NGC 3201 and found that apparent iron spread is due to non-LTE effects in asymptotic giant branch stars, concluding the cluster has no intrinsic iron variation.

Contribution

It demonstrates that the previously reported iron spread in NGC 3201 is caused by non-LTE effects in AGB stars, not an actual metallicity variation.

Findings

Iron spread disappears when using photometric gravities and Fe II lines.

The metal-poor component is due to AGB stars affected by non-LTE effects.

NGC 3201 is a chemically homogeneous globular cluster.

Abstract

NGC 3201 is a globular cluster suspected to have an intrinsic spread in the iron content. We re-analysed a sample of 21 cluster stars observed with UVES-FLAMES at the Very Large Telescope and for which Simmerer et al. found a 0.4 dex wide [Fe/H] distribution with a metal-poor tail. We confirmed that when spectroscopic gravities are adopted, the derived [Fe/H] distribution spans ~0.4 dex. On the other hand, when photometric gravities are used, the metallicity distribution from Fe I lines remains large, while that derived from Fe II lines is narrow and compatible with no iron spread. We demonstrate that the metal-poor component claimed by Simmerer et al. is composed by asymptotic giant branch stars that could be affected by non local thermodynamical equilibrium effects driven by iron overionization. This leads to a decrease of the Fe I abundance, while leaving the Fe II abundance unaltered. A similar finding has been already found in asymptotic giant branch stars of the globular clusters M5 and 47 Tucanae. We conclude that NGC 3201 is a normal cluster, with no evidence of intrinsic iron spread.