A chemical trompe-l'\oe{}il: no iron spread in the globular cluster M22

TL;DR

This study analyzes high-resolution spectra of stars in globular cluster M22, revealing no intrinsic iron spread when using photometric gravities, and suggests non-LTE effects cause apparent iron abundance variations.

Contribution

The paper demonstrates that apparent iron spreads in M22 are due to non-LTE effects affecting FeI lines, not actual abundance variations, clarifying the cluster's chemical homogeneity.

Findings

No evidence of intrinsic iron spread in M22 when using photometric gravities.

Non-LTE effects cause FeI lines to underestimate iron abundance.

s-process element variations correlate with FeI and FeII abundance discrepancies.

Abstract

We present the analysis of high-resolution spectra obtained with UVES and UVES-FLAMES at the Very Large Telescope of 17 giants in the globular cluster M22, a stellar system suspected to have an intrinsic spread in the iron abundance. We find that when surface gravities are derived spectroscopically (by imposing to obtain the same iron abundance from FeI and FeII lines) the [Fe/H] distribution spans ~0.5 dex, according to previous analyses. However, the gravities obtained in this way correspond to unrealistic low stellar masses (0.1-0.5 Msun) for most of the surveyed giants. Instead, when photometric gravities are adopted, the [FeII/H] distribution shows no evidence of spread at variance with the [FeI/H] distribution. This difference has been recently observed in other clusters and could be due to non-local thermodynamical equilibrium effects driven by over-ionization mechanisms, that mainly affect the neutral species (thus providing lower [FeI/H]) but leave [FeII/H] unaltered. We confirm that the s-process elements show significant star-to-star variations and their abundances appear to be correlated with the difference between [FeI/H] and [FeII/H]. This puzzling finding suggests that the peculiar chemical composition of some cluster stars may be related to effects able to spuriously decrease [FeI/H]. We conclude that M22 is a globular cluster with no evidence of intrinsic iron spread, ruling out that it has retained the supernovae ejecta in its gravitational potential well.