M22: A [Fe/H] Abundance Range Revealed

TL;DR

This study reveals that the globular cluster M22 has a broad and intrinsic distribution of iron abundances among its stars, similar to omega Centauri, suggesting a complex formation history involving multiple stellar populations.

Contribution

First spectroscopic measurement of [Fe/H] distribution in M22 showing intrinsic metallicity spread, indicating complex formation history.

Findings

M22 has a broad [Fe/H] distribution with a peak at -1.9

The highest measured [Fe/H] in M22 is -1.45

The abundance spread is not due to observational errors or reddening

Abstract

Intermediate resolution spectra at the Ca II triplet have been obtained for 55 candidate red giants in the field of the globular cluster M22 with the VLT/FORS instrument. Spectra were also obtained for a number of red giants in standard globular clusters to provide a calibration of the observed line strengths with overall abundance [Fe/H]. For the 41 M22 member stars that lie within the V-V_HB bounds of the calibration, we find an abundance distribution that is substantially broader than that expected from the observed errors alone. We argue that this broad distribution cannot be the result of differential reddening. Instead we conclude that, as has long been suspected, M22 is similar to omega Cen in having an intrinsic dispersion in heavy element abundance. The observed M22 abundance distribution rises sharply to a peak at [Fe/H] = -1.9 with a broad tail to higher abundances: the highest abundance star in our sample has [Fe/H] = -1.45 dex. If the unusual properties of omega Cen have their origin in a scenario in which the cluster is the remnant nucleus of a disrupted dwarf galaxy, then such a scenario likely applies also to M22.