The iron dispersion of the globular cluster M2, revised.

Abstract

M2 has been claimed to possess three distinct stellar components that are enhanced in iron relative to each other. We use equivalent width measurements from 14 red giant branch stars from which Yong etal. detect a ∼0.8 dex wide, trimodal iron distribution to redetermine the metallicity of the cluster. In contrast to Yong etal., which derive atmospheric parameters following only the classical spectroscopic approach, we perform the chemical analysis using three different methods to constrain effective temperatures and surface gravities. When atmospheric parameters are derived spectroscopically, we measure a trimodal metallicity distribution, that well resembles that by Yong etal. We find that the metallicity distribution from Feii lines strongly differs from the distribution obtained from Fei features when photometric gravities are adopted. The Fei distribution mimics the metallicity distribution obtained using spectroscopic parameters, while the Feii shows the presence of only two stellar groups with metallicity [Fe/H] ≃ -1.5 and -1.1 dex, which are internally homogeneous in iron. This finding, when coupled with the high-resolution photometric evidence, demonstrates that M2 is composed by a dominant population (∼99percent) homogeneous in iron and a minority component (∼1percent) enriched in iron with respect to the main cluster population.