Iron and neutron-capture element abundance variations in the globular cluster M2 (NGC 7089)

TL;DR

This study analyzes chemical abundance variations in 14 giant stars of globular cluster M2, revealing complex multiple populations with significant s-process element variations, metallicity dispersion, and star-to-star abundance correlations, indicating a complex formation history.

Contribution

It provides detailed chemical abundance analysis of M2, highlighting its multiple populations and complex formation history, similar to other peculiar globular clusters.

Findings

Significant CN and CH line strength variations.

Metallicity dispersion with peaks at -1.7, -1.5, -1.0.

Star-to-star abundance variations in light and s-process elements.

Abstract

We present CN and CH indices and CaII triplet metallicities for 34 giant stars and chemical abundances for 33 elements in 14 giants in the globular cluster M2. Assuming the program stars are cluster members, our analysis reveals (i) an extreme variation in CN and CH line strengths, (ii) a metallicity dispersion with a dominant peak at [Fe/H] = -1.7 and smaller peaks at -1.5 and -1.0, (iii) star-to-star abundance variations and correlations for the light elements O, Na, Al and Si and (iv) a large (and possibly bimodal) distribution in the abundances of all elements produced mainly via the s-process in solar system material. Following Roederer et al. (2011), we define two groups of stars, "r+s" and "r-only", and subtract the average abundances of the latter from the former group to obtain a "s-process residual". This s-process residual is remarkably similar to that found in M22 and in M4 despite the range in metallicity covered by these three systems. With recent studies identifying a double subgiant branch in M2 and a dispersion in Sr and Ba abundances, our spectroscopic analysis confirms that this globular cluster has experienced a complex formation history with similarities to M22, NGC 1851 and omega Centauri.