A spectroscopic study of the Globular Cluster M28 (NGC~6626)

TL;DR

This study provides the first detailed spectroscopic analysis of 17 red giant stars in the metal-poor globular cluster M28, revealing multiple stellar populations and chemical abundance patterns indicative of complex formation history.

Contribution

It offers the first extensive high-resolution spectroscopic data for M28, identifying multiple sub-populations through detailed chemical abundance analysis.

Findings

Metallicity of [Fe/H]=-1.29+-0.01

Detection of Na-O and Na-Al anticorrelations

Identification of at least 3 distinct stellar sub-populations

Abstract

We present the abundance analysis for a sample of 17 red giant branch stars in the metal-poor globular cluster M28 based on high resolution spectra. This is the first extensive spectroscopic study of this cluster. We derive abundances of O, Na, Mg, Al, Si, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Ba, La, Ce, and Eu. We find a metallicity of [Fe/H]=-1.29+-0.01 and an alpha-enhancement of +0.34+-0.01 (errors on the mean), typical of Halo Globular Clusters in this metallicity regime. A large spread is observed in the abundances of light elements O, Na, and Al. Mg also shows an anticorrelation with Al with a significance of 3 sigma. The cluster shows a Na-O anticorrelation and a Na-Al correlation. This correlation is not linear but "segmented" and that the stars are not distributed continuously, but form at least 3 well separated sub-populations. In this aspect M28 resembles NGC~2808 that was found to host at least 5 sub-populations. The presence of a Mg-Al anticorrelation favor massive AGB stars as the main polluters responsible for the multiple-population phenomenon.