Abundance variations in the globular cluster M71 (NGC 6838)

TL;DR

This study analyzes spectral data from 89 stars in the globular cluster M71 to investigate chemical abundance variations, confirming previous CN bimodality and uncovering new correlations that shed light on cluster formation and internal mixing processes.

Contribution

It provides detailed measurements of multiple elemental indices in M71 stars and discusses the implications for internal mixing and primordial pollution in globular clusters.

Findings

Confirmed CN bimodality and CN-CH anticorrelation.

Discovered CN-Na correlation and Al-Na, Mg2-Al anticorrelations.

Suggests a combination of convective mixing and primordial pollution explains observations.

Abstract

Abundance variations in moderately metal-rich globular clusters can give clues about the formation and chemical enrichment of globular clusters. CN, CH, Na, Mg and Al indices in spectra of 89 stars of the template metal-rich globular cluster M71 are measured and implications on internal mixing are discussed. Stars from the turn-off up to the Red Giant Branch (0.87 $<$ log g $<$ 4.65) observed with the GMOS multi-object spectrograph at the Gemini-North telescope are analyzed. Radial velocities, colours, effective temperatures, gravities and spectral indices are determined for the sample. Previous findings related to the CN bimodality and CN-CH anticorrelation in stars of M71 are confirmed. We also find a CN-Na correlation, and Al-Na, as well as an Mg$_2$-Al anticorrelation. A combination of convective mixing and a primordial pollution by AG or massive stars in the early stages of globular cluster formation is required to explain the observations.