News from the Galactic suburbia: the chemical composition of the remote globular cluster NGC 2419

TL;DR

This study analyzes 49 giant stars in the globular cluster NGC 2419, revealing a unique bimodal magnesium distribution and its impact on calcium triplet lines, challenging existing nucleosynthesis models.

Contribution

It provides the first detailed chemical abundance analysis of NGC 2419, uncovering a large Mg spread and its effects on spectral features, which was not previously understood.

Findings

All stars share similar Fe, Ca, and Ti abundances.

A bimodal Mg distribution with ~40% of stars highly Mg-deficient.

Mg deficiency influences calcium triplet line strengths via atmospheric effects.

Abstract

We present the chemical analysis of 49 giant stars of the globular cluster NGC 2419, using medium resolution spectra collected with the multi-object spectrograph DEIMOS@Keck. Previous analysis of this cluster revealed a large dispersion in the line strength of the infrared Ca II triplet, suggesting an intrinsic star-to-star scatter in its Fe or Ca content. From our analysis, we assess that all the investigated stars share the same [Fe/H], [Ca/Fe] and [Ti/Fe] abundance ratios, while a large spread in Mg and K abundances is detected. The distribution of [Mg/Fe] is bimodal, with ~40% of the observed targets having subsolar [Mg/Fe], down to [Mg/Fe] ~ -1 dex, a level of Mg-deficiency never observed before in globular clusters. It is found that the large dispersion in Mg abundances is likely the main origin of the observed dispersion of the Ca II triplet lines strengths (that can be erroneously interpreted in terms of Fe or Ca abundance scatter) because Mg plays a relevant role in the atmosphere of giant stars as an electron donor. A strong depletion in the Mg abundance leads to an increase of the line strength of the Ca II triplet, due to the variation in the electronic pressure, at a constant Fe and Ca abundance. Finally, we detect an anti-correlation between Mg and K abundances, not easily explainable within the framework of the current nucleosynthesis models.