Journal Article

News from the Galactic suburbia: the chemical composition of the remote globular cluster NGC 2419<sup>⋆</sup>

A. Mucciarelli, M. Bellazzini, R. Ibata, T. Merle, S. C. Chapman, E. Dalessandro and A. Sollima

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 426, issue 4, pages 2889-2900
Published in print November 2012 | ISSN: 0035-8711
Published online November 2012 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2012.21847.x
News from the Galactic suburbia: the chemical composition of the remote globular cluster NGC 2419⋆

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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 per cent 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.

Keywords: stars: abundances; stars: atmospheres; stars: evolution; stars: Population II; globular clusters: individual: NGC 2419

Journal Article.  9959 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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