Potassium in globular cluster stars: comparing normal clusters to the peculiar cluster NGC 2419

TL;DR

This study investigates potassium abundances in globular cluster stars, comparing normal clusters to the peculiar NGC 2419, revealing that NGC 2419's chemical composition is unique among globular clusters.

Contribution

It provides a detailed comparison of potassium and magnesium abundances across multiple clusters, establishing NGC 2419's distinct chemical signature.

Findings

NGC 2419 has a unique chemical composition with extreme Mg depletion and K enhancement.

Other clusters show K-Mg abundance patterns similar to field stars and Mg-normal populations.

Potassium abundance behavior is consistent across different stellar evolutionary stages within clusters.

Abstract

Two independent studies recently uncovered two distinct populations among giants in the distant, massive globular cluster (GC) NGC 2419. One of these populations has normal magnesium (Mg) and potassium (K) abundances for halo stars: enhanced Mg and roughly solar K. The other population has extremely depleted Mg and very enhanced K. To better anchor the peculiar NGC 2419 chemical composition, we have investigated the behavior of K in a few red giant branch stars in NGC 6752, NGC 6121, NGC 1904, and omega Cen. To verify that the high K abundances are intrinsic and not due to some atmospheric features in giants, we also derived K abundances in less evolved turn-off and subgiant stars of clusters 47 Tuc, NGC 6752, NGC 6397, and NGC 7099. We normalized the K abundance as a function of the cluster metallicity using 21 field stars analyzed in a homogeneous manner. For all GCs of our sample, the stars lie in the K-Mg abundance plane on the same locus occupied by the Mg-normal population in NGC 2419 and by field stars. This holds both for giants and less evolved stars. At present, NGC 2419 seems unique among GCs.