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

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