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