The Bizarre Chemical Inventory of NGC 2419, An Extreme Outer Halo
  Globular Cluster
  We present new Keck/HIRES observations of six red giants in the globular
cluster NGC 2419. Although the cluster is among the most distant and most
luminous in the Milky Way, it was considered chemically ordinary until very
recently. Our previous work showed that the near-infrared Ca II triplet line
strength varied more than expected for a chemically homogeneous cluster, and
that at least one star had unusual abundances of Mg and K. Here, we confirm
that NGC 2419 harbors a population of stars, comprising about one third of its
mass, that is depleted in Mg by a factor of 8 and enhanced in K by a factor of
6 with respect to the Mg-normal population. Although the majority, Mg-normal
population appears to have a chemical abundance pattern indistinguishable from
ordinary, inner halo globular clusters, the Mg-poor population exhibits
dispersions of several elements. The abundances of K and Sc are strongly
anti-correlated with Mg, and some other elements (Si and Ca among others) are
weakly anti-correlated with Mg. These abundance patterns suggest that the
different populations of NGC 2419 sample the ejecta of diverse supernovae in
addition to AGB ejecta. However, the abundances of Fe-peak elements except Sc
show no star-to-star variation. We find no nucleosynthetic source that
satisfactorily explains all of the abundance variations in this cluster.
Because NGC 2419 appears like no other globular cluster, we reiterate our
previous suggestion that it is not a globular cluster at all, but rather the
core of an accreted dwarf galaxy.
