Chemical composition of the outer halo globular cluster Palomar 15
  Globular clusters (GCs) in the outer Milky Way halo are important tracers of
the assembly history of our Galaxy. Only a few of these objects show spreads in
heavier elements beyond the canonical light-element variations that have
essentially been found throughout the entire Galactic GC system, suggesting a
more complex origin and evolution of these objects. Here, we present the first
abundance analysis of three red giants in the remote ($R_{\rm GC}=38$ kpc)
outer halo GC Palomar 15, based on medium-resolution spectra obtained with the
Keck/ESI instrument. Our results ascertain a low iron abundance of
$-$1.94$\pm$0.06 dex with no evidence of any significant abundance spreads,
although this is based on low number statistics. Overall, abundance ratios of
16 species were measured, including carbon, Na, Al, $\alpha$-peak (Mg,Si,Ca,Ti)
and Fe-peak (Sc,V,Cr,Fe,Co,Ni) elements, and the three neutron-capture elements
Sr, Ba, and Eu. The majority of abundances are compatible with those of halo
field stars and those found in other GCs in the outer and inner halos at
similar metallicity. Pal 15 is enhanced to [Mg/Fe]=0.45 dex, while other
$\alpha$-elements, Ca and Ti, are lower by 0.3 dex. Taking Mg as a
representative for [$\alpha$/Fe], and coupled with the lack of any significant
spread in any of the studied elements we conclude that Pal 15 is typical of the
outer halo, as is bolstered by its chemical similarity to the benchmark outer
halo cluster NGC 7492. One star shows evidence of elevated Na and Al
abundances, hinting at the presence of multiple stellar populations in this
cluster.
