Complete element abundances of nine stars in the r-process galaxy
  Reticulum II
  We present chemical abundances derived from high-resolution Magellan/MIKE
spectra of the nine brightest known red giant members of the ultra-faint dwarf
galaxy Reticulum II. These stars span the full metallicity range of Ret II
(-3.5 < [Fe/H] < -2). Seven of the nine stars have extremely high levels of
r-process material ([Eu/Fe]~1.7), in contrast to the extremely low
neutron-capture element abundances found in every other ultra-faint dwarf
galaxy studied to date. The other two stars are the most metal-poor stars in
the system ([Fe/H] < -3), and they have neutron-capture element abundance
limits similar to those in other ultra-faint dwarf galaxies. We confirm that
the relative abundances of Sr, Y, and Zr in these stars are similar to those
found in r-process halo stars but ~0.5 dex lower than the solar r-process
pattern. If the universal r-process pattern extends to those elements, the
stars in Ret II display the least contaminated known r-process pattern. The
abundances of lighter elements up to the iron peak are otherwise similar to
abundances of stars in the halo and in other ultra-faint dwarf galaxies.
However, the scatter in abundance ratios is large enough to suggest that
inhomogeneous metal mixing is required to explain the chemical evolution of
this galaxy. The presence of low amounts of neutron-capture elements in other
ultra-faint dwarf galaxies may imply the existence of additional r-process
sites besides the source of r-process elements in Ret II. Galaxies like Ret II
may be the original birth sites of r-process enhanced stars now found in the
halo.
