Detailed Chemical Abundances in the r-Process-Rich Ultra-Faint Dwarf
  Galaxy Reticulum 2
  The ultra-faint dwarf galaxy Reticulum 2 (Ret 2) was recently discovered in
images obtained by the Dark Energy Survey. We have observed the four brightest
red giants in Ret 2 at high spectral resolution using the Michigan/Magellan
Fiber System. We present detailed abundances for as many as 20 elements per
star, including 12 elements heavier than the Fe group. We confirm previous
detection of high levels of r-process material in Ret 2 (mean
[Eu/Fe]=+1.69+/-0.05) found in three of these stars (mean [Fe/H]=-2.88+/-0.10).
The abundances closely match the r-process pattern found in the well-studied
metal-poor halo star CS22892-052. Such r-process-enhanced stars have not been
found in any other ultra-faint dwarf galaxy, though their existence has been
predicted by at least one model. The fourth star in Ret 2 ([Fe/H]=-3.42+/-0.20)
contains only trace amounts of Sr ([Sr/Fe]=-1.73+/-0.43) and no detectable
heavier elements. One r-process enhanced star is also enhanced in C (natal
[C/Fe]=+1.1). This is only the third such star known, which suggests that the
nucleosynthesis sites leading to C and r-process enhancements are decoupled.
The r-process-deficient star is enhanced in Mg ([Mg/Fe]=+0.81+/-0.14), and the
other three stars show normal levels of alpha-enhancement (mean
[Mg/Fe]=+0.34+/-0.03). The abundances of other alpha and Fe-group elements
closely resemble those in ultra-faint dwarf galaxies and metal-poor halo stars,
suggesting that the nucleosynthesis that led to the large r-process
enhancements either produced no light elements or produced light-element
abundance signatures indistinguishable from normal supernovae.
