The Origin of the Heaviest Metals in Most Ultra-Faint Dwarf Galaxies
  The heaviest metals found in stars in most ultra-faint dwarf (UFD) galaxies
in the Milky Way halo are generally underabundant by an order of magnitude or
more when compared with stars in the halo field. Among the heavy elements
produced by n-capture reactions, only Sr and Ba can be detected in red giant
stars in most UFD galaxies. This limited chemical information is unable to
identify the nucleosynthesis process(es) responsible for producing the heavy
elements in UFD galaxies. Similar [Sr/Ba] and [Ba/Fe] ratios are found in three
bright halo field stars, BD-18 5550, CS 22185-007, and CS 22891-200. Previous
studies of high-quality spectra of these stars report detections of additional
n-capture elements, including Eu. The [Eu/Ba] ratios in these stars span +0.41
to +0.86. These ratios and others among elements in the rare earth domain
indicate an r-process origin. These stars have some of the lowest levels of
r-process enhancement known, with [Eu/H] spanning -3.95 to -3.32, and they may
be considered nearby proxies for faint stars in UFD galaxies. Direct
confirmation, however, must await future observations of additional heavy
elements in stars in the UFD galaxies themselves.
