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 degrees 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.