Neutron-capture element deficiency of the Hercules dwarf spheroidal
  galaxy
  We present an assessment of the barium abundance ratios for red giant member
stars in the faint Hercules dwarf spheroidal (dSph) galaxy. Our results are
drawn from intermediate-resolution FLAMES/GIRAFFE spectra around the Ba II
6141.71 AA absorption line at low signal-to-noise ratios. For three brighter
stars we were able to gain estimates from direct equivalent-width measurements,
while for the remaining eight stars only upper limits could be obtained. These
results are investigated in a statistical manner and indicate very low Ba
abundances of log epsilon (Ba) < 0.7 dex (3 sigma). We discuss various possible
systematic biasses, first and foremost, a blend with the Fe I 6141.73 AA-line,
but most of those would only lead to even lower abundances. A better match with
metal-poor halo and dSph stars can only be reached by including a large
uncertainty in the continuum placement. This contrasts with the high
dispersions in iron and calcium (in excess of 1 dex) in this galaxy. While the
latter spreads are typical of the very low luminosity, dark-matter dominated
dSphs, a high level of depletion in heavy elements suggests that chemical
enrichment in Hercules was governed by very massive stars, coupled with a very
low star formation efficiency. While very low abundances of some heavy elements
are also found in individual stars of other dwarf galaxies, this is the first
time that a very low Ba abundance is found within an entire dSph over a broad
metallicity range.
