Ages and heavy element abundances from very metal-poor stars in the
  Sagittarius dwarf galaxy
  Sagittarius (Sgr) is a massive disrupted dwarf spheroidal galaxy in the Milky
Way halo that has undergone several stripping events. Previous chemical studies
were restricted mainly to a few, metal- rich ([Fe/H]~ -1) stars that suggested
a top-light initial mass function (IMF). Here we present the first
high-resolution, very metal-poor ([Fe/H]=-1 to -3) sample of 13 giant stars in
the main body of Sgr. We derive abundances of 13 elements namely C, Ca, Co, Fe,
Sr, Ba, La, Ce, Nd, Eu, Dy, Pb, and Th which challenge the interpretation based
on previous studies. Our abundances from Sgr mimic those of the metal-poor halo
and our most metal-poor star ([Fe/H]~ -3) indicates a pure r-process pollution.
Abundances of Sr, Pb, and Th are presented for the first time in Sgr, allowing
for age determination using nuclear cosmochronology. We calculate ages of
9$\pm$2.5 Gyr. Most of the sample stars have been enriched by a range of
asymptotic giant branch (AGB) stars with masses between 1.3 and 5 M$_{\odot}$.
Sgr J190651.47-320147.23 shows a large overabundance of Pb (2.05dex) and a
peculiar abundance pattern best fit by a 3 M$_{\odot}$ AGB star. Based on
star-to-star scatter and observed abundance patterns a mixture of low- and
high-mass AGB stars and supernovae (15-25 M$_{\odot}$) are necessary to explain
these patterns. The high level (0.29$\pm$0.05 dex) of Ca indicates that massive
supernovae must have existed and polluted the early ISM of Sgr before it lost
its gas. This result is in contrast with a top-light IMF with no massive stars
polluting Sgr.
