Evolution of neutron capture elements in dwarf galaxies
  We study the evolution of Eu and Ba abundances in local group dwarf
spheroidal and ultra faint dwarf galaxies by means of detailed chemical
evolution models and compare our results with new sets of homogeneous
abundances. The adopted models include gas infall and outflow and have been
previously tested. We investigate several production scenarios for r-process
elements: merging neutron stars and magneto-rotational driven supernovae.
Production of Ba through the main s-process acting in low- and intermediate-
mass stars is considered as well. We also test different sets of
nucleosynthesis yields. For merging neutron stars we adopt either a constant
and short delay time for merging or a delay time distribution function. Our
simulations show that: i) if r-process elements are produced only by a quick
source, it is possible to reproduce the [Eu/Fe] vs [Fe/H], but those models
fail in reproducing the [Ba/Fe] vs [Fe/H]. ii) If r-process elements are
produced only with longer delays the opposite happens. iii) If both a quick
source and a delayed one are adopted, such as magneto-rotational driven
supernovae and merging neutron stars with a delay time distribution, the
[Eu/Fe] abundance pattern is successfully reproduced, but models still fail in
reproducing the [Ba/Fe]. iv) On the other hand, the characteristic abundances
of Reticulum II can be reproduced only if both the Eu and the r-process
fraction of Ba are produced on short and constant time delays during a single
merging event. We discuss also other possible interpretations, including an
inhomogeneous mixing of gas which might characterize this galaxy.
