The contribution from rotating massive stars to the enrichment in Sr and
  Ba of the Milky Way
  Most neutron capture elements have a double production by r- and s-processes,
but the question of production sites is complex and still open. Recent studies
show that including stellar rotation can have a deep impact on nucleosynthesis.
We studied the evolution of Sr and Ba in the Milky Way. A chemical evolution
model was employed to reproduce the Galactic enrichment. We tested two
different nucleosynthesis prescriptions for s-process in massive stars, adopted
from the Geneva group and the Rome group. Rotation was taken into account,
studying the effects of stars without rotation or rotating with different
velocities. We also tested different production sites for the r-process:
magneto rotational driven supernovae and neutron star mergers. The evolution of
the abundances of Sr and Ba is well reproduced. The comparison with the most
recent observations shows that stellar rotation is a good assumption, but
excessive velocities result in overproduction of these elements. In particular,
the predicted evolution of the [Sr/Ba] ratio at low metallicity does not
explain the data at best if rotation is not included. Adopting different
rotational velocities for different stellar mass and metallicity better
explains the observed trends. Despite the differences between the two sets of
adopted stellar models, both show a better agreement with the data assuming an
increase of rotational velocity toward low metallicity. Assuming different
r-process sources does not alter this conclusion.
