Constraints on stellar rotation from the evolution of Sr and Ba in the
  Galactic halo
  Recent studies show that the chemical evolution of Sr and Ba in the Galaxy
can be explained if different production sites, hosting r- and s-processes, are
taken into account. However, the question of unambiguously identifying these
sites is still unsolved. Massive stars are shown to play an important role in
the production of s-material if rotation is considered. In this work, we study
in detail the contribution of rotating massive stars to the production of Sr
and Ba, in order to explain their chemical evolution, but also to constrain the
rotational behaviour of massive stars. A stochastic chemical evolution model
was employed to reproduce the enrichment of the Galactic halo. We developed new
methods for model-data comparison which help to objectively compare the
stochastic results to the observations. We employed these methods to estimate
the value of free parameters which describe the rotation of massive stars,
assumed to be dependent on the stellar metallicity. We constrain the parameters
using the observations for Sr and Ba. Employing these parameters for rotating
massive stars in our stochastic model, we are able to correctly reproduce the
chemical evolution of Sr and Ba, but also Y, Zr and La. The data supports a
decrease of both the mean rotational velocities and their dispersion with
increasing metallicity. Our results show that a metallicity-dependent rotation
is a necessary assumption to explain the s-process in massive stars. Our novel
methods of model-data comparison represent a promising tool for future galactic
chemical evolution studies.
