An inhomogeneous model for the Galactic halo:: a possible explanation for the spread observed in s- and r-process elements

Aims. We propose an explanation for the considerable scatter of the abundances of neutron capture elements observed in low-metallicity stars in the solar vicinity, compared to the small star-to-star scatter observed for the alpha-elements. Methods. We have developed a stochastic chemical evolution model in which the main assumption is a random formation of new stars subject to the condition that the cumulative mass distribution follows a given initial mass function. Results. With our model, we are able to reproduce the different spreads of neutron capture elements and alpha-elements in low-metallicity stars. Conclusions. The reason for different observed spreads in neutron capture elements and alpha-elements resides in the random birth of stars, coupled with different stellar mass ranges, from which a alpha-elements and neutron capture elements originate. In particular, the site of production of alpha-elements is the whole range of massive stars, from 10 to 80 M(circle dot) whereas the mass range of production for neutron capture elements lies between 12 and 30 M(circle dot).