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

Abstract

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. 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. With our model, we are able to reproduce the different spreads of neutron capture elements and alpha-elements in low-metallicity stars. The reason for different observed spread in neutron capture elements and alpha-elements resides in the random birth of stars, coupled with different stellar mass ranges, from which 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 Msun whereas the mass range of production for neutron capture elements lies between 12 and 30 Msun.