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

Dipartimento di Astronomia, Universitá di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy e-mail: cescutti@oats.inaf.it

Received: 29 August 2007
Accepted: 14 January 2008

Abstract

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 α-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 α-elements in low-metallicity stars.

Conclusions. The reason for different observed spreads in neutron capture elements and α-elements resides in the random birth of stars, coupled with different stellar mass ranges, from which α-elements and neutron capture elements originate. In particular, the site of production of α-elements is the whole range of massive stars, from 10 to 80  whereas the mass range of production for neutron capture elements lies between 12 and 30 .