The chemical evolution of the Galaxy is followed for the elements affected by neutron capture , in particular for those in the atomic number range 56 to 63 ( Ba , La , Ce , Pr , Nd , Sm and Eu ) .
Neutrons by the major ^ { 13 } C source , released in radiative conditions in the interpulse periods of TP-AGB stars , give rise to an efficient s -processing , making low mass AGB the major contributors to the chemical evolution of heavy elements .
The s -process scenario , characterized by the combined operation of the two neutron sources ^ { 13 } C ( \alpha , n ) ^ { 16 } O and ^ { 22 } Ne ( \alpha , n ) ^ { 25 } Mg , is analyzed using AGB stellar evolutionary calculations with the FRANEC Code ( FRASCATI Raphson-Newton Evolutionary Code ) , and are applied over a wide range of stellar masses and metallicities .
The presence of r -process elements in low metallicity stars is indicative of a prompt enrichment of the Galaxy by early generation of stars , and low mass SNII appear to be good candidates for primary production of r -nuclei .
The chemical evolution model used here is organized over three-zone , halo , thick and thin disk .
A comparison between model abundance predictions of the r - and s -process elements observed in unevolved halo and disk stars confirms the overall consistency of the theoretical framework and reveals a number of striking features deserving a careful analysis .