In this paper we follow the Galactic enrichment of three easily observed light n -capture elements – Sr , Y , and Zr .
Input stellar yields have been first separated into their respective main and weak s -process components , and r -process component .
The s -process yields from Asymptotic Giant Branch ( AGB ) stars of low to intermediate mass are computed , exploring a wide range of efficiencies of the major neutron source , ^ { 13 } C , and covering both disk and halo metallicities .
AGB stars have been shown to reproduce the main s -component in the solar system , i.e. , the s -process isotopic distribution of all heavy isotopes with atomic mass number A > 90 , with a minor contribution to the light s -process isotopes up to A \sim 90 .
The concurrent weak s -process , which accounts for the major fraction of the light s -process isotopes in the solar system and occurs in massive stars by the operation of the ^ { 22 } Ne neutron source , is discussed in detail .
Neither the main s - , nor the weak s -components are shown to contribute significantly to the neutron capture element abundances observed in unevolved halo stars .
Knowing the s -process distribution at the epoch of the solar system formation , we first employed the r -process residuals method to infer the isotopic distribution of the r -process .
We assumed a primary r -process production in the Galaxy from moderately massive Type II supernovae that best reproduces the observational Galactic trend of metallicity versus Eu , an almost pure r -process element .
We present a detailed analysis of a large published database of spectroscopic observations of Sr , Y , Zr , Ba , and Eu for Galactic stars at various metallicities , showing that the observed trends versus metallicity can be understood in light of a multiplicity of stellar neutron-capture components .
Spectroscopic observations of the Sr , Y , and Zr to Ba and Eu abundance ratios versus metallicity provide useful diagnostics of the types of neutron-capture processes forming Sr , Y and Zr .
In particular , the observed [ Sr , Y , Zr/Ba , Eu ] ratio is clearly not flat at low metallicities , as we would expect if Ba , Eu and Sr , Y , Zr all had the same r -process nucleosynthetic origin .
We discuss our chemical evolution predictions , taking into account the interplay between different processes to produce Sr-Y-Zr .
Making use of the very r -process-rich and very metal-poor stars like CS 22892-052 and CS 31082-001 , we find hints , and discuss the possibility of a primary process in low-metallicity massive stars , different from the ‘ classical s -process ’ and from the ‘ classical r -process ’ , that we tentatively define LEPP ( Lighter Element Primary Process ) .
This allows us to revise the estimates of the r -process contributions to the solar Sr , Y and Zr abundances , as well as of the contribution to the s -only isotopes ^ { 86 , 87 } Sr and ^ { 96 } Mo .