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Galactic Evolution of Sr, Y, Zr: A Multiplicity of Nucleosynthetic Processes

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,and r-process components.The s-process yields from AGB stars are computed,exploring a wide range of efficiencies of the major neutron source,13C,and covering both disk and halo metallicities.AGBs have been shown to reproduce the main s-component in the solar system.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 22Ne n-source,is discussed in detail.Neither the main s-,nor the weak s-components are shown to contribute significantly to the n-capture element abundances observed in unevolved halo stars.We present a detailed analysis of a large database of spectroscopic observations of Sr,Y,Zr, Ba,and Eu for Galactic stars at various metallicities.Spectroscopic observations of Sr,Y,and Zr to Ba and Eu abundance ratios versus metallicity provide useful diagnostics of the types of n-capture processes forming Sr,Y and Zr.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 origin.We discuss our chemical evolution predictions, taking into account the interplay between different processes to produce Sr-Y-Zr.We find hints for 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 86Sr,87Sr,96Mo.