CONTRIBUTION TO THE HEAVY-ELEMENT ABUNDANCES IN THE GALACTIC HALO FROM S-PROCESS NUCLEOSYNTHESIS IN MASSIVE STARS

A quantitative estimate is made for heavy-elements enrichment in the early Galaxy that are due to the s-process nucleosynthesis in massive stars. It is based on a realistic calculation of s-process yields from massive stars with varied initial composition, and a simple one-zone model of chemical evolution of the Galactic halo. In particular, we compare the predicted time-variations of Cu, Sr, Y and Zr abundances with those implied from observation. Unlike the others, the present work does not attempt to fit the observational data by parametrized models for various nucleosynthesis processes. Instead, we intend to set the limit to the contributions just from the s-process in massive stars by utilizing results of realistic nucleosynthesis calculation. We bring explicitly the remaining theoretical uncertainties to light, particularly those stemming from the initial O/Fe ratios and in at least one of the key nuclear reaction rates. Using oxygen as our theoretical reference element, we could minimize the effects of the chemical evolution model parameter values. The present results substantiate the idea that the abundance distribution patterns in metal-poor stars could hardly be accounted for by the s-process in massive stars. Finally, we strongly urge observers to clarify the uncertainties in their analyses of heavy elements (Sr, for one) in metal-poor stars.