Early Star Formation, Nucleosynthesis, and Chemical Evolution in Proto-Galactic Clouds

Abstract

We present numerical simulations to describe the nucleosynthesis and evolution of pre-Galactic clouds in a model which is motivated by cold dark matter simulations of hierarchical galaxy formation. We adopt a SN-induced star-formation mechanism and follow the chemical enrichment and energy input by Type II and Type Ia SNe. We utilize metallicity-dependent yields and include finite stellar lifetimes. We derive the metallicity distribution functions, the age-metallicity relation, and relative elemental abundances for a number of alpha- and Fe-group elements. We find that the dispersion of the metallicity distribution function of the outer halo is reproduced by contributions from clouds with different initial conditions. Clouds with initial masses greater than that of present globular clusters are found to survive the first 0.1 Gyr, suggesting that such systems may have contributed to the formation of the first stars, and could have been self-enriched. More massive clouds are only stable when one assumes an initial mass function that is not biased towards massive stars. The predicted relative abundances of some alpha- and Fe-group elements show good agreement with the observed values down to metallicities below [Fe/H] = -4 when the iron yields are reduced relative to stellar models. The observed scatter is also reproduced for many elements including the observed bifurcation in [alpha/Fe] for stars with low [Fe/H]. However, the predicted dispersion may be too large for some alpha elements unless a limited range of progenitor masses is assumed. The results suggest that the low-mass SNeII were absent at the very low metallicities, and that the upper mass limit for the first stars that contributed to nucleosynthesis may be < 40 solar masses.