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 within a model that follows the evolution of chemical enrichment and energy input to the clouds by Type II and Type Ia supernovae. We utilize metallicity-dependent yields for all elements at all times and include effects of finite stellar lifetimes. We derive the metallicity distribution functions for stars in the clouds, their age-metallicity relation and relative elemental abundances for a number of alpha- and Fe-group elements. The stability of these clouds against destruction is discussed, and results are compared for different initial mass functions. We find that the dispersion of the metallicity distribution function observed in the outer halo is naturally reproduced by contributions from many clouds with different initial conditions. The scatter in metallicity as a function of age for these stars is very large, implying that no age-metallicity relation exists in the early stages of galaxy formation. Clouds with initial masses ≳ presently observed globular clusters are found to survive the first 0.1 Gyr from the onset of star formation, 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, indicating that even if the first stars were formed according to a top-heavy mass function, subsequent star formation was likely to have proceeded with a present-day mass function or happened in an episodic manner. 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 most elements including the observed bifurcation in [α/Fe] for stars with low [Fe/H]. However, the predicted dispersion may be too large for some elements (particularly alpha elements) unless a limited range of progenitor masses contributing to the abundances of these elements is assumed. The contributions to the abundances from supernovae with different progenitor masses and metallicity are discussed. The results suggest that the low-mass end of SNeII was probably absent at the very lowest metallicities and that the upper mass limit for the first stars that contributed to nucleosynthesis may be ≲40 M⊙.