Stochastic chemical enrichment in metal-poor systems I. Theory

A stochastic model of the chemical enrichment of metal-poor systems by core-collapse (Type II) supernovae is presented, allowing for large-scale mixing of the enriched material by turbulent motions and cloud collisions in the interstellar medium. Infall of pristine material is taken into account by following the evolution of the gas density in the medium. Analytical expressions were derived for the number of stars enriched by a given number of supernovae, as well as for the amount of mass with which the ejected material from a supernova is mixed before being locked up in a subsequently formed star. It is shown that for reasonable values of the gas density (∼ 0.1 cm) and of the supernova rate (∼ 0.25 kpc Myr) of the Galactic halo, the resulting metallicity distributions of the extreme Population II stars show a distinct cut-off at [Fe/H] ≃ −4. In fact, by assuming no low-mass Population III stars were able to form out of the primordial interstellar medium, the derived fraction of stars below [Fe/H] = −4 is in agreement with observations. Moreover, the probability is high that even the most metal-poor stars observed to date have been enriched by several contributing supernovae. This partly explains the relatively small starto-star scatter in many chemical-abundance ratios for stars down to [Fe/H] = −4, as recently found in several observational studies. Contribution from the thermonuclear (Type Ia) supernovae is found to be negligible over almost the entire extremely metal-poor regime. Although the fraction of contaminated stars may increase rapidly towards [Fe/H] = −2.5, the fraction of stars with iron primarily from Type Ia supernovae remains small. The stars that are heavily polluted by Type Ia supernovae are pushed towards higher metallicities, creating a hole/bump in the metallicity distribution. Such features could be used to reveal the possible presence of subpopulations of Galactic halo stars that have been enriched by Type Ia supernovae.