The number and metallicities of the most metal-poor stars

M.S. Oey

A Simple Inhomogeneous Model (SIM) for chemical evolution of the Galactic halo is applied to two low-metallicity problems: (1) The predicted number fraction F_III of zero-metallicity, Population III stars; and (2) The existence and value of a low-metallicity threshold. This semi-analytic model modifies the homogeneous Simple Model by including a metallicity distribution function for the parent, individual enrichment events, f(z₀). Thus, this SIM model is suited for analysis of the lowest-metallicity regimes that are dominated by stochastic events. The current empirical upper limit on the Population III fraction is F_III < 4e-4. We find that the SIM predicts values of F_III that are two orders of magnitude above this observed limit. Thus, accounting for simple inhomogeneity greatly worsens the discrepancy between observations and predictions than found from the Simple Model.

We investigate interstellar mixing and mass transport, since the values of z₀ depend on metal dispersal processes. These processes determine the lowest metallicities and existence, if any, of a low-metallicity threshold. Diffusion is found to be ineffective for all phases, including the hot phase, of the interstellar medium (ISM): relevant diffusion lengths are 1 - 3 orders of magnitude smaller than corresponding length scales for turbulent mixing. Rough relations for dispersal processes are given for multiphase ISM. These suggest that the expected low-metallicity threshold above zero is consistent with the currently observed limit of -4 < [Fe/H], although with large uncertainties.