The number and metallicities of the most metalpoor stars
M.S. Oey
A Simple Inhomogeneous Model (SIM) for chemical evolution of the
Galactic halo is applied to two lowmetallicity problems: (1) The
predicted number fraction F_{III} of zerometallicity,
Population III
stars; and (2) The existence and value of a lowmetallicity
threshold. This semianalytic model modifies the homogeneous Simple
Model by including a metallicity distribution function for the parent,
individual enrichment events, f(z_{0}). Thus, this
SIM model is suited for analysis of the
lowestmetallicity regimes that are dominated by stochastic events.
The current empirical upper limit on the Population III fraction is
F_{III} < 4e4. 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 z_{0} depend on metal dispersal processes.
These processes determine the lowest metallicities and existence, if
any, of a lowmetallicity 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 lowmetallicity threshold above zero is consistent
with the currently observed limit of 4 < [Fe/H], although with
large uncertainties.
MNRAS, 339, 849
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