Abstract
The initial mass function of the first (Population III) stars and Population
II (Pop II) stars is poorly known due to a lack of observations of the period
between recombination and re-ionization. In simulations of the formation of the
first stars, it has been shown that, due to the limited ability of metal-free
primordial gas to cool, the IMF of the first stars is a few orders of magnitude
more massive than the current IMF. The transition from a high-mass IMF of the
first stars to a lower-mass current IMF is thus important to understand. To
study the underlying physics of this transition, we performed several
simulations using the cosmological hydrodynamical adaptive mesh refinement code
Enzo for metallicities of 10^-4, 10^-3, 10^-2, 10^-1 Z_ødot. In our
simulations we include a star formation prescription that is derived from a
metallicity dependent multi-phase ISM structure, an external UV radiation
field, and a mechanical feedback algorithm. We also implement cosmic ray
heating, photoelectric heating and gas-dust heating/cooling, and follow the
metal enrichment of the ISM. It is found that the interplay between metallicity
and UV radiation leads to the co-existence of Pop III and Pop II star formation
in non-zero metallicity (Z/Z_ødot \geq10^-2) gas. A cold (T<100 K) and
dense (\rho>10^-22 g cm^-3) gas phase is fragile to ambient UV radiation.
In a metal-poor (Z/Z_ødot łeq10^-3) gas, the cold and dense gas phase
does not form in the presence of a radiation field of F_0\sim10^-5-10^-4
erg cm^-2 s^-1. Therefore, metallicity by itself is not a good indicator of
the Pop III-Pop II transition. Metal-rich (Z/Z_ødot\geq10^-2) gas
dynamically evolves two to three orders of magnitude faster than metal poor gas
(Z/Z_ødotłeq10^-3). The simulations including SNe show that
pre-enrichment of the halo does not affect the mixing of metals
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