Аннотация
Population III stars are believed to have been more massive than typical
stars today and to have formed in relative isolation. The thermodynamic impact
of metals is expected to induce a transition leading to clustered, low-mass
Population II star formation. In this work, we present results from three
cosmological simulations, only differing in gas metallicity, that focus on the
impact of metal fine-structure line cooling on the formation of stellar
clusters in a high-redshift atomic cooling halo. Introduction of sink particles
allows us to follow the process of gas hydrodynamics and accretion onto cluster
stars for 4 Myr corresponding to multiple local free-fall times. At
metallicities at least 10^-3 Zsun, gas is able to reach the CMB temperature
floor and fragment pervasively resulting in a stellar cluster of size ~1 pc and
total mass ~1000 Msun. The masses of individual sink particles vary, but are
typically ~100 Msun, consistent with the Jeans mass when gas cools to the CMB
temperature, though some solar mass fragments are also produced. At the low
metallicity of 10^-4 Zsun, fragmentation is completely suppressed on scales
greater than 0.01 pc and total stellar mass is lower by a factor of 3 than in
the higher metallicity simulations. The sink particle accretion rates, and thus
their masses, are determined by the mass of the gravitationally unstable gas
cloud and the prolonged gas accretion over many Myr. The simulations thus
exhibit features of both monolithic collapse and competitive accretion. Even
considering possible dust induced fragmentation that would occur at higher
densities, the formation of a bona fide stellar cluster seems to require metal
line cooling and metallicities of at least 10^-3 Zsun.
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