The Fragmentation of Pre-enriched Primordial Objects
Abstract
Recent theoretical investigations have suggested that the formation of the very first stars, forming out of metal-free gas, was fundamentally different from the present-day case. In this paper, we study the effect of metallicity on the evolution of the gas in a collapsing dark matter mini-halo. We model such a system as an isolated 3σ peak of mass 2x106 Msun that collapses at zcoll=30, using smoothed particle hydrodynamics. The gas has a supposed level of pre-enrichment of either 10-4 Zsun or 10-3 Zsun. We find that the evolution proceeds very differently for the two cases. The gas in the lower metallicity simulation fails to undergo continued collapse and fragmentation, whereas the gas in the higher metallicity case dissipatively settles into the center of the dark matter halo. The central gas, characterized by densities n > 104 cm-3, and a temperature, T 90 K, which closely follows that of the CMB, is gravitationally unstable and undergoes vigorous fragmentation. We discuss the physical reason for the existence of a critical metallicity, Zcrit 5x10-4 Zsun, and its possible dependence on redshift. Compared to the pure H/He case, the fragmentation of the 10-3 Zsun gas leads to a larger relative number of low-mass clumps.
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