The fragmentation of pre-enriched primordial objects

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. The question then arises which effect was responsible for this transition in the star formation properties. 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 3 sigma peak of mass 2 x 10(6) M. that collapses at z(coll) similar or equal to 30, using smoothed particle hydrodynamics. The gas has a supposed level of pre-enrichment of either Z = 10(-4) Z. or 10(-3) Z. We assume that H-2 has been radiatively destroyed by the presence of a soft UV background. Metals therefore provide the only viable cooling at temperatures below 10(4) K. 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 centre of the dark matter halo. The central gas, characterized by densities n(H) greater than or similar to 10(4) cm(-3), and a temperature, T similar or equal to 90 K, that closely follows that of the cosmic microwave background, is gravitationally unstable and undergoes vigorous fragmentation. We discuss the physical reason for the existence of a critical metallicity, Z(crit) similar to 5 x 10(-4) Z., and its possible dependence on redshift. Compared with the pure H/He case, the fragmentation of the Z = 10(-3) Z. gas leads to a larger relative number of low-mass clumps.