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. 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 2x10^6 M_sun that collapses at z_coll=30, using smoothed particle hydrodynamics. The gas has a supposed level of pre-enrichment of either 10^-4 Z_sun or 10^-3 Z_sun. 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 > 10^4 cm^-3, and a temperature, T \sim 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, Z_crit \sim 5x10^-4 Z_sun, and its possible dependence on redshift. Compared to the pure H/He case, the fragmentation of the 10^-3 Z_sun gas leads to a larger relative number of low-mass clumps.