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V. Bromm (University of Cambridge), A. Ferrara (Osservatorio Astrofisico di Arcetri), P.S. Coppi, R.B. Larson (Yale University)
One of the grand challenges in modern astrophysics is to understand how the cosmic ``dark ages'' ended. Recent investigations have suggested that the formation of the very first stars was fundamentally different from the present-day case. The question then arises: How did the transition in the star formation properties take place? The arguably most important effect is the presence of a trace amount of heavy elements.
We investigate the evolution of primordial objects using smoothed particle hydrodynamics. The gas has a supposed level of pre-enrichment of Z=10-4Z\odot and Z=10-3Z\odot. 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 and undergoes vigorous fragmentation. We discuss the physical reason for the existence of a critical metallicity, Zcrit~5\times 10-4Z\odot, below which the presence of metals has no effect on the fragmentation.
We find evidence suggesting a smooth progression towards the formation of more massive clumps with decreasing metallicity.