The transition from the first stars to the second stars in the early universe

We observe a sharp transition from a singular, high-mass mode of star formation to a low-mass-dominated mode, in numerical simulations, at a metallicity of 10^-3 Z_⊙. We incorporate a new method for including the radiative cooling from metals into adaptive mesh refinement hydrodynamic simulations. Our results illustrate how metals, produced by the first stars, led to a transition from the high-mass star formation mode of Population III stars to the low-mass mode that dominates today. We ran hydrodynamic simulations with cosmological initial conditions in the standard ACDM model, with metallicities, from zero to 10^-2 Z_⊙, beginning at redshift z = 99. The simulations were run until a dense core forms at the center of a 5 × 10⁵ M_⊙ dark matter halo, at z ∼ 18. Analysis of the central 1 M_⊙ core reveals that the two simulations with the lowest metallicities, Z = 0 and 10 ^-4 Z_⊙, contain one clump with 99% of the mass, while the two with metallicities Z = 10^-3 and 10^-2 Z _⊙ each contain two clumps that share most of the mass. The Z = 10^-3 Z_⊙ simulation also produced two low-mass protostellar objects with masses between 10^-2 and 10^-1 M_⊙. Gas with Z ≥ 10^-3 Z_⊙ is able to cool to the temperature of the cosmic microwave background (CMB), which sets a lower limit to the minimum fragmentation mass. This suggests that the second-generation stars produced a spectrum of lower mass stars but were still more massive, on average, than stars formed in the local universe.