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 _ { \odot } .
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 Pop III stars , to the low-mass mode that dominates today .
We ran hydrodynamic simulations with cosmological initial conditions in the standard \Lambda CDM model , with metallicities , from zero to 10 ^ { -2 } Z _ { \odot } , beginnning at redshift , z = 99 .
The simulations were run until a dense core forms at the center of a 5 \times 10 ^ { 5 } M _ { \odot } dark matter halo , at z \sim 18 .
Analysis of the central 1 M _ { \odot } core reveals that the two simulations with the lowest metallicities , Z = 0 and 10 ^ { -4 } Z _ { \odot } , contain one clump with 99 % of the mass , while the two with metallicities , Z = 10 ^ { -3 } and 10 ^ { -2 } Z _ { \odot } , each contain two clumps that share most of the mass .
The Z = 10 ^ { -3 } Z _ { \odot } simulation also produced two low-mass proto-stellar objects with masses between 10 ^ { -2 } and 10 ^ { -1 } M _ { \odot } .
Gas with Z \geq 10 ^ { -3 } Z _ { \odot } is able to cool to the temperature of the 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 .