# The Transition from the First Stars to the Second Stars in the Early Universe

## TL;DR

This study uses advanced hydrodynamic simulations to show how metal enrichment from the first stars caused a transition from high-mass to low-mass star formation in the early universe, with implications for the initial mass function.

## Contribution

It introduces a new method for including metal radiative cooling in simulations and demonstrates the transition in star formation modes at a specific metallicity threshold.

## Key findings

- Metallicity of 10^-3 Zsolar marks the transition to low-mass star formation.
- Simulations show a shift from single to multiple clumps with increasing metallicity.
- Low-metallicity environments produce more massive stars, while higher metallicity allows for lower mass stars.

## Abstract

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 Zsolar. 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 LambdaCDM model, with metallicities, from zero to 10^-2 Zsolar, beginnning at redshift, z = 99. The simulations were run until a dense core forms at the center of a 5 x 10^5 Msolar dark matter halo, at z ~ 18. Analysis of the central 1 Msolar core reveals that the two simulations with the lowest metallicities, Z = 0 and 10^-4 Zsolar, contain one clump with 99% of the mass, while the two with metallicities, Z = 10^-3 and 10^-2 Zsolar, each contain two clumps that share most of the mass. The Z = 10^-3 Zsolar simulation also produced two low-mass proto-stellar objects with masses between 10^-2 and 10^-1 Msolar. Gas with Z >= 10^-3 Zsolar 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.