Three modes of metal-enriched star formation in the early universe

We study how the collapse of gas clouds is altered by the addition of metals by performing a series of simulations of pre-enriched star formation at various metallicities using the adaptive mesh refinement code Enzo. In order to directly compare with the case of metal-free star formation, we use initial conditions identical to the formation of a first star, including only radiation from the high redshift cosmic microwave background (CMB). We find that there were three distinct modes of star formation at high redshift (z > 4): a 'primordial' mode, producing massive stars (10s to 100s M_⊙) at very low metallicities (log(Z/Z_⊙) < -3.75); a 'CMB-regulated' mode, producing moderate mass (10s of M_⊙) stars at high metallicites (log(Z/Z_⊙) > -2.5) at redshift z ∼ 15-20); and a 'metallicity-regulated' low-mass (a few M_⊙) mode existing between those two metallicities. As the universe ages and the CMB temperature decreases, the range of the low mass mode extends to higher metallicities, eventually becoming the sole mode of star formation. Recent studies have suggested that if dust grains are produced in Pop III supernovae, they can provide an additional coolant during late stages of collapse that could induce additional fragmentation. If this is true, the conventional value of Z _cr (∼10^-3.5Z_⊙) could be as much as two orders of magnitude lower. However, the exact amount of dust created in Pop III supernovae is very uncertain. Even if created in significant amounts, dust can be quickly destroyed via sputtering as dust-rich clumps of outflowing ejecta are impacted by reverse shocks. We report on preliminary results from simulations of dust-destruction in supernova reverse shocks.