The first Population II stars formed in externally enriched mini-haloes

We present a simulation of the formation of the earliest Population II stars, starting from cosmological initial conditions and ending when metals created in the first supernovae are incorporated into a collapsing gas cloud. This occurs after a supernova blast-wave collides with a nearby mini-halo, inducing further turbulence that efficiently mixes metals into the dense gas in the centre of the halo. The gas that first collapses has been enriched to a metallicity of Z ∼ 2 × 10⁻⁵ Z_⊙. Due to the extremely low metallicity, collapse proceeds similarly to metal-free gas until dust cooling becomes efficient at high densities, causing the cloud to fragment into a large number of low-mass objects. This external enrichment mechanism provides a plausible origin for the most metal-poor stars observed, such as SMSS J031300.36-670839.3, that appear to have formed out of gas enriched by a single supernova. This mechanism operates on shorter time-scales than the time for low-mass mini-haloes (M ≤ 5 × 10⁵ M_⊙) to recover their gas after experiencing a supernova. As such, metal-enriched stars will likely form first via this channel if the conditions are right for it to occur. We identify a number of other externally enriched haloes that may form stars in this manner. These haloes have metallicities as high as 0.01 Z_⊙, suggesting that some members of the first generation of metal-enriched stars may be hiding in plain sight in current stellar surveys.