The origin of the most iron-poor star

TL;DR

This paper explores the origins of the most iron-poor star, demonstrating that faint, metal-free supernovae can explain its elemental abundances and that dust cooling from supernova ejecta likely triggered low-mass star formation in the early universe.

Contribution

It introduces calibrated faint supernova models to study dust formation in iron-poor ejecta and links dust cooling to the formation of CEMP stars, a novel approach.

Findings

Amorphous carbon is the primary dust grain formed in such supernovae.

Dust yields from these supernovae range between 0.025 and 2.25 solar masses.

Dust cooling can trigger low-mass star formation in the early universe.

Abstract

We investigate the origin of carbon-enhanced metal-poor (CEMP) stars starting from the recently discovered $\rm [Fe/H]<-7.1$ star SMSS J031300 (Keller et al. 2014). We show that the elemental abundances observed on the surface of SMSS J031300 can be well fit by the yields of faint, metal free, supernovae. Using properly calibrated faint supernova explosion models, we study, for the first time, the formation of dust grains in such carbon-rich, iron-poor supernova ejecta. Calculations are performed assuming both unmixed and uniformly mixed ejecta and taking into account the partial destruction by the supernova reverse shock. We find that, due to the paucity of refractory elements beside carbon, amorphous carbon is the only grain species to form, with carbon condensation efficiencies that range between (0.15-0.84), resulting in dust yields in the range (0.025-2.25)M$_{\odot}$. We follow the collapse and fragmentation of a star forming cloud enriched by the products of these faint supernova explosions and we explore the role played by fine structure line cooling and dust cooling. We show that even if grain growth during the collapse has a minor effect of the dust-to-gas ratio, due to C depletion into CO molecules at an early stage of the collapse, the formation of CEMP low-mass stars, such as SMSS J031300, could be triggered by dust cooling and fragmentation. A comparison between model predictions and observations of a sample of C-normal and C-rich metal-poor stars supports the idea that a single common pathway may be responsible for the formation of the first low-mass stars.