Carbon-Enhanced Hyper-metal-poor Stars and the Stellar IMF at Low Metallicity

TL;DR

This paper investigates the origins of hyper-metal-poor stars and suggests that low-mass star formation was possible at extremely low metallicities, challenging previous assumptions about the stellar initial mass function and its dependence on metallicity.

Contribution

It provides evidence supporting binary mass transfer as the origin of hyper-metal-poor stars and discusses implications for the stellar initial mass function at low metallicity.

Findings

Binary mass transfer likely explains hyper-metal-poor stars.

Low-mass stars can form at metallicities around 10^{-5.5} Zsun.

A top-heavy IMF may persist at very low metallicities.

Abstract

The two known ``hyper-metal-poor'' (HMP) stars, HE0107-5240 and HE1327-2326, have extremely high enhancements of the light elements C, N, and O relative to Fe and appear to represent a statistically significant excess population relative to the halo metallicity distribution extrapolated from [Fe/H] > -3. This study weighs the available evidence for and against three hypothetical origins for these stars: (1) that they formed from gas enriched by a primordial ``faint supernova'', (2) that they formed from gas enriched by core-collapse supernovae and C-rich gas ejected in rotation-driven winds from massive stars, and (3) that they formed as the low-mass secondaries in binary systems at Z ~ 10^{-5.5} Zsun and acquired their light-element enhancements from an intermediate-mass companion as it passed through an AGB phase. The observations interpreted here, especially the depletion of lithium seen in HE1327-2326, favor the binary mass-transfer hypothesis. If HE0107-5240 and HE1327-2326 formed in binary systems, the statistically significant absence of isolated and/or C-normal stars at similar [Fe/H] implies that low-mass stars could form at that metallicity, but that masses M ~< 1.4 Msun were disfavored in the IMF. This result is also explained if the abundance-derived top-heavy IMF for primordial stars persists to [Fe/H] ~ -5.5. This finding indicates that low-mass star formation was possible at extremely low metallicity, and that the typical stellar mass may have had a complex dependence on metallicity rather than a sharp transition driven solely by gas cooling.