An extremely primitive halo star

TL;DR

This paper reports the discovery of a very low metallicity star with a typical ancient star chemical pattern, demonstrating that low mass stars can form at extremely low metallicities, challenging previous theories.

Contribution

It provides observational evidence that low mass stars can form at metallicities lower than previously thought, with detailed chemical composition analysis of such a star.

Findings

Star has metallicity Z ≤ 6.9E-7, lower than previous limits.

Star's chemical pattern is typical of extremely metal poor stars.

Lithium is not detected, indicating possible depletion at low metallicity.

Abstract

The early Universe had a chemical composition consisting of hydrogen, helium and traces of lithium1, almost all other elements were created in stars and supernovae. The mass fraction, Z, of elements more massive than helium, is called "metallicity". A number of very metal poor stars have been found some of which, while having a low iron abundance, are rich in carbon, nitrogen and oxygen. For theoretical reasons and because of an observed absence of stars with metallicities lower than Z=1.5E-5, it has been suggested that low mass stars (M<0.8M\odot, the ones that survive to the present day) cannot form until the interstellar medium has been enriched above a critical value, estimated to lie in the range 1.5E-8\leqZ\leq1.5E-6, although competing theories claiming the contrary do exist. Here we report the chemical composition of a star with a very low Z\leq6.9E-7 (4.5E-5 of that of the Sun) and a chemical pattern typical of classical extremely metal poor stars, meaning without the enrichment of carbon, nitrogen and oxygen. This shows that low mass stars can be formed at very low metallicity. Lithium is not detected, suggesting a low metallicity extension of the previously observed trend in lithium depletion. Lithium depletion implies that the stellar material must have experienced temperatures above two million K in its history, which points to rather particular formation condition or internal mixing process, for low Z stars.