A second-generation star in a relic dwarf galaxy

TL;DR

This paper reports the discovery of a second-generation star in an ancient dwarf galaxy with extreme metal deficiency and carbon enhancement, providing insights into early supernovae and galaxy formation.

Contribution

It presents the first confirmed second-generation star in a relic dwarf galaxy, supporting theories about low-energy supernovae origins of carbon enhancement.

Findings

Star has the lowest iron and calcium outside the Milky Way.

Shows >3000x carbon enhancement relative to other elements.

Supports hypothesis that low-energy supernovae produce carbon enhancement.

Abstract

Stars that contain only trace amounts of elements heavier than helium, referred to as having low "metallicity", preserve the chemical fingerprints of the first generation of stars and supernovae. In the Milky Way, the lowest metallicity stars show an extreme over-abundance of carbon relative to other elements, which has been hypothesized to be a unique result of the first low-energy supernovae. However, the origin of this signature has remained a mystery, since no such stars have been discovered in the ancient dwarf galaxies where they are thought to have formed. Here, we present observations of a star in the >10 billion year old ultra-faint dwarf galaxy Pictor II, that shows the lowest iron and calcium abundances outside the Milky Way (<1/43,000th solar and ~1/160,000th solar), with a factor of >3000x relative carbon enhancement. As the first unambiguous second-generation star in a relic dwarf galaxy, this object demonstrates that carbon-enhanced second-generation stars can originate in primordial small-scale systems. This star supports the hypothesis that carbon-enhancement is produced by low-energy-supernovae, since the yields of energetic supernovae are harder to retain in small-scale environments. This key local signature of chemical enrichment by the first stars traces a regime inaccessible to current high-redshift observations, which cannot detect the early enrichment of the smallest galaxies.