Zero-metallicity hypernova uncovered by an ultra metal-poor star in the Sculptor dwarf spheroidal galaxy

TL;DR

This paper reports the discovery of the most metal-poor star in an external galaxy, revealing evidence of a zero-metallicity hypernova and offering insights into Population III stars through its unique chemical signature.

Contribution

It presents the first observational evidence of a zero-metallicity hypernova through detailed chemical analysis of an ultra-metal-poor star in a dwarf galaxy.

Findings

Star has [Fe/H] = -4.11, the lowest in an external galaxy.

Star shows no carbon enhancement, unlike similar stars.

Chemical patterns suggest enrichment by a high-energy 20 M_sun hypernova.

Abstract

Although true metal-free "Population III" stars have so-far escaped discovery, their nature, and that of their supernovae, is revealed in the chemical products left behind in the next generations of stars. Here we report the detection of an ultra-metal poor star in the Sculptor dwarf spheroidal galaxy, AS0039. With [Fe/H]$_{\rm LTE}=-4.11$, it is the most metal-poor star so far discovered in any external galaxy. Contrary to the majority of Milky Way stars at this metallicity, AS0039 is clearly not enhanced in carbon, with [C/Fe]$_{\rm LTE}=-0.75$ and A(C)=+3.60, making it the lowest detected carbon abundance in any star to date. It furthermore lacks $\alpha$-element uniformity, having extremely low [Mg/Ca]$_{\rm NLTE}=-0.60$ and [Mg/Ti]$_{\rm NLTE}=-0.86$, in stark contrast with the near solar ratios observed in C-normal stars within the Milky Way halo. The unique abundance pattern indicates that AS0039 formed out of material that was predominantly enriched by a $\sim$20$ M_\odot$ progenitor star with an unusually high explosion energy $E=10\times10^{51}$ erg. The star AS0039 is thus one of the first observational evidence for zero-metallicity hypernovae and provides a unique opportunity to investigate the diverse nature of Population III stars.