Tracing Pop III supernovae with extreme energies through the Sculptor dwarf spheroidal galaxy

The Sculptor dwarf spheroidal galaxy is old and metal-poor, making it ideal to study the earliest chemical enrichment in the Local Group. We followed up the most metal-poor star known in this (or any external) galaxy, AS0039, with high-resolution ESO VLT/UVES spectra. Our new analysis confirmed its low metallicity, [Fe/H]=-3.90, and that it is extremely C-poor, with A(C)=+3.60, which corresponds to [C/Fe]=-0.33 (accounting for internal mixing). This adds to the evidence of Sculptor being intrinsically C-poor at low [Fe/H]. However, here we also report a new discovery of a carbon-enhanced metal-poor star in Sculptor, DR20080, with no enhancement of Ba (CEMP-no), indicative of enrichment by zero-metallicity low-energy supernovae. This is the first evidence of a dual population of CEMP-no and C-normal stars in Sculptor at [Fe/H]≤−3. The fraction of CEMP-no stars is still low, 9+11−8% at −4≤[Fe/H]≤−3, compared to the significantly higher fraction in the Milky Way halo, ≈40%. In addition, we re-derive chemical abundances of light, α-, iron peak, and neutron-capture elements in all Sculptor stars at [Fe/H]≤−2.8, with available high-resolution spectra. Our results show that at these low [Fe/H], Sculptor is deficient in light elements (e.g. C, Na, Al, Mg) relative to both the Milky Way halo, and ultra-faint dwarf galaxies, pointing towards significant contribution of high-energy supernovae. Furthermore, the abundance pattern of the star AS0039 is best fitted with a zero-metallicity hypernova progenitor, with a mass of M=20M ⊙. Our results in Sculptor, at [Fe/H]≤−3, therefore suggest significant enrichment by both very low-energy supernovae and hypernovae, solidifying this galaxy as one of the benchmarks for understanding the energy distribution of the first supernova in the Universe.