2024A&A...681A..44S

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 on 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]_LTE = -3.90 ± 0.15, and that it is extremely C-poor, with A(C) = + 3.60, which corresponds to [C/Fe]_LTE = -0.33 ± 0.17 (accounting for internal mixing). This adds to the evidence of Sculptor being intrinsically C-poor at low [Fe/H] ≲ -3. However, here we also report a new discovery of a carbon-enhanced metal-poor (CEMP-no) star in Sculptor, DR20080, with no enhancement of Ba, indicative of enrichment by zero-metallicity low-energy supernovae, E_SN < 1 × 10⁵¹. This is the first piece of 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, $ f^{\mathrm{Scl}}_{\mathrm{CEMP}}=9^{+11}_{-8}\% $ at -4 ≤ [Fe/H] ≤ -3, compared to the significantly higher fraction in the Milky Way halo, $ f^{\mathrm{MW}}_{\mathrm{CEMP}}\approx40\% $. To further investigate the early chemical enrichment of Sculptor, we re-derived 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 a significant contribution from high-energy supernovae. Furthermore, the abundance pattern of the star AS0039 is best fitted with a zero-metallicity hypernova progenitor, E_SN = 10 × 10⁵¹, with a mass of M = 20 M_⊙. 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.