We investigate the frequency and origin of carbon-enhanced metal-poor (CEMP) stars in Local Group dwarf galaxies by means of a statistical, data-calibrated cosmological model for the hierarchical build-up of the Milky Way and its dwarf satellites. The model self-consistently explains the variation with dwarf galaxy luminosity of the observed: i) frequency and [Fe/H] range of CEMP stars; ii) metallicity distribution functions; iii) star formation histories. We show that if primordial faint supernovae dominated the early metal enrichment, then CEMP-no stars enriched by the first stellar generations should be present in all dwarf galaxies, with similar number of stars and CEMP fractions at [Fe/H]< −4. We demonstrate that the probability to observe a star that is carbon-enhanced within a given [Fe/H] range strongly depends on the luminosity of the dwarf galaxy and, on average, it is an order of magnitude lower in "classical" Sculptor-like dSph galaxies (P 0.02) than in the least luminous ultrafaint dwarfs (P ≈ 0.1). In addition, we explain why it may be easier to find CEMP-no stars at [Fe/H]≈ −2 in classical dSph galaxies than in ultra-faint dwarfs. These are consequences of the dramatic variation in the fraction of stars at [Fe/H]< −3 with galaxy luminosity: 40% for galaxies with L < 10 5 L ⊙ , and 0.2% for L > 10 7 L ⊙ . We present model predictions for the low Fe-tail and CEMP fraction of stars in dwarf galaxies, with particular emphasis on the Sculptor dSph, that can be used to shed light on the properties of the first stars.

We present detailed chemical abundances for 99 red-giant branch stars in the centre of the Sculptor dwarf spheroidal galaxy, which have been obtained from high-resolution VLT/FLAMES spectroscopy. The abundances of Li, Na, α-elements (O, Mg, Si, Ca Ti), iron-peak elements (Sc, Cr, Fe, Co, Ni, Zn), and r-and s-process elements (Ba, La, Nd, Eu) were all derived using stellar atmosphere models and semi-automated analysis techniques. The iron abundances populate the whole metallicity distribution of the galaxy with the exception of the very low metallicity tail, −2.3 ≤ [Fe/H] ≤ −0.9. There is a marked decrease in [α/Fe] over our sample, from the Galactic halo plateau value at low [Fe/H] and then, after a 'knee', a decrease to sub-solar [α/Fe] at high [Fe/H]. This is consistent with products of core-collapse supernovae dominating at early times, followed by the onset of supernovae type Ia as early as ∼12 Gyr ago. The s-process products from low-mass AGB stars also participate in the chemical evolution of Sculptor on a timescale comparable to that of supernovae type Ia. However, the r-process is consistent with having no time delay relative to core-collapse supernovae, at least at the later stages of the chemical evolution in Sculptor. Using the simple and well-behaved chemical evolution of Sculptor, we further derive empirical constraints on the relative importance of massive stars and supernovae type Ia to the nucleosynthesis of individual iron-peak and α-elements. The most important contribution of supernovae type Ia is to the iron-peak elements: Fe, Cr, and Mn. There is, however, also a modest but non-negligible contribution to both the heavier α-elements: S, Ca and Ti, and some of the iron-peak elements: Sc and Co. We see only a very small or no contribution to O, Mg, Ni, and Zn from supernovae type Ia in Sculptor. The observed chemical abundances in Sculptor show no evidence of a significantly different initial mass function, compared to that of the Milky Way. With the exception of neutron-capture elements at low [Fe/H], the scatter around mean trends in Sculptor for [Fe/H] > −2.3 is extremely low, and compatible with observational errors. Combined with the small scatter in the age-elemental abundances relation, this calls for an efficient mixing of metals in the gas in the centre of Sculptor since ∼12 Gyr ago.

The origin of carbon-enhanced metal-poor (CEMP) stars and their possible connection with the chemical elements produced by the first stellar generation is still highly debated. In contrast to the Galactic halo, not many CEMP stars have been found in the dwarf spheroidal galaxies around the Milky Way. Here we present detailed abundances from ESO VLT/UVES high-resolution spectroscopy for ET0097, the first CEMP star found in the Sculptor dwarf spheroidal, which is one of the best studied dwarf galaxies in the Local Group. This star has [Fe/H] = −2.03 ± 0.10, [C/Fe] = 0.51 ± 0.10 and [N/Fe] = 1.18 ± 0.20, which is the first nitrogen measurement in this galaxy. The traditional definition of CEMP stars is [C/Fe] ≥ 0.70, but taking into account that this luminous red giant branch star has undergone mixing, it was intrinsically less nitrogen enhanced and more carbon-rich when it was formed, and so it falls under the definition of CEMP stars, as proposed by Aoki et al. (2007, ApJ, 655, 492) to account for this effect. By making corrections for this mixing, we conclude that the star had [C/Fe] ≈ 0.8 during its earlier evolutionary stages. Apart from the enhanced C and N abundances, ET0097 shows no peculiarities in other elements lighter than Zn, and no enhancement of the heavier neutron-capture elements (Ba, La, Ce, Nd, Sm, Eu, Dy), making this a CEMP-no star. However, the star does show signs of the weak r-process, with an overabundance of the lighter neutron-capture elements (Sr, Y, Zr). To explain the abundance pattern observed in ET0097, we explore the possibility that this star was enriched by primordial stars. In addition to the detailed abundances for ET0097, we present estimates and upper limits for C abundances in 85 other stars in Sculptor derived from CN molecular lines, including 11 stars with [Fe/H] ≤ −2. Combining these limits with observations from the literature, the fraction of CEMP-no stars in Sculptor seems to be significantly lower than in the Galactic halo.

The abundances of carbon, oxygen, and iron in late-type stars are important parameters in exoplanetary and stellar physics, as well as key tracers of stellar populations and Galactic chemical evolution. However, standard spectroscopic abundance analyses can be prone to severe systematic errors, based on the assumption that the stellar atmosphere is one-dimensional (1D) and hydrostatic, and by ignoring departures from local thermodynamic equilibrium (LTE). In order to address this, we carried out three-dimensional (3D) non-LTE radiative transfer calculations for C I and O I, and 3D LTE radiative transfer calculations for Fe II, across the STAGGER-grid of 3D hydrodynamic model atmospheres. The absolute 3D non-LTE versus 1D LTE abundance corrections can be as severe as − 0.3 dex for C I lines in low-metallicity F dwarfs, and − 0.6 dex for O I lines in high-metallicity F dwarfs. The 3D LTE versus 1D LTE abundance corrections for Fe II lines are less severe, typically less than + 0.15 dex. We used the corrections in a re-analysis of carbon, oxygen, and iron in 187 F and G dwarfs in the Galactic disk and halo. Applying the differential 3D non-LTE corrections to 1D LTE abundances visibly reduces the scatter in the abundance plots. The thick disk and high-α halo population rise in carbon and oxygen with decreasing metallicity, and reach a maximum of [C/Fe] ≈ 0.2 and a plateau of [O/Fe] ≈ 0.6 at [Fe/H] ≈ −1.0. The low-α halo population is qualitatively similar, albeit offset towards lower metallicities and with larger scatter. Nevertheless, these populations overlap in the [C/O] versus [O/H] plane, decreasing to a plateau of [C/O] ≈ −0.6 below [O/H] ≈ −1.0. In the thin-disk, stars having confirmed planet detections tend to have higher values of C∕O at given [O/H]; this potential signature of planet formation is only apparent after applying the abundance corrections to the 1D LTE results. Our grids of line-by-line abundance corrections are publicly available and can be readily used to improve the accuracy of spectroscopic analyses of late-type stars.

From ESO VLT/FLAMES/GIRAFFE spectra, abundance measurements of Zn have been made in ≈100 individual red giant branch (RGB) stars in the Sculptor dwarf spheroidal galaxy. This is the largest sample of individual Zn abundance measurements within a stellar system beyond the Milky Way. In the observed metallicity range, −2.7 ≤ [Fe/H] ≤ −0.9, the general trend of Zn abundances in Sculptor is similar to that of α-elements. That is, super-solar abundance ratios of [Zn/Fe] at low metallicities, which decrease with increasing [Fe/H], eventually reaching subsolar values. However, at the higher metallicities in Sculptor, [Fe/H] −1.8, we find a significant scatter, −0.8 [Zn/Fe] +0.4, which is not seen in any α-element. Our results are consistent with previous observations of a limited number of stars in Sculptor and in other dwarf galaxies. These results suggest that zinc has a complex nucleosynthetic origin, behaving neither completely like an α-nor an iron-peak element.