[en] We report the discovery of the carbon-rich hyper metal-poor unevolved star J0815+4729. This dwarf star was selected from SDSS/BOSS as a metal-poor candidate and follow-up spectroscopic observations at medium resolution were obtained with the Intermediate dispersion Spectrograph and Imaging System (ISIS) at William Herschel Telescope and the Optical System for Imaging and low-intermediate-Resolution Integrated Spectroscopy (OSIRIS) at Gran Telescopio de Canarias. We use the FERRE code to derive the main stellar parameters, $T_{\mathrm{e}\mathrm{f}\mathrm{f}}=6215\pm <!--\; ??\; -->82$ K, and $\mathrm{l}\mathrm{o}\mathrm{g}g=4.7\pm <!--\; ??\; -->0.5$, an upper limit to the metallicity of [Fe/H] ≤ −5.8, and a carbon abundance of [C/Fe] ≥ +5.0, while $[\mathrm{\alpha <!--\; ??\; -->}/\mathrm{F}\mathrm{e}]=0.4$ is assumed. The metallicity upper limit is based on the Ca ii K line, which at the resolving power of the OSIRIS spectrograph cannot be resolved from possible interstellar calcium. The star could be the most iron-poor unevolved star known and also be among the ones with the largest overabundances of carbon. High-resolution spectroscopy of J0815+4729 will certainly help to derive other important elemental abundances, possibly providing new fundamental constraints on the early stages of the universe, the formation of the first stars, and the properties of the first supernovae.

[en] We present an analysis of high-resolution Keck/HIRES spectroscopic observations of J0815+4729, an extremely carbon-enhanced, iron-poor dwarf star. These high-quality data allow us to derive a metallicity of [Fe/H] = −5.49 ± 0.14 from the three strongest Fe i lines and to measure a high [Ca/Fe] = 0.75 ± 0.14. The large carbon abundance of A(C) = 7.43 ± 0.17 (or [C/Fe] ∼ 4.49 ± 0.11) places this star in the upper boundary of the low-carbon band in the A(C)–[Fe/H] diagram, suggesting no contamination from a binary AGB companion. We detect the oxygen triplet at 777 nm for the first time in an ultra-metal-poor star, indicating a large oxygen-to-iron abundance ratio of [O/Fe] = 4.03 ± 0.12 (A(O) = 7.23 ± 0.14), significantly higher than the previously most metal-poor dwarf J2209–0028 with an oxygen triplet detection with [O/Fe] ∼ 2.2 dex at [Fe/H] ∼ −3.9. Nitrogen is also dramatically enhanced with (A(N) = 6.75 ± 0.08) and an abundance ratio [N/Fe] ∼ 4.41 ± 0.08. We also detect Ca, Na, and Mg, while providing upper limits for eight other elements. The abundance pattern of J0815+4729 resembles that of HE 1327–2326, indicating that both are second-generation stars contaminated by a ∼21–27 M _⊙ single, zero-metallicity, low-energy supernova with very little mixing and substantial fallback. The absence of lithium implies an upper limit abundance A(Li) < 1.3 dex, about 0.7 dex below the detected Li abundance in J0023+0307, which has a similar metallicity, exacerbating the cosmological lithium problem.

[en] The Gaia Sausage (GS) and the Sequoia represent the major accretion events that formed the stellar halo of the Milky Way. A detailed chemical study of these main building blocks provides a pristine view of the early steps of the Galaxy’s assembly. We present the results of the analysis of the UVES high-resolution spectroscopic observations at the 8.2 m VLT of nine Sausage/Sequoia members selected kinematically using Gaia DR2. We season this set of measurements with archival data from Nissen & Schuster and GALAH DR3 (2020). Here, we focus on the neutron-capture process by analyzing Sr, Y, Ba, and Eu behavior. We detect clear enhancement in Eu abundance ([Eu/Fe] ∼ 0.6–0.7) indicative of large prevalence of the r-process in the stellar n-capture makeup. We are also able to trace the evolution of the heavy element production across a wide range of metallicity. The barium to europium changes from a tight, flat sequence with [Ba/Eu] = −0.7 reflecting dominant contribution from exploding massive stars, to a clear upturn at higher iron abundances, betraying the onset of contamination from asymptotic giant branch (AGB) ejecta. Additionally, we discover two clear sequences in the [Fe/H]−[Ba/Fe] plane likely caused by distinct levels of s-process pollution and mixing within the GS progenitor.