Carbon-enhanced Metal-poor Stars Research Articles

Four-hundred Very Metal-poor Stars Studied with LAMOST and Subaru. III. Dynamically Tagged Groups and Chemodynamical Properties

Very metal-poor (VMP) stars record the signatures of early accreted galaxies, making them essential tools for unraveling the early stages of Galaxy formation. Understanding the origin of VMP stars requires comprehensive studies of their chemical compositions and kinematics, which are currently lacking. Hence, we conduct a chemodynamical analysis of 352 VMP stars selected from one of the largest uniform high-resolution VMP star samples, jointly obtained from LAMOST and Subaru. We apply a friends-of-friends clustering algorithm to the master catalog of this high-resolution sample, which consists of 5778 VMP stars. It results in 131 dynamically tagged groups with 89 associated with known substructures in the Milky Way, including Gaia-Sausage-Enceladus (GSE), Thamnos, Helmi streams, Sequoia, Wukong, Pontus, and the very metal-poor disk (VMPD). Our findings are: (i) the VMPD shows lower Zn abundances than the rest, which indicates that it could be a relic of small stellar systems; (ii) Sequoia shows moderately high r-process abundances; (iii) Helmi streams show deficiencies in carbon and light neutron-capture elements; (iv) the fraction of carbon-enhanced metal-poor stars with no enhancement in heavy elements (CEMP-no stars) seems low in the VMPD and the Helmi streams; and (v) a subgroup in GSE exhibits a very high fraction of r-process enhanced stars, with four out of five showing [Eu/Fe]> +1.0. The abundance patterns of other elements in VMP substructures largely match the whole VMP sample. We also study large-scale correlations between abundance ratios and kinematics without classifying stars into substructures, but it does not yield significant correlations once the overall chemical evolution is considered for most elements.

New evidence of binarity in young α-rich turn-off and subgiant stars: fast rotation and strong magnetic activity

ABSTRACT Young α-rich (YAR) stars within the old Galactic thick disc exhibit a dual characteristic of relative youth determined with asteroseismology and abundance enhancement in α elements measured from high-resolution spectroscopy. The youth origin of YAR stars has been proposed to be binary evolution via mass transfer or stellar mergers. If that is the case, YAR stars should spin rapidly and thus be magnetically active, because they are mass and angular momentum gainers. In this study, to seek this binary footprint, we select YAR stars on the main-sequence turn-off or the subgiant branch (MSTO-SGB) from APOGEE DR17, whose ages and projected rotation velocities (vsin i) can be precisely measured. With APOGEE vsin i and LAMOST spectra, we find that YAR stars are indeed fast rotators and magnetically active. In addition, we observe low [C/N] ratios and high Gaia RUWE in some YAR stars, suggesting that these MSTO-SGB stars probably have experienced mass transfer from red-giant companions. Our findings underscore that magnetic activity can serve as a valuable tool for probing the binary evolution for other chemically peculiar stars, such as red giants with lithium anomalies and carbon-enhanced metal-poor stars.

The Pristine Inner Galaxy Survey – VIII. Characterizing the orbital properties of the ancient, very metal-poor inner Milky Way

ABSTRACT The oldest stars in the Milky Way (born in the first few billion years) are expected to have a high density in the inner few kpc, spatially overlapping with the Galactic bulge. We use spectroscopic data from the Pristine Inner Galaxy Survey (PIGS) to study the dynamical properties of ancient, metal-poor inner Galaxy stars. We compute distances using starhorse, and orbital properties in a barred Galactic potential. With this paper, we release the spectroscopic AAT/PIGS catalogue (13 235 stars). We find that most PIGS stars have orbits typical for a pressure-supported population. The fraction of stars confined to the inner Galaxy decreases with decreasing metallicity, but many very metal-poor stars (VMP; [Fe/H] <−2.0) stay confined ($\sim 60~{{\ \rm per \, cent}}$ stay within 5 kpc). The azimuthal velocity vϕ also decreases between [Fe/H] = −1.0 and −2.0, but is constant for VMP stars (at ∼+40 km s−1). The carbon-enhanced metal-poor (CEMP) stars in PIGS appear to have similar orbital properties compared to normal VMP stars. Our results suggest a possible transition between two spheroidal components – a more metal-rich, more concentrated, faster rotating component, and a more metal-poor, more extended and slower/non-rotating component. We propose that the former may be connected to pre-disc in-situ stars (or those born in large building blocks), whereas the latter may be dominated by contributions from smaller galaxies. This is an exciting era where large metal-poor samples, such as in this work (as well as upcoming surveys, e.g. 4MOST), shed light on the earliest evolution of our Galaxy.

The impact of overshoot on the i-process in AGB stars

The production of neutron-rich elements at neutron densities intermediate to those of the s - and r -processes, the so-called has been identified as possibly being responsible for the observed abundance pattern found in certain carbon-enhanced metal-poor (CEMP) stars. The production site may be low-metallicity stars on the asymptotic giant branch (AGB) where the physical processes during the thermal pulses are not well known. We investigate the impact of overshoot from various convective boundaries during the AGB phase on proton ingestion events (PIEs) and the neutron densities as a necessary precondition for the as well as on the structure and continued evolution of the models. We therefore analyzed models of a $ msun $, Z= 5e-5 star. A fiducial model without overshoot on the AGB (overshoot was applied during the pre-AGB evolution) serves as a reference. The same model was then run with various overshoot values and the resulting models were compared to one another. Light element nucleosynthesis is also discussed. Additionally, we introduce a new timescale argument to predict PIE occurrence to discriminate between a physical and a numerical reason for a nonoccurrence. A comparison to observations as well as previous studies was conducted before finally presenting the most promising choice of overshoot parameters for the occurrence of the in low-mass, low-metallicity models. The fiducial model reveals high neutron densities and a persistent split of the pulse-driven convection zone (PDCZ). Overshoot from the PDCZ results in either temporary or permanent remerging of the split PDCZ, influencing the star's structure and evolution. While both overshoot and non-overshoot models exhibit PIEs generating neutron densities suitable for the they lead to varied and $ N O $ ratios and notable Li enhancements. Comparison with previous studies and observations of CEMP-r/s stars suggests that while surface enhancements in our models may be exaggerated, abundance ratios align well. Though, for high values of overshoot from the PDCZ the agreement becomes worse.

Extremely metal-poor stars in the Fornax and Carina dwarf spheroidal galaxies

We present our analysis of VLT/UVES and X-shooter observations of six very metal-poor stars, including four stars at Fe/H approx $-3$ in the Fornax and Carina dwarf spheroidal (dSph) galaxies. To date, this metallicity range in these two galaxies has not yet been investigated fully, or at all in some cases. The chemical abundances of 25 elements are presented, based on 1D and local thermodynamic equilibrium (LTE) model atmospheres. We discuss the different elemental groups, and find that alpha - and iron-peak elements in these two systems are generally in good agreement with the Milky Way halo at the same metallicity. Our analysis reveals that none of the six stars we studied exhibits carbon enhancement, which is noteworthy given the prevalence of carbon-enhanced metal-poor stars without s-process enhancement (CEMP-no) in the Galaxy at similarly low metallicities. Our compilation of literature data shows that the fraction of CEMP-no stars in dSph galaxies is significantly lower than in the Milky Way, and than in ultra-faint dwarf galaxies. Furthermore, we report the discovery of the lowest metallicity Fe/H = $-2.92$, r-process rich (r-I) star in a dSph galaxy. This star, fnx\_06\_019, has $ Eu/Fe =+0.8$, and also shows enhancement of La, Nd, and Dy, $ X/Fe >+0.5$. Our new data in Carina and Fornax help populate the extremely low metallicity range in dSph galaxies, and add to the evidence of a low fraction of CEMP-no stars in these systems.

Extending the chemical reach of the H3 survey: detailed abundances of the dwarf-galaxy stellar stream Wukong/LMS-1

ABSTRACT We present the first detailed chemical-abundance analysis of stars from the dwarf-galaxy stellar stream Wukong/LMS-1 covering a wide metallicity range ($-3.5 \lt \rm [Fe/H] \lesssim -1.3$). We find abundance patterns that are effectively indistinguishable from the bulk of Indus and Jhelum, a pair of smaller stellar streams proposed to be dynamically associated with Wukong/LMS-1. We confirmed a carbon-enhanced metal-poor star ($\rm [C/Fe] \gt +0.7$ and $\rm [Fe/H] \sim -2.9$) in Wukong/LMS-1 with strong enhancements in Sr, Y, and Zr, which is peculiar given its solar-level [Ba/Fe]. Wukong/LMS-1 stars have high abundances of α elements up to $\rm [Fe/H] \gtrsim -2$, which is expected for relatively massive dwarfs. Towards the high-metallicity end, Wukong/LMS-1 becomes α-poor, revealing that it probably experienced fairly standard chemical evolution. We identified a pair of N- and Na-rich stars in Wukong/LMS-1, reminiscent of multiple stellar populations in globular clusters. This indicates that this dwarf galaxy contained at least one globular cluster that was completely disrupted in addition to two intact ones previously known to be associated with Wukong/LMS-1, which is possibly connected to similar evidence found in Indus. From these ≥3 globular clusters, we estimate the total mass of Wukong/LMS-1 to be ${\approx }10^{10} \, \mathrm{M}_\odot$, representing ∼1 per cent of the present-day Milky Way. Finally, the [Eu/Mg] ratio in Wukong/LMS-1 continuously increases with metallicity, making this the first example of a dwarf galaxy where the production of r-process elements is clearly dominated by delayed sources, presumably neutron-star mergers.

The intermediate neutron capture process

Context. The observed surface abundance distributions of carbon-enhanced metal-poor (CEMP) r/s stars suggest that these stars could have been polluted by an intermediate neutron capture process (the so-called i-process) occurring at intermediate neutron densities between the r- and s-processes. Triggered by the ingestion of protons inside a convective He-burning zone, the i-process could be hosted in several sites, a promising one being the early AGB phase of low-mass, low-metallicity stars. The i-process remains affected however by many uncertainties, including those of nuclear origin, since it involves hundreds of nuclei for which reaction rates have not yet been determined experimentally. Aims. We investigate both the systematic and statistical uncertainties associated with theoretical nuclear reaction rates of relevance during the i-process and explore their impact on the i-process elemental production, and subsequently on the surface enrichment, of a low-mass, low-metallicity star during the early AGB phase. Methods. We used the TALYS reaction code to estimate both the model and parameter uncertainties affecting the photon strength function and the nuclear level densities, and hence the radiative neutron capture rates. The impact of correlated systematic uncertainties was estimated by considering different nuclear models, as was detailed in Paper II. In contrast, the uncorrelated uncertainties associated with local variation in model parameters were estimated using a variant of the backward-forward Monte Carlo method to constrain the parameter changes to experimentally known cross sections before propagating them consistently to the neutron capture rates. The STAREVOL code (Siess 2006, A&A, 448, 717) was used to determine the impact of nuclear uncertainties on the i-process nucleosynthesis in a 1 M⊙ [Fe/H] = –2.5 model star during the proton ingestion event in the early AGB phase. A large nuclear network of 1160 species coherently coupled to the transport processes was solved to follow the i-process nucleosynthesis. Results. We find that the uncorrelated parameter uncertainties lead the surface abundance uncertainties of elements with Z ≥ 40 to range between 0.5 and 1.0 dex, with odd-Z elements displaying higher uncertainties. The correlated model uncertainties are of the same order of magnitude, and both model and parameter uncertainties have an important impact on potential observable tracers such as Eu and La. We find around 125 important (n, γ) reactions impacting the surface abundances, including 28 reactions that have a medium to high impact on the surface abundance of elements that are taken as observable tracers of i-process nucleosynthesis in CEMP stars. Conclusions. Both the correlated model and uncorrelated parameter uncertainties need to be estimated coherently before being propagated to astrophysical observables through multi-zone stellar evolution models. Many reactions are found to affect the i-process predictions and will require improved nuclear models guided by experimental constraints. Priority should be given to the reactions influencing the observable tracers.

Study of the Astrophysical Origins of Neutron-captue Elements in CEMP Star HE 1005-1439

HE 1005-1439 is a carbon-enhanced metal-poor (CEMP) star with [Fe/H]∼−3.0, which is largely enhanced in elements produced by slow neutron-capture process (s-process; e.g., [Ba/Fe]=1.16±0.31, [Pb/Fe]=1.98±0.19), and mildly enhanced in elements produced by rapid neutron-capture process (r-process; e.g., [Eu/Fe]=0.46±0.22). The neutron-capture element patterns of the star can not be fitted by the s-process model or intermediate process (i-process) model. Studying the astrophysical origins of chemical elements in the star based on the method of abundance decomposition can help understanding the formation and evolution of the CEMP stars. Using the combination of s-process and r-process abundances, we fully fit the abundance patterns of the neutron-capture elements of this star. We find that the neutron-capture elements are mainly produced by the s-process of the AGB companion with low mass and low metallicity, and the r-process nucleosynthesis also contributes a little to this star.

Tracing Population 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 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, ESN < 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, fCEMPScl = 9−8+11% at −4 ≤ [Fe/H] ≤ −3, compared to the significantly higher fraction in the Milky Way halo, fCEMPMW ≈ 40%. 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, ESN = 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.

Chemodynamical study of two CEMP-no stars from the Hamburg/ESO Survey

ABSTRACT The Carbon-Enhanced Metal-Poor (CEMP) stars with no enhancement of neutron-capture elements, the so-called CEMP-no stars are believed to be the direct descendants of first-generation stars and provide a unique opportunity to probe the early Galactic nucleosynthesis. We present a detailed chemical and kinematic analysis for two extremely metal-poor stars HE 1243 − 2408 and HE 0038 − 0345 using high-resolution (R∼86 000) HERMES spectra. For the object HE 1243 − 2408, we could make a detailed comparison with the available literature values; however, only limited information is available for the other object HE 0038 − 0345. Our estimated metallicity for these two objects are −3.05 and −2.92, respectively. With estimated [C/Fe] (1.03 and 1.05) and [Ba/Fe] (−0.18 and −0.11), respectively, the objects are found to be bonafide CEMP-no stars. From the observed abundances of C, Na, Mg, and Ba (i.e. A(C), A(Na), A(Mg), A(Ba)), the objects are found to belong to Group II CEMP-no stars. A detailed abundance profile analysis indicates that the objects are accreted from dSph satellite galaxies that support hierarchical galaxy assembly. Further, our analysis shows that the progenitors of the stars are likely Pop II Core-Collapse Supernovae. The object HE 0038 − 0345 is found to be a high-energy, prograde, outer-halo object, and HE 1243 − 2408 is found to be a high-energy, retrograde, inner-halo object. Our detailed chemodynamical analysis shows that HE 1243 − 2408 is related to I’itoi structure, where as HE 0038 − 0345 is likely related to Sgr or GSE events. The mass of the progenitor galaxies of the programme stars inferred from their dynamics is at par with their likely origin in massive dSph galaxies.

A comparative study of three modes of s-process nucleosynthesis in extremely metal-poor AGB stars

Abstract Carbon-enhanced metal-poor (CEMP) stars in the Galactic halo have a wide range of neutron-capture element abundance patterns. To identify their origin, we investigate three modes of s-process nucleosynthesis that have been proposed to operate in extremely metal-poor asymptotic giant branch (AGB) stars: convective 13C burning, which occurs when hydrogen is engulfed by helium-flash convection in low-mass AGB stars; convective 22Ne burning, which occurs in helium-flash convection of intermediate-mass AGB stars; and radiative 13C burning, which occurs in the 13C pocket that is formed during inter-pulse periods. We show that the production of s-process elements per iron seed (s-process efficiency) does not depend on metallicity below [Fe$/$H] = −2, because 16O in the helium zone dominates the neutron poison. The convective 13C mode can produce a variety of s-process efficiencies for Sr, Ba, and Pb, including the maxima observed among CEMP stars. The 22Ne mode only produces the lowest end of s-process efficiencies among CEMP models. We show that the combination of these two modes can explain the full range of observed enrichment of s-process elements in CEMP stars. In contrast, the 13C pocket mode can hardly explain the high level of enrichment observed in some CEMP stars, even if considering star-to-star variations of the mass of the 13C pocket. These results provide a basis for discussing the binary mass transfer origin of CEMP stars and their subgroups.

Rapidly rotating massive Population III stars: a solution for high carbon enrichment in CEMP-no stars

ABSTRACT Very metal-poor stars that have [Fe/H] < −2 and that are enhanced in C relative to Fe ([C/Fe] > +0.7) but have no enhancement of heavy elements ([Ba/Fe] < 0) are known as carbon-enhanced metal-poor (CEMP-no) stars. These stars are thought to be produced from a gas that was polluted by the supernova (SN) ejecta of the very first generation (Population III) massive stars. The very high enrichment of C (A(C) ≳ 6) observed in many of the CEMP-no stars is difficult to explain by current models of SN explosions from massive Population III stars when a reasonable dilution of the SN ejecta, which is consistent with detailed simulation of metal mixing in minihaloes, is adopted. We explore rapidly rotating Population III stars that undergo efficient mixing and reach a quasi-chemically homogeneous (QCH) state. We find that QCH stars can eject large amounts of C in the wind and that the resulting dilution of the wind ejecta in the interstellar medium can lead to a C enrichment of A(C) ≲ 7.75. The core of QCH stars can produce up to an order of magnitude of more C than non-rotating progenitors of similar mass and the resulting SN can lead to a C enrichment of A(C) ≲ 7. Our rapidly rotating massive Population III stars cover almost the entire range of A(C) observed in CEMP-no stars and are a promising site for explaining the high C enhancement in the early Galaxy. Our work indicates that a substantial fraction of Population III stars were likely rapid rotators.

3D Stagger model atmospheres with FreeEOS

Three-dimensional radiation-hydrodynamics (3D RHD) simulations of stellar surface convection provide valuable insights into many problems in solar and stellar physics. However, almost all 3D near-surface convection simulations to date are based on solar-scaled chemical compositions, which limits their relevance when applied to stars with peculiar abundance patterns. To overcome this difficulty, we implement the robust and widely used FreeEOS equation of state and our Blue opacity package into the Stagger 3D radiation-magnetohydrodynamics code. We present a new 3D RHD model of the solar atmosphere, and demonstrate that the mean stratification as well as the distributions of key physical quantities are in good agreement with those of the latest Stagger solar model atmosphere. The new model is further validated by comparisons with solar observations. The new model atmospheres reproduce the observed flux spectrum, continuum centre-to-limb variation, and hydrogen line profiles at a satisfactory level, thereby confirming the realism of the model and the underlying input physics. These implementations open the prospect for studying other stars with different α-element abundance, carbon-enhanced metal-poor stars, and population II stars with peculiar chemical compositions using 3D Stagger model atmospheres.

Abundance Analysis of Stars at Large Radius in the Sextans Dwarf Spheroidal Galaxy* *This paper includes data gathered at the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile. Other observations reported here were obtained at the MMT Observatory, a joint facility of the Smithsonian Institution and the University of Arizona. This paper is also based on archival observations

We present the stellar parameters and chemical abundances of 30 elements for five stars located at large radii (3.5–10.7 times the half-light radius) in the Sextans dwarf spheroidal galaxy. We selected these stars using proper motions, radial velocities, and metallicities, and we confirm them as metal-poor members of Sextans with −3.34 ≤ [Fe/H] ≤ −2.64 using high-resolution optical spectra collected with the Magellan Inamori Kyocera Echelle spectrograph. Four of the five stars exhibit normal abundances of C (−0.34 ≤ [C/Fe] ≤ + 0.36), mild enhancement of the α elements Mg, Si, Ca, and Ti ([α/Fe] = +0.12 ± 0.03), and unremarkable abundances of Na, Al, K, Sc, V, Cr, Mn, Co, Ni, and Zn. We identify three chemical signatures previously unknown among stars in Sextans. One star exhibits large overabundances ([X/Fe] > +1.2) of C, N, O, Na, Mg, Si, and K, and large deficiencies of heavy elements ([Sr/Fe] = −2.37 ± 0.25, [Ba/Fe] = −1.45 ± 0.20, [Eu/Fe] < + 0.05), establishing it as a member of the class of carbon-enhanced metal-poor stars with no enhancement of neutron-capture elements. Three stars exhibit moderate enhancements of Eu (+0.17 ≤ [Eu/Fe] ≤ + 0.70), and the abundance ratios among 12 neutron-capture elements are indicative of r-process nucleosynthesis. Another star is highly enhanced in Sr relative to heavier elements ([Sr/Ba] = +1.21 ± 0.25). These chemical signatures can all be attributed to massive, low-metallicity stars or their end states. Our results, the first for stars at large radius in Sextans, demonstrate that these stars were formed in chemically inhomogeneous regions, such as those found in ultra-faint dwarf galaxies.

Carbon-enhanced metal-poor star candidates from BP/RP spectra in Gaia DR3

ABSTRACT Carbon-enhanced metal-poor (CEMP) stars comprise almost a third of stars with [Fe/H] &amp;lt; −2, although their origins are still poorly understood. It is highly likely that one sub-class (CEMP-s stars) is tied to mass-transfer events in binary stars, while another sub-class (CEMP-no stars) are enriched by the nucleosynthetic yields of the first generations of stars. Previous studies of CEMP stars have primarily concentrated on the Galactic halo, but more recently they have also been detected in the thick disc and bulge components of the Milky Way. Gaia DR3 has provided an unprecedented sample of over 200 million low-resolution (R ≈ 50) spectra from the BP and RP photometers. Training on the CEMP catalogue from the SDSS/SEGUE database, we use XGBoost to identify the largest all-sky sample of CEMP candidate stars to date. In total, we find 58 872 CEMP star candidates, with an estimated contamination rate of 12 per cent. When comparing to literature high-resolution catalogues, we positively identify 60–68 per cent of the CEMP stars in the data, validating our results and indicating a high completeness rate. Our final catalogue of CEMP candidates spans from the inner to outer Milky Way, with distances as close as r ∼ 0.8 kpc from the Galactic centre, and as far as r &amp;gt; 30 kpc. Future higher resolution spectroscopic follow-up of these candidates will provide validations of their classification and enable investigations of the frequency of CEMP-s and CEMP-no stars throughout the Galaxy, to further constrain the nature of their progenitors.

Evidence for multiple nucleosynthetic processes from carbon-enhanced metal-poor stars in the Carina dwarf spheroidal galaxy

Context. Carbon-enhanced metal-poor (CEMP) stars ([C/Fe] &gt; 0.7) are known to exist in large numbers at low metallicity in the Milky Way halo and are important tracers of early Galactic chemical evolution. However, very few stars of this kind have been identified in the classical dwarf spheroidal (dSph) galaxies, and detailed abundances, including neutron-capture element abundances, have only been reported for 13 stars. Aims. We aim to derive detailed abundances of six CEMP stars identified in the Carina dSph and compare the abundances to CEMP stars in other dSph galaxies and the Milky Way halo. This is the largest sample of CEMP stars in a dSph galaxy analysed to date. Methods. One-dimensional local thermodynamic equilibrium (LTE) elemental abundances are derived via equivalent width and spectral synthesis using high-resolution spectra of the six stars obtained with the MIKE spectrograph at Las Campanas Observatory. Results. We derived abundances or upper limits for up to 27 elements from C to Os in the six stars. Our analysis reveals one of the stars to be a CEMP-no star with very low neutron-capture element abundances. In contrast, the other five stars all show enhancements in neutron-capture elements in addition to their carbon enhancement, classifying them as CEMP-s and -r/s stars. The six stars have similar α and iron-peak element abundances to other stars in Carina, except for the CEMP-no star, which shows enhancement in Na, Mg, and Si. We explored the absolute carbon abundances (A(C)) of CEMP stars in dSph galaxies and find similar behaviour to that seen for Milky Way halo CEMP stars, but highlight that CEMP-r/s stars primarily have very high A(C) values. We also compared the neutron-capture element abundances of the CEMP-r/s stars in our sample to recent i-process yields, which provide a good match to the derived abundances.

The Pristine Inner Galaxy Survey (PIGS) VII: a discovery of the first inner Galaxy CEMP-r/s star

ABSTRACT Well-studied very metal-poor (VMP, [Fe/H] &amp;lt;−2) stars in the inner Galaxy are few in number, and they are of special interest because they are expected to be among the oldest stars in the Milky Way. We present high-resolution spectroscopic follow-up of the carbon-enhanced metal-poor (CEMP) star Pristine_184237.56-260624.5 (hereafter Pr184237) identified in the Pristine Inner Galaxy Survey. This star has an apocentre of ∼2.6 kpc. Its atmospheric parameters (Teff = 5100 K, log g = 2.0, and [Fe/H] = −2.60) were derived based on the non-local thermodynamic equilibrium (NLTE) line formation. We determined abundances for 32 elements, including 15 heavy elements beyond the iron group. The NLTE abundances were calculated for 13 elements from Na to Pb. Pr184237 is strongly enhanced in C, N, and O, and both s- and r-process elements from Ba to Pb; it reveals a low carbon isotope ratio of 12C/13C = 7. The element abundance pattern in the Na–Zn range is typical of halo stars. With [Ba/Eu] = 0.32, Pr184237 is the first star of the CEMP-r/s subclass identified in the inner Galaxy. Variations in radial velocity suggest binarity. We tested whether a pollution by the s- or i-process material produced in the more massive and evolved companion can form the observed abundance pattern and find that an i-process in the asymptotic giant branch star with a progenitor mass of 1.0–2.0 $\, {\rm M}_{\odot }$ can be the solution.

Search for Extremely Metal-poor Stars with Gemini-N/Graces. I. Chemical-abundance Analysis

We present stellar parameters and abundances of 13 elements for 18 very metal-poor (VMP; [Fe/H] < –2.0) stars, selected as extremely metal-poor (EMP; [Fe/H] < –3.0) candidates from the Sloan Digital Sky Survey and Large sky Area Multi-Object Fiber Spectroscopic Telescope survey. High-resolution spectroscopic observations were performed using GEMINI-N/GRACES. We find 10 EMP stars among our candidates, and we newly identify three carbon-enhanced metal-poor stars with [Ba/Fe] < 0. Although chemical abundances of our VMP/EMP stars generally follow the overall trend of other Galactic halo stars, there are a few exceptions. One Na-rich star ([Na/Fe] = +1.14) with low [Mg/Fe] suggests a possible chemical connection with second-generation stars in a globular cluster. The progenitor of an extremely Na-poor star ([Na/Fe] = –1.02) with high K- and Ni-abundance ratios may have undergone a distinct nucleosynthesis episode, associated with core-collapse supernovae (SNe) having a high explosion energy. We have also found a Mg-rich star ([Mg/Fe] = +0.73) with slightly enhanced Na and extremely low [Ba/Fe], indicating that its origin is not associated with neutron-capture events. On the other hand, the origin of the lowest Mg abundance ([Mg/Fe] = –0.61) star could be explained by accretion from a dwarf galaxy, or formation in a gas cloud largely polluted by SNe Ia. We have also explored the progenitor masses of our EMP stars by comparing their chemical-abundance patterns with those predicted by Population III SNe models, and find a mass range of 10–26 M ⊙, suggesting that such stars were primarily responsible for the chemical enrichment of the early Milky Way.

Evidence of First Stars-enriched Gas in High-redshift Absorbers*

The first stars were born from chemically pristine gas. They were likely massive, and thus they rapidly exploded as supernovae, enriching the surrounding gas with the first heavy elements. In the Local Group, the chemical signatures of the first stellar population were identified among low-mass, long-lived, very metal-poor ([Fe/H] < −2) stars, characterized by high abundances of carbon over iron ([C/Fe] > +0.7): the so-called carbon-enhanced metal-poor stars. Conversely, a similar carbon excess caused by first-star pollution was not found in dense neutral gas traced by absorption systems at different cosmic time. Here we present the detection of 14 very metal-poor, optically thick absorbers at redshift z ∼ 3–4. Among these, 3 are carbon-enhanced and reveal an overabundance with respect to Fe of all the analyzed chemical elements (O, Mg, Al, and Si). Their relative abundances show a distribution with respect to [Fe/H] that is in very good agreement with those observed in nearby very metal-poor stars. All the tests we performed support the idea that these C-rich absorbers preserve the chemical yields of the first stars. Our new findings suggest that the first-star signatures can survive in optically thick but relatively diffuse absorbers, which are not sufficiently dense to sustain star formation and hence are not dominated by the chemical products of normal stars.

Chemodynamically Tagged Groups of CEMP Stars in the Halo of the Milky Way. I. Untangling the Origins of CEMP-s and CEMP-no Stars

We construct a sample of 644 carbon-enhanced metal-poor (CEMP) stars with abundance analyses based on moderate- to high-resolution spectroscopic studies. Dynamical parameters for these stars are estimated based on radial velocities, Bayesian parallax-based distance estimates, and proper motions from Gaia EDR3 and DR3, supplemented by additional available information where needed. After separating our sample into the different CEMP morphological groups in the Yoon–Beers diagram of absolute carbon abundance versus metallicity, we used the derived specific energies and actions (E, J r , J ϕ , J z ) to cluster them into Chemodynamically Tagged Groups (CDTGs). We then analyzed the elemental-abundance dispersions within these clusters by comparing them to the dispersion of clusters that were generated at random. We find that, for the Group I (primarily CEMP-s and CEMP-r/s) clustered stars, there exist statistically insignificant intracluster dispersions in [Fe/H], [C/Fe] c (evolution corrected carbon), and [Mg/Fe] when compared to the intracluster dispersions of randomly clustered Group I CEMP stars. In contrast, the Group II (primarily CEMP-no) stars exhibit clear similarities in their intracluster abundances, with very low, statistically significant, dispersions in [C/Fe] c and marginally significant results in [Mg/Fe]. These results strongly indicate that Group I CEMP stars received their carbon enhancements from local phenomena, such as mass transfer from an evolved binary companion in regions with extended star formation histories, while the CDTGs of Group II CEMP stars formed in low-metallicity environments that had already been enriched in carbon, likely from massive rapidly rotating ultra- and hyper-metal-poor stars and/or supernovae associated with high-mass early-generation stars.