CEMP-no Stars Research Articles

Chemical Diversity on Small Scales: Abundance Analysis of the Tucana V Ultrafaint Dwarf Galaxy

The growing number of Milky Way satellites detected in recent years has introduced a new focus for stellar abundance analysis. Abundances of stars in satellites have been used to probe the nature of these systems and their chemical evolution. However, for most satellites, only centrally located stars have been examined. This paper presents an analysis of three stars in the Tucana V system, one in the inner region and two at ∼10′ (7–10 half-light radii) from the center. We find a remarkable chemical diversity between the stars. One star exhibits enhancements in rapid neutron-capture elements (an r-I star), and another is highly enhanced in C, N, and O but with low neutron-capture abundances (a CEMP-no star). The metallicities of the stars analyzed span more than 1 dex from [Fe/H] = −3.55 to −2.46. This, combined with a large abundance range of other elements like Ca, Sc, and Ni, confirms that Tuc V is an ultrafaint dwarf (UFD) galaxy. The variation in abundances, highlighted by [Mg/Ca] ratios ranging from +0.89 to −0.75, among the stars demonstrates that the chemical enrichment history of Tuc V was very inhomogeneous. Tuc V is only the second UFD galaxy in which stars located at large distances from the galactic center have been analyzed, along with Tucana II. The chemical diversity seen in these two galaxies, driven by the composition of the noncentral member stars, suggests that distant member stars are important to include when classifying faint satellites and that these systems may have experienced more complex chemical enrichment histories than previously anticipated.

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.

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.

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.

On the potential of Carbon-Enhanced Metal-poor Stars for Galactic Archaeology

The low-mass metal-poor stars in the Galaxy that preserve in their atmosphere, the chemical imprints of the gas clouds from which they were formed can be used as probes to get insight into the origin and evolution of elements in the early galaxy, early star formation and nucleosynthesis. Among the metal-poor stars, a large fraction, the so-called carbon-enhanced metal-poor (CEMP) stars exhibits high abundance of carbon. These stars exhibit diverse abundance patterns, particularly for heavy elements, based on which they are classified into different groups. The diversity of abundance patterns points at different formation scenarios. Hence, accurate classification of CEMP stars and knowledge of their distribution is essential to understand the role and contribution of each group. While CEMP-s and CEMP-r/s stars can be used to get insight into binary interactions at very low metallicity, CEMP-no stars can be used to probe the properties of the first stars and early nucleosynthesis. To exploit the full potential of CEMP stars for Galactic archaeology a homogeneous analysis of each class is extremely important. Our efforts towards, and contributions to providing an improved classification scheme for accurate classification of CEMP-s and CEMP-r/s stars and in characterizing the companion asymptotic giant branch (AGB) stars of CH, CEMP-no, CEMP-s and CEMP-r/s binary systems are discussed. Some recent results obtained based on low- and high-resolution spectroscopic analysis of a large number of potential CH and CEMP star candidates are highlighted.

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.

The 12C/13C isotopic ratio at the dawn of chemical evolution

Context. The known mega metal-poor (MMP) and hyper metal-poor (HMP) stars, with [Fe/H] < −6.0 and < −5.0, respectively, likely belong to the CEMP-no class, namely, carbon-enhanced stars with little or no second peak neutron-capture elements. They are likely second-generation stars, and the few elements measurable in their atmospheres are used to infer the properties of a single or very few progenitors. Aims. The high carbon abundance in the CEMP-no stars offers a unique opportunity to measure the carbon isotopic ratio, which directly indicates the presence of mixing between the He- and H-burning layers either within the star or in the progenitor(s). By means of high-resolution spectra acquired with the ESPRESSO spectrograph at the VLT, we aim to derive values for the 12C/13C ratio at the lowest metallicities. Methods. We used a spectral synthesis technique based on the SYNTHE code and on ATLAS models within a Markov chain Monte Carlo methodology to derive 12C/13C in the stellar atmospheres of four of the most metal-poor stars known: the MMP giant SMSS J0313–6708 ([Fe/H] < −7.1), the HMP dwarf HE 1327–2326 ([Fe/H] = −5.8), the HMP giant SDSS J1313–0019 ([Fe/H] = −5.0), and the ultra metal-poor subgiant HE0233 -0343 ([Fe/H] = −4.7). We also revised a previous value for the MMP giant SMSS J1605–1443 ([Fe/H] = −6.2). Results. In four stars we derive an isotopic value while for HE 1327–2326 we provide a lower limit. All measurements are in the range 39 < 12C/13C < 100, showing that the He- and H-burning layers underwent partial mixing either in the stars or, more likely, in their progenitors. This provides evidence of a primary production of 13C at the dawn of chemical evolution. CEMP-no dwarf stars with slightly higher metallicities show lower isotopic values, <30 and even approaching the CNO cycle equilibrium value. Thus, extant data suggest the presence of a discontinuity in the 12C/13C ratio at around [Fe/H] ≈ − 4, which could mark a real difference between the progenitor pollution captured by stars with different metallicities. We also note that some MMP and HMP stars with high 12C/13C show low 7Li values, providing an indication that mixing in the CEMP-no progenitors is not responsible for the observed Li depletion.

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.

GHOST Commissioning Science Results: Identifying a New Chemically Peculiar Star in Reticulum II

The Gemini High-resolution Optical SpecTrograph (GHOST) is the newest high-resolution spectrograph to be developed for a large-aperture telescope, recently deployed and commissioned at the Gemini-South telescope. In this paper, we present the first science results from the GHOST spectrograph taking during its commissioning runs. We have observed the bright metal-poor benchmark star HD 122563, along with two stars in the ultrafaint dwarf galaxy Reticulum II (Ret ii), one of which was previously identified as a candidate member, but did not have a previous detailed chemical abundance analysis. We find that this candidate (GDR3 0928) to be a bona fide member of Ret ii, and from a spectral synthesis analysis it is also revealed to be a CEMP-r star, with significant enhancements in several light elements (C, N, O, Na, Mg, and Si), in addition to featuring an r-process enhancement like many other Ret ii stars. The light-element enhancements in this star resemble the abundance patterns seen in the CEMP-no stars of other ultrafaint dwarf galaxies, and are thought to have been produced by an independent source from the r-process. These unusual abundance patterns are thought to be produced by faint supernovae, which may be produced by some of the earliest generations of stars.

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] < −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 > 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] > 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.

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.

Understanding the origin of CEMP – no stars through ultra-faint dwarfs

ABSTRACT The origin of Carbon Enhanced Metal-Poor (CEMP-no) stars with low abundances of neutron-capture elements is still unclear. These stars are ubiquitous, found primarily in the Milky Way halo and ultra-faint dwarf galaxies (UFDs). To make a major step forward, we developed a data-calibrated model for Böotes I that simultaneously includes all carbon sources: supernovae and asymptotic giant branch (AGB) stars both from first (Pop III) stars, and subsequent normal star formation (Pop II). We demonstrate that each of these sources leaves a specific chemical signature in the gas, allowing us to identify the origin of present day CEMP-no stars through their location in the A(C)–[Fe/H] diagram. The CEMP stars with A(C) > 6 are predominantly enriched by AGB Pop II stars. We identify a new class of moderate CEMP-s stars with A(C) ∼7 and 0 < [Ba/Fe] < + 1, imprinted by winds from AGB stars. True Pop III descendants are predicted to have A(C) < 6 and a constant [C/Mg] with [Fe/H], in perfect agreement with observations in Böotes I and the Milky Way halo. For the first time we now have a complete picture of the origins of CEMP-no stars which can and will be verified with future observations.

The Pristine survey – XX. GTC follow-up observations of extremely metal-poor stars identified from Pristine and LAMOST

ABSTRACT Ultra-metal-poor stars ($\rm {[Fe/H]} \lt -4.0$) are very rare, and finding them is a challenging task. Both narrow-band photometry and low-resolution spectroscopy have been useful tools for identifying candidates, and in this work, we combine both approaches. We cross-matched metallicity-sensitive photometry from the Pristine survey with the low-resolution spectroscopic Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) data base, and re-analysed all LAMOST spectra with $\rm {[Fe/H]} _{\rm Pristine} \lt -2.5$. We find that ∼1/3rd of this sample (selected without $\rm {[Fe/H]} _{\rm Pristine}$ quality cuts) also have spectroscopic $\rm {[Fe/H]} \lt -2.5$. From this sample, containing many low signal-to-noise ratio (S/N) spectra, we selected 11 stars potentially having $\rm {[Fe/H]} \lt -4.0$ or $\rm {[Fe/H]} \lt -3.0$ with very high carbon abundances, and we performed higher S/N medium-resolution spectroscopic follow-up with the Optical System for Imaging and low Resolution Integrated Spectroscopy (OSIRIS) on the 10.4-m Gran Telescopio Canarias (GTC). We confirm their extremely low metallicities, with a mean of $\rm {[Fe/H]} = -3.4$, and the most metal-poor star having $\rm {[Fe/H]} = -3.8$. Three of these are clearly carbon-enhanced metal-poor (CEMP) stars with $+1.65 \lt \rm {[C/Fe]} \lt +2.45$. The two most carbon-rich stars are either among the most metal-poor CEMP-s stars or the most carbon-rich CEMP-no stars known, the third is likely a CEMP-no star. We derived orbital properties for the OSIRIS sample and find that only one of our targets can be confidently associated with known substructures/accretion events, and that three out of four inner halo stars have prograde orbits. Large spectroscopic surveys may contain many hidden extremely and ultra-metal-poor stars, and adding additional information from e.g. photometry as in this work can uncover them more efficiently and confidently.

ESPRESSO observations of HE 0107−5240 and other CEMP-no stars with [Fe/H] ≤ –4.5

Context. HE 0107−5240 is a hyper metal-poor star with [Fe/H] = −5.39, one of the lowest-metallicity stars known. Its stellar atmosphere is enhanced in carbon, with [C/Fe] = +4.0, without a detectable presence of neutron-capture elements. Therefore, it belongs to the carbon-enhanced metal-poor (CEMP−no) group, along with the majority of the most metal-poor stars known to date. Recent studies have revealed variations in the line-of-sight velocity of HE 0107−5240, suggesting it belongs to a binary system. CEMP-no stars are the closest descendants of the very first Pop III stars, and binarity holds important clues for the poorly known mechanism that leads to their formation. Aims. We performed high-resolution observations with the ESPRESSO spectrograph at the VLT to constrain the kinematical properties of the binary system HE 0107−5240 and to probe the binarity of the sample of the eight most metal-poor stars with [Fe/H] ≤ −4.5. Methods. Radial velocities are obtained by using a cross-correlation function in the interval 4200−4315 Å , which contains the relatively strong CH band, against a template that could be either a synthetic spectrum or a combined observed spectrum in an iterative process. A Bayesian method is applied to calculate the orbit using the ESPRESSO measurements and others from the literature. Chemical analysis has also been performed for HE 0107−5240, employing spectral synthesis with the SYNTHE and ATLAS codes. Results. Observations of HE 0107−5240 spanning more than 3 years show a monotonic decreasing trend in radial velocity at a rate of approximately 0.5 m s−1 d−1. A maximum vrad was reached between March 13, 2012, and December 8, 2014. The period is constrained at Porb = 13009−1370+1496 d. New, more stringent upper limits have been found for several elements: (a) [Sr/Fe] and [Ba/Fe] are lower than −0.76 and +0.2, respectively, confirming the star is a CEMP-no; (b) A(Li) < 0.5 is well below the plateau at A(Li) = 1.1 found in the lower red giant branch stars, suggesting Li was originally depleted; and (c) the isotopic ratio 12C/13C is 87 ± 6, showing very low 13C in contrast to what is expected from a ‘spinstar’ progenitor. Conclusions. We confirm that HE 0107−5240 is a binary star with a long period of about 13 000 d (∼36 yr). The carbon isotopic ratio excludes the possibility that the companion has gone through the asymptotic giant branch phase and transferred mass to the currently observed star. The binarity of HE 0107−5240 implies that some of the first generations of low-mass stars formed in multiple systems and indicates that the low metallicity does not preclude the formation of binaries. Finally, a solid indication of vrad variation has also been found in SMSS 1605−1443.

Metal-poor Stars Observed with the Southern African Large Telescope II. An Extended Sample

Abstract We present results from high-resolution (R ∼ 40,000) spectroscopic observations of over 200 metal-poor stars, mostly selected from the RAVE survey, using the Southern African Large Telescope. We were able to derive stellar parameters for a total of 108 stars; an additional sample of 50 stars from this same effort was previously reported on by Rasmussen et al. Among our newly reported observations, we identify 84 very metal-poor (VMP; [Fe/H] < −2.0, 53 newly identified) stars and three extremely metal-poor (EMP; [Fe/H] < −3.0, one newly identified) stars. The elemental abundances were measured for carbon, as well as several other α-elements (Mg, Ca, Sc, and Ti), iron-peak elements (Mn, Co, Ni, and Zn), and neutron-capture elements (Sr, Ba, and Eu). Based on these measurements, the stars are classified by their carbon and neutron-capture abundances into carbon-enhanced metal-poor (CEMP; [C/Fe] > +0.70), CEMP subclasses, and by the level of their r-process abundances. A total of 17 are classified as CEMP stars. There are 11 CEMP-r stars (eight newly identified), one CEMP-s star (newly identified), two possible CEMP-i stars (one newly identified), and three CEMP-no stars (all newly identified) in this work. We found 11 stars (eight newly identified) that are strongly enhanced in r-process elements (r-II; [Eu/Fe] > +0.70), 38 stars (31 newly identified) that are moderately enhanced in r-process elements (r-I; +0.30 < [Eu/Fe] ≤ + 0.70), and one newly identified limited-r star.

Stellar winds and metal enrichment from fast-rotating Population III stars

ABSTRACT Stellar winds from fast-rotating Population III (Pop III) stars have long been suspected to make important contributions to early metal enrichment, as features in the nucleosynthesis of such ‘spinstars’ are consistent with the chemical abundance patterns of some metal-poor stars in the local Universe. Particularly, stellar winds rich in light elements can provide another pathway towards explaining the carbon enhancement in carbon-enhanced metal-poor (CEMP) stars. In this work, we focus on the feedback of Pop III stellar winds combined with supernovae (SNe), and derive the resulting chemical signatures in the enriched medium. We explore a large parameter space of Pop III star formation, feedback, yields from winds, and SNe with a semi-analytical model. The predicted pattern of carbon and iron abundances of second-generation stars agrees well with observations of CEMP-no stars ($[\rm Ba/Fe]\lt 0$) at $[\rm Fe/H]\lesssim -3$ and A(C) ≲ 7, under the optimistic assumption of significant mass-loss by winds from massive ($\gtrsim 25\ \rm M_{\odot }$) stars that collapse into BHs without SNe. In this scenario, carbon-rich but iron-free second-generation stars can form in systems dominated by enrichment from winds, gaining trace amounts of iron by accretion from the interstellar medium, to become the most iron-poor and carbon-enhanced stars seen in observations ($[\rm Fe/H]\lesssim -4$, $[\rm C/Fe]\gtrsim 2$). We conclude that the observed CEMP-no stars can be explained by both our winds + ISM accretion channel as well as the well-studied faint SN scenario. Wind feedback from Pop III spinstars deserves more detailed modelling in early cosmic structure formation.

The Pristine survey – XII. Gemini-GRACES chemo-dynamical study of newly discovered extremely metal-poor stars in the Galaxy

ABSTRACTHigh-resolution optical spectra of 30 metal-poor stars selected from the Pristine survey are presented, based on observations taken with the Gemini Observatory GRACES spectrograph. Stellar parameters Teff and log g are determined using a Gaia DR2 colour–temperature calibration and surface gravity from the Stefan–Boltzmann equation. GRACES spectra are used to determine chemical abundances (or upper limits) for 20 elements (Li, O, Na, Mg, K, Ca, Ti, Sc, Cr, Mn, Fe, Ni, Cu, Zn, Y, Zr, Ba, La, Nd, Eu). These stars are confirmed to be metal-poor ([Fe/H] < −2.5), with higher precision than from earlier medium-resolution analyses. The chemistry for most targets is similar to other extremely metal-poor stars in the Galactic halo. Three stars near [Fe/H] = −3.0 have unusually low Ca and high Mg, suggestive of contributions from few SN II where alpha-element formation through hydrostatic nucleosynthesis was more efficient. Three new carbon-enhanced metal-poor (CEMP) stars are also identified (two CEMP-s and one potential CEMP-no star) when our chemical abundances are combined with carbon from previous medium-resolution analyses. The GRACES spectra also provide precision radial velocities (σRV ≤ 0.2 km s−1) for dynamical orbit calculations with the Gaia DR2 proper motions. Most of our targets are dynamically associated with the Galactic halo; however, five stars with [Fe/H] < −3 have planar-like orbits, including one retrograde star. Another five stars are dynamically consistent with the Gaia-Sequoia accretion event; three have typical halo [α/Fe] ratios for their metallicities, whereas two are [Mg/Fe]-deficient, and one is a new CEMP-s candidate. These results are discussed in terms of the formation and early chemical evolution of the Galaxy.

The Pristine Inner Galaxy Survey (PIGS) III: carbon-enhanced metal-poor stars in the bulge

ABSTRACT The most metal-deficient stars hold important clues about the early buildup and chemical evolution of the Milky Way, and carbon-enhanced metal-poor (CEMP) stars are of special interest. However, little is known about CEMP stars in the Galactic bulge. In this paper, we use the large spectroscopic sample of metal-poor stars from the Pristine Inner Galaxy Survey (PIGS) to identify CEMP stars ($\rm {[C/Fe]} \geqslant +0.7$) in the bulge region and to derive a CEMP fraction. We identify 96 new CEMP stars in the inner Galaxy, of which 62 are very metal-poor ($\rm {[Fe/H]} \lt -2.0$); this is more than a 10-fold increase compared to the seven previously known bulge CEMP stars. The cumulative fraction of CEMP stars in PIGS is $42^{\, +14\, }_{\, -13} {{\ \rm per\ cent}}$ for stars with $\rm {[Fe/H]} \lt -3.0$, and decreases to $16^{\, +3\, }_{\, -3} {{\ \rm per\ cent}}$ for $\rm {[Fe/H]} \lt -2.5$ and $5.7^{\, +0.6\, }_{\, -0.5} {{\ \rm per\ cent}}$ for $\rm {[Fe/H]} \lt -2.0$. The PIGS inner Galaxy CEMP fraction for $\rm {[Fe/H]} \lt -3.0$ is consistent with the halo fraction found in the literature, but at higher metallicities, the PIGS fraction is substantially lower. While this can partly be attributed to a photometric selection bias, such bias is unlikely to fully explain the low CEMP fraction at higher metallicities. Considering the typical carbon excesses and metallicity ranges for halo CEMP-s and CEMP-no stars, our results point to a possible deficiency of both CEMP-s and CEMP-no stars (especially the more metal-rich) in the inner Galaxy. The former is potentially related to a difference in the binary fraction, whereas the latter may be the result of a fast chemical enrichment in the early building blocks of the inner Galaxy.

The role of faint population III supernovae in forming CEMP stars in ultra-faint dwarf galaxies

ABSTRACT Carbon enhanced metal poor (CEMP)-no stars, a subset of CEMP stars ($\rm [C/Fe]\ge 0.7$ and $\rm [Fe/H]\lesssim -1$) have been discovered in ultra-faint dwarf (UFD) galaxies, with $M_{\rm vir}\approx 10^8{\, \mathrm{ M}_\odot }$ and $M_{\ast }\approx 10^3-10^4{\, \mathrm{ M}_\odot }$ at z = 0, as well as in the halo of the Milky Way (MW). These CEMP-no stars are local fossils that may reflect the properties of the first (Pop III) and second (Pop II) generation of stars. However, cosmological simulations have struggled to reproduce the observed level of carbon enhancement of the known CEMP-no stars. Here, we present new cosmological hydrodynamic zoom-in simulations of isolated UFDs that achieve a gas mass resolution of $m_{\rm gas}\approx 60{\, \mathrm{ M}_\odot }$. We include enrichment from Pop III faint supernovae (SNe), with ESN = 0.6 × 1051 erg, to understand the origin of CEMP-no stars. We confirm that Pop III and Pop II stars are mainly responsible for the formation of CEMP and C-normal stars, respectively. New to this study, we find that a majority of CEMP-no stars in the observed UFDs and the MW halo can be explained by Pop III SNe with normal explosion energy (ESN = 1.2 × 1051 erg) and Pop II enrichment, but faint SNe might also be needed to produce CEMP-no stars with $\rm [C/Fe]\gtrsim 2$, corresponding to the absolute carbon abundance of $\rm A(C)\gtrsim 6.0$. Furthermore, we find that while we create CEMP-no stars with high carbon ratio $\rm [C/Fe]\approx 3-4$, by adopting faint SNe, it is still challenging to reproduce CEMP-no stars with extreme level of carbon abundance of $\rm A(C)\approx 7.0-7.5$, observed both in the MW halo and UFDs.