Metal-poor Halo Stars Research Articles

The R-process Alliance: Fifth Data Release from the Search for R-process-enhanced Metal-poor Stars in the Galactic Halo with the GTC* * This paper includes data gathered with the 10.4 m Gran Telescopio Canarias located at La Palma, Canary Islands, Spain.

Understanding the abundance pattern of metal-poor stars and the production of heavy elements through various nucleosynthesis processes offers crucial insights into the chemical evolution of the Milky Way, revealing primary sites and major sources of rapid neutron-capture process (r-process) material in the Universe. In this fifth data release from the R-Process Alliance (RPA), we present the detailed chemical abundances of 41 faint (down to V = 15.8) and extremely metal-poor (down to [Fe/H] = −3.3) halo stars selected from the RPA. We obtained high-resolution spectra for these objects with the HORuS spectrograph on the Gran Telescopio Canarias. We measure the abundances of light, α, Fe-peak, and neutron-capture elements. We report the discovery of five carbon-enhanced metal-poor, one limited-r, three r-I, and four r-II stars, and six Mg-poor stars. We also identify one star of a possible globular cluster origin at an extremely low metallicity at [Fe/H] = −3.0. This adds to the growing evidence of a lower-limit metallicity floor for globular cluster abundances. We use the abundances of Fe-peak elements and the α-elements to investigate the contributions from different nucleosynthesis channels in the progenitor supernovae. We find the distribution of [Mg/Eu] as a function of [Fe/H] to have different enrichment levels, indicating different possible pathways and sites of their production. We also reveal differences in the trends of the neutron-capture element abundances of Sr, Ba, and Eu of various r-I and r-II stars from the RPA data releases, which provide constraints on their nucleosynthesis sites and subsequent evolution.

The oldest stars with low neutron-capture element abundances and origins in ancient dwarf galaxies

ABSTRACT We present a detailed chemical abundance and kinematic analysis of six extremely metal-poor (−4.2 ≤ [Fe/H] ≤−2.9) halo stars with very low neutron-capture abundances ([Sr/H] and [Ba/H]) based on high-resolution Magellan/MIKE spectra. Three of our stars have [Sr/Ba] and [Sr/H] ratios that resemble those of metal-poor stars in ultra-faint dwarf galaxies (UFDs). Since early UFDs may be the building blocks of the Milky Way, extremely metal-poor halo stars with low, UFD-like Sr and Ba abundances may thus be ancient stars from the earliest small galactic systems that were accreted by the proto-Milky Way. We label these objects as Small Accreted Stellar System (SASS) stars, and we find an additional 61 similar ones in the literature. A kinematic analysis of our sample and literature stars reveals them to be fast-moving halo objects, all with retrograde motion, indicating an accretion origin. Because SASS stars are much brighter than typical UFD stars, identifying them offers promising ways towards detailed studies of early star formation environments. From the chemical abundances of SASS stars, it appears that the earliest accreted systems were likely enriched by a few supernovae whose light element yields varied from system to system. Neutron-capture elements were sparsely produced and/or diluted, with r-process nucleosynthesis playing a role. These insights offer a glimpse into the early formation of the Galaxy. Using neutron-capture elements as a distinguishing criterion for early formation, we have access to a unique metal-poor population that consists of the oldest stars in the universe.

SPLUS J142445.34–254247.1: An r-process–enhanced, Actinide-boost, Extremely Metal-poor Star Observed with GHOST

We report on a chemo-dynamical analysis of SPLUS J142445.34−254247.1 (SPLUS J1424−2542), an extremely metal-poor halo star enhanced in elements formed by the rapid neutron-capture process (r-process). This star was first selected as a metal-poor candidate from its narrowband S-PLUS photometry and followed up spectroscopically in medium resolution with Gemini-South/GMOS, which confirmed its low-metallicity status. High-resolution spectroscopy was gathered with GHOST at Gemini-South, allowing for the determination of the chemical abundances for 36 elements, from carbon to thorium. At [Fe/H] = −3.39, SPLUS J1424−2542 is one of the lowest-metallicity stars with measured Th and has the highest observed to date, making it part of the “actinide-boost” category of r-process–enhanced stars. The analysis presented here suggests that the gas cloud from which SPLUS J1424−2542 formed must have been enriched by at least two progenitor populations. The light-element (Z ≤ 30) abundance pattern is consistent with the yields from a supernova explosion of metal-free stars with 11.3–13.4 M ⊙, and the heavy-element (Z ≥ 38) abundance pattern can be reproduced by the yields from a neutron star merger (1.66 M ⊙ and 1.27 M ⊙) event. A kinematical analysis also reveals that SPLUS J1424−2542 is a low-mass, old halo star with a likely in situ origin, not associated with any known early merger events in the Milky Way.

A geometric calibration of the tip of the red giant branch in the Milky Way using Gaia DR3

ABSTRACT We use the latest parallaxes measurements from Gaia DR3 to obtain a geometric calibration of the tip of the red giant branch (TRGB) in Cousins I magnitudes as a standard candle for cosmology. We utilize the following surveys: SkyMapper DR3, APASS DR9, ATLAS Refcat2, and Gaia DR3 synthetic photometry to obtain multiple zero-point calibrations of the TRGB magnitude, $M_{I}^{TRGB}$. Our sample contains Milky Way halo stars at high galactic latitudes (|b| > 36) where the impact of metallicity, dust, and crowding are minimized. The magnitude of the TRGB is identified using Sobel edge detection, but this approach introduced a systematic offset. To address this issue, we utilized simulations with parsec isochrones and showed how to calibrate and remove this bias. Applying our method within the colour range where the slope of the TRGB is relatively flat for metal-poor halo stars (1.55 < (BP − RP) < 2.25), we find a weighted average $M_{I}^{TRGB} = -4.042 \pm 0.041$ (stat) ±0.031 (sys) mag. A geometric calibration of the Milky Way TRGB has the benefit of being independent of other distance indicators and will help probe systematics in the local distance ladder, leading to improved measurements of the Hubble constant.

Implications of the non-observation of 6Li in halo stars for the primordial 7Li problem

The primordial Lithium Problem is intimately connected to the assumption that the 7Li abundance observed in metal-poor halo stars is unchanged from its primordial value, which lies significantly below the predictions of standard big-bang nucleosynthesis. Two key lines of evidence have argued that these stars have not significantly depleted their initial (mostly primordial) 7Li: i) the lack of dispersion in Li abundance measurements at low metallicity (and high surface temperature); and ii) the detection of the more fragile 6Li isotope in at least two halo stars. The purported 6Li detections were in good agreement with predictions from cosmic-ray nucleosynthesis which is responsible for the origin of 6Li. This concordance left little room for 6Li depletion, and the apparent 6Li survival implied that 7Li largely evaded destruction, because stellar interiors destroy 6Li more vigorously then than 7Li. Recent (re)-observations of halo stars challenge the evidence against 7Li depletion: i) lithium elemental abundances now show significant dispersion, and ii) sensitive 6Li searches now reveal only upper limits to the 6Li/7Li ratio. We discuss the consequences of these 6Li non-detections on the primordial 7Li Problem, Galactic cosmic-ray nucleosynthesis, and the question of differential depletion of Li in stars. The tight new 6Li upper limits generally fall far below the predictions of cosmic-ray nucleosynthesis, implying that substantial 6Li depletion has occurred — by factors up to 50. We show that in stars with 6Li limits and thus lower bounds on 6Li depletion, an equal amount of 7Li depletion is more than sufficient to resolve the primordial 7Li Problem. This picture is consistent with well-studied stellar models in which 7Li is less depleted than 6Li, and strengthen the case that the Lithium Problem has an astrophysical solution. We conclude by suggesting future observations that could test these ideas.

Supernova-driven Turbulent Metal Mixing in High-redshift Galactic Disks: Metallicity Fluctuations in the Interstellar Medium and its Imprints on Metal-poor Stars in the Milky Way

The extent to which turbulence mixes gas in the face of recurrent infusions of fresh metals by supernovae (SN) could help provide important constraints on the local star formation conditions. This includes predictions of the metallicity dispersion among metal-poor stars, which suggests that the interstellar medium was not very well mixed at these early times. The purpose of this Letter is to help isolate, via a series of numerical experiments, some of the key processes that regulate turbulent mixing of SN elements in galactic disks. We study the gas interactions in small simulated patches of a galaxy disk with the goal of resolving the small-scale mixing effects of metals at parsec scales, which enables us to measure the turbulent diffusion coefficient in various galaxy environments. By investigating the statistics of variations of α elements in these simulations, we are able to derive constraints not only on the allowed range of intrinsic yield variations in SN explosions but also on the star formation history of the Milky Way. We argue that the observed dispersion of [Mg/Fe] in metal-poor halo stars is compatible with the star-forming conditions expected in dwarf satellites or in an early low-star-forming Milky Way progenitor. In particular, metal variations in stars that have not been phase-mixed can be used to infer the star-forming conditions of disrupted dwarf satellites.

Correlations ofr-process elements in very metal-poor stars as clues to their nucleosynthesis sites

Aims.Various nucleosynthesis studies have pointed out that ther-process elements in very metal-poor (VMP) halo stars might have different origins. By means of familiar concepts from statistics (correlations, cluster analysis, and rank tests of elemental abundances), we look for causally correlated elemental abundance patterns and attempt to link them to astrophysical events. Some of these events produce ther-process elements jointly with iron, while others do not have any significant iron contribution. We try to (a) characterize these different types of events by their abundance patterns and (b) identify them among the existing set of suggestedr-process sites.Methods.The Pearson and Spearman correlation coefficients were used in order to investigate correlations amongr-process elements (X,Y) as well as their relation to iron (Fe) in VMP halo stars. We gradually tracked the evolution of those coefficients in terms of the element enrichments [X/Fe] or [X/Y] and the metallicity [Fe/H]. This approach, aided by cluster analysis to find different structures of abundance patterns and rank tests to identify whether several events contributed to the observed pattern, is new and provides deeper insights into the abundances of VMP stars.Results.In the early stage of our Galaxy, at least threer-process nucleosynthesis sites have been active. The first two produce and eject iron and the majority of the lighterr-process elements. We assign them to two different types of core-collapse events, not identical to regular core-collapse supernovae (CCSNe), which produce only light trans-Fe elements. The third category is characterized by a strongr-process and is responsible for the major fraction of the heavy mainr-process elements without a significant coproduction of Fe. It does not appear to be connected to CCSNe, in fact most of the Fe found in the relatedr-process enriched stars must come from previously occurring CCSNe. The existence of actinide boost stars indicates a further division among strongr-process sites. We assign these two strongr-process sites to neutron star mergers without fast black hole formation and to events where the ejecta are dominated by black hole accretion disk outflows. Indications from the lowest-metallicity stars hint at a connection with massive single stars (collapsars) forming black holes in the early Galaxy.

The Local Stellar Halo is Not Dominated by a Single Radial Merger Event

We use halo dwarf stars with photometrically determined metallicities that are located within 2 kpc of the Sun to identify local halo substructure. The kinematic properties of these stars do not indicate a single, dominant radial merger event (RME). The retrograde Virgo Radial Merger (VRM) component has [Fe/H] = −1.7. A second, nonrotating RME component we name Nereus is identified with [Fe/H] = −2.1 and has similar energy to the VRM. We identify a possible third RME, which we name Cronus, that is corotating with the disk, has lower energy than the VRM, and has [Fe/H] = −1.2. We identify the Nyx Stream in the data. In addition to these substructures, we observe metal-poor halo stars ([Fe/H] ∼ −2.0 and σ v ∼ 180 km s−1) and a disk/Splash component with lower rotational velocity than the disk and lower metallicity than typically associated with the Splash. An additional excess of halo stars with low velocity and metallicity of [Fe/H] = −1.5 could be associated with the shell of a lower-energy RME or indicate that lower-energy halo stars have higher metallicity. Stars that comprise the “Gaia Sausage” velocity structure are a combination of the components identified in this work.

Be, V, and Cu in the halo star CS 31082-001 from near-UV spectroscopy

ABSTRACT The ‘First Stars’ programme revealed the metal-poor halo star CS 31082-001 to be r-process and actinide rich, including the first measurement of a uranium abundance for an old star. To better characterize and understand such rare objects, we present the first abundance estimates of three elements (Be, V, Cu) for CS 31082-001 from analysis of its near-ultraviolet spectrum. Beryllium is rarely measured in giant stars, and we confirm that its abundance in this star is low due to the rather cool effective temperature that causes destruction of both Be and Li in its atmosphere. Vanadium and copper are iron-peak elements that are starting to be used as chemical-tagging indicators to investigate the origin of stellar populations. We find V and Cu abundances for CS 31082-001 that are comparable to other metal-poor stars, and present new chemical evolution models to investigate our results. In the case of V, extra nucleosynthesis due to interaction of neutrinos with matter is included in the models to be able to reproduce the measured abundance. Given the availability of high-quality spectroscopy of CS 31082-001, we also explore other atomic lines in the near-ultraviolet as a template for future studies of metal-poor stars with the planned CUBES instrument in development for the Very Large Telescope.

R-Process Contributions to Low-Metallicity Stars

Various nucleosynthesis studies have pointed out that the rapid neutron capture r-process elements in very metal-poor (VMP) halo stars might have different origins. It has been known that an r-process can either be obtained in neutron-rich low Ye conditions or in high entropy environments [see e.g. 1–5], an overview over many investigations has appeared recently [6]. In the present article we analyze with statistical methods the observational abundance patterns from trans-Fe elements up to the actinides and come to the conclusion that four to five categories of astrophysical events must have contributed. These include the ejection of Fe and trans-Fe elements Sr, Y, Zr (continuing possibly beyond to slightly higher mass numbers) in category 0 events (hereafter "C0"), Fe and weak r-process contributions (including Eu in moderate to slightly larger but varying amounts) in CI and CII events, strong r-process abundance patterns with no or negligible (in comparison to solar) Fe production in CIIIa and CIIIb events, where category CIIIb shows a tendency for an actinide boost behavior. When comparing these categories with presently existing nucleosynthesis predictions, we suggest to identify them (despite remaining uncertainties) with regular core-collapse supernovae, quark deconfinement supernovae, magneto-rotational supernovae, neutron star mergers, and outflows from black hole accretion tori.

Lithium on the lower red giant branch of five Galactic globular clusters

Context. Lithium is one of the few elements produced during Big Bang nucleosynthesis in the early universe. Moreover, its fragility makes it useful as a proxy for stellar environmental conditions. As such, the lithium abundance in old systems is at the core of various astrophysical investigations. Aims. Stars on the lower red giant branch are key to studies of globular clusters where main sequence stars are too faint to be observed. We use these stars to analyze the initial Li content of the clusters and compare it to cosmological predictions, to measure spreads in Li between different stellar populations, and to study signs of extra depletion in these giants. Methods. We used the GIRAFFE spectra to measure the lithium and sodium abundances of lower red giant branch stars in five globular clusters. These cover an extensive range in metallicity, from [Fe/H] ∼ −0.7 to [Fe/H] ∼ −2.3 dex. Results. We find that the lithium abundance in these lower red giant branch stars forms a plateau, with values from A(Li)NLTE = 0.84 to 1.03 dex, showing no clear correlation with metallicity. When using stellar evolutionary models to calculate the primordial abundance of these clusters, we recover values of A(Li)NLTE = 2.1 − 2.3 dex, consistent with the constant value observed in warm metal-poor halo stars, namely the Spite plateau. Additionally, we find no difference in the lithium abundance of first and second population stars in each cluster. We also report the discovery of a Li-rich giant in the cluster NGC 3201, with A(Li)NLTE = 1.63 ± 0.18 dex, where the enrichment mechanism is probably pollution from external sources.

Searching For Transiting Planets Around Halo Stars. I. Sample Selection and Validation

Abstract By measuring the elemental abundances of a star, we can gain insight into the composition of its initial gas cloud—the formation site of the star and its planets. Planet formation requires metals, the availability of which is determined by the elemental abundance. In the case where metals are extremely deficient, planet formation can be stifled. To investigate such a scenario requires a large sample of metal-poor stars and a search for planets therein. This paper focuses on the selection and validation of a halo star sample. We select ∼17,000 metal-poor halo stars based on their Galactic kinematics, and confirm their low metallicities ([Fe/H] < −0.5), using spectroscopy from the literature. Furthermore, we perform high-resolution spectroscopic observations using LBT/PEPSI and conduct detailed metallicity ([Fe/H]) analyses on a sample of 13 previously-known halo stars that also have hot kinematics. We can use the halo star sample presented here to measure the frequency of planets and to test planet formation in extremely metal-poor environments. The result of the planet search and its implications will be presented and discussed in a companion paper by Boley et al.

The Abundance of Lead in Four Metal-poor Stars

Abstract Cowan et al. review how roughly half the elements heavier than iron found in the Sun are produced by rapid neutron capture and half by slow neutron capture, the r- and s-processes. In the Sun, their relative contribution to individual elemental abundances is well understood, except for the lightest and heaviest elements beyond iron. Their contributions are especially uncertain for the heaviest nonradioactive element, lead (Pb, Z = 82). This is constrained by deriving lead abundances in metal-poor stars. For in the most metal-poor halo stars, strontium and heavier elements are found in the solar r-process proportion; s-process elements appear only at metallicities above one-thirtieth solar. In unevolved metal-poor stars of roughly solar heavy-element content, only two UV Pb lines are detectable. Four such stars have high-resolution spectra of the strongest line, Pb ii at 2203.53 Å. Roederer et al. analyzed this line in one star, deriving a lead-to-iron abundance ratio 10 times solar. This and its blueshifted profile suggested strong s-process production. This work analyzes the UV spectra of all four stars. Calculations including a predicted Fe i line blueward of the Pb ii line, and assuming the lead abundance scales with r-process abundances, match all four profiles extremely well. A scaled s-process contribution might improve the match to the much lower lead abundance found in the unevolved star analyzed previously, but its s-process excess is modest. An Fe ii line blends the other lead line, Pb i at 2833.05 Å, which constrains the lead abundance only in the coolest star.

Light Elements in the Universe

Due to their production sites, as well as to how they are processed and destroyed in stars, the light elements are excellent tools to investigate a number of crucial issues in modern astrophysics: from stellar structure and non-standard processes at work in stellar interiors to age dating of stars; from pre-main sequence evolution to the star formation histories of young clusters and associations and to multiple populations in globular clusters; from Big Bang nucleosynthesis to the formation and chemical enrichment history of the Milky Way Galaxy and its populations, just to cite some relevant examples. In this paper, we focus on lithium, beryllium, and boron (LiBeB) and on carbon, nitrogen, and oxygen (CNO). LiBeB are rare elements, with negligible abundances with respect to hydrogen; on the contrary, CNO are among the most abundant elements in the Universe, after H and He. Pioneering observations of light-element surface abundances in stars started almost 70 years ago and huge progress has been achieved since then. Indeed, for different reasons, precise measurements of LiBeB and CNO are difficult, even in our Sun; however, the advent of state-of-the-art ground- and space-based instrumentation has allowed the determination of high-quality abundances in stars of different type, belonging to different Galactic populations, from metal-poor halo stars to young stars in the solar vicinity and from massive stars to cool dwarfs and giants. Noticeably, the recent large spectroscopic surveys performed with multifiber spectrographs have yielded detailed and homogeneous information on the abundances of Li and CNO for statistically significant samples of stars; this has allowed us to obtain new results and insights and, at the same time, raise new questions and challenges. A complete understanding of the light-element patterns and evolution in the Universe has not been still achieved. Perspectives for further progress will open up soon thanks to the new generation instrumentation that is under development and will come online in the coming years.

Reconstructing Masses of Merging Neutron Stars from Stellar r-process Abundance Signatures

Abstract Neutron star mergers (NSMs) are promising astrophysical sites for the rapid neutron-capture (“r”) process, but can their integrated yields explain the majority of heavy-element material in the Galaxy? One method to address this question implements a forward approach that propagates NSM rates and yields along with stellar formation rates and compares those results with observed chemical abundances of r-process-rich, metal-poor stars. In this work, we take the inverse approach by utilizing r-process-element abundance ratios of metal-poor stars as input to reconstruct the properties—especially the masses—of their neutron star (NS) binary progenitors. This novel analysis provides an independent avenue for studying the population of the original NS binary systems that merged and produced the r-process material now incorporated in Galactic metal-poor halo stars. We use ratios of elements typically associated with the limited-r-process and the actinide region to those in the lanthanide region (i.e., Zr/Dy and Th/Dy) to probe the NS masses of the progenitor merger. We find that NSMs can account for all r-process material in metal-poor stars that display r-process signatures, while simultaneously reproducing the present-day distribution of double-NS systems. Notably, with our model assumptions and the studied stellar sample, we postulate that the most r-process enhanced stars (the r–II stars) on their own would require progenitor NSMs of asymmetric systems that are distinctly different from present ones in the Galaxy. We also explore variations to the model and find that the predicted degree of asymmetry is most sensitive to the electron fraction of the remnant disk wind.

Dynamically Tagged Groups of Very Metal-poor Halo Stars from the HK and Hamburg/ESO Surveys

Abstract We analyze the dynamical properties of ∼1500 very metal-poor (VMP; [Fe/H] ≲ −2.0) halo stars, based primarily on medium-resolution spectroscopic data from the HK and Hamburg/ESO surveys. These data, collected over the past 30 yr, are supplemented by a number of calibration stars and other small samples, along with astrometric information from Gaia DR2. We apply a clustering algorithm to the 4D energy-action space of the sample, and identify a set of 38 dynamically tagged groups (DTGs), containing between 5 and 30 member stars. Many of these DTGs can be associated with previously known prominent substructures such as Gaia-Sausage/Enceladus (GSE), Sequoia, Helmi Stream (HStr), and Thamnos. Others are associated with previously identified smaller dynamical groups of stars and streams. We identify 10 new DTGs as well, many of which have strongly retrograde orbits. We also investigate possible connections between our DTGs and ∼300 individual r-process-enhanced (RPE) stars from a recent literature compilation. We find that several of these objects have similar dynamical properties to GSE (5), the HStr (4), Sequoia (1), and Rg5 (1), indicating that their progenitors might have been important sources of RPE stars in the Galaxy. Additionally, a number of our newly identified DTGs are shown to be associated with at least two RPE stars each (DTG-2: 3, DTG-7: 2; DTG-27: 2). Taken as a whole, these results are consistent with ultra-faint and/or dwarf spheroidal galaxies as birth environments in which r-process nucleosynthesis took place, and then were disrupted by the Milky Way.

The stochastic enrichment of Population II stars

ABSTRACT We investigate the intrinsic scatter in the chemical abundances of a sample of metal-poor ([Fe/H] < −2.5) Milky Way halo stars. We draw our sample from four historic surveys and focus our attention on the stellar Mg, Ca, Ni, and Fe abundances. Using these elements, we investigate the chemical enrichment of these metal-poor stars using a model of stochastic chemical enrichment. Assuming that these stars have been enriched by the first generation of massive metal-free stars, we consider the mass distribution of the enriching population alongside the stellar mixing and explosion energy of their supernovae. For our choice of stellar yields, our model suggests that the most metal-poor stars were enriched, on average, by $\hat{N}_{\star }=5^{+13}_{-3}~(1\sigma)$ Population III stars. This is comparable to the number of enriching stars inferred for the most metal-poor DLAs. Our analysis therefore suggests that some of the lowest mass structures at z ∼ 3 contain the chemical products from < 13(2σ) Population III enriched minihaloes. The inferred IMF is consistent with that of a Salpeter distribution and there is a preference towards ejecta from minimally mixed hypernovae. However, the estimated enrichment model is sensitive to small changes in the stellar sample. An offset of ∼ 0.1 dex in the [Mg/Ca] abundance is shown to be sensitive to the inferred number of enriching stars. We suggest that this method has the potential to constrain the multiplicity of the first generation of stars, but this will require: (1) a stellar sample whose systematic errors are well understood; and, (2) documented uncertainties associated with nucleosynthetic yields.

Stellar signatures of inhomogeneous big bang nucleosynthesis

We evaluate abundance anomalies generated in patches of the universe where the baryon-to-photon ratio was locally enhanced by possibly many orders of magnitude in the range $\eta = 10^{-10} - 10^{-1}$. Our study is motivated by the possible survival of rare dense regions in the early universe, the most extreme of which, above a critical threshold, collapsed to form primordial black holes. If this occurred, one may expect there to also be a significant population of early-forming stars that formed in similar but subthreshold patches. We derive a range of element abundance signatures by performing BBN simulations at high values of the baryon-to-photon ratio that may be detectable in any surviving first generation stars of around a solar mass. Our predictions apply to metal-poor galactic halo stars, to old globular star clusters and to dwarf galaxies, and we compare with observations in each of these cases.

Trans-iron Ge, As, Se, and heavier elements in the dwarf metal-poor stars HD 19445, HD 84937, HD 94028, HD 140283, and HD 160617

Context. The spectra of unevolved metal-poor halo stars uniquely reflect the elemental abundances incorporated during the earliest Galactic epoch. Their heavy-element content is well understood as the products of neutron capture on iron-peak elements. However, for the lightest trans-iron elements with atomic number 30 < Z < 52, they show striking abundance patterns that defy model predictions. Understanding their sources may illuminate the diverse halo, thick disk, or extragalactic origins of metal-poor stars. Aims. The primary goal is the derivation of halo dwarf abundances and their uncertainties for six trans-iron elements from UV spectra, plus optical abundances for four additional trans-Fe elements and two well-understood heavier elements. Methods. For five metal-poor dwarfs, we analyzed high-resolution UV spectra from the Hubble Space Telescope Imaging Spectrograph, supplemented by archival optical echelle spectra. Two independent analyses adopted different programs, models, and line lists, clarifying systematic errors. Results. The results from the separate UV analyses are in good agreement. The largest source of discrepancy is the placement of the UV continuum. Once rectified, the separate results agree to 0.2 dex for moderately unblended, moderately strong lines. Similar agreement is found with previous works, except where new data and line selection become important, notably our exclusion of trans-Fe lines blended by newly identifed Fe I lines. Conclusions. Improved line lists lead to low As/Ge ratios that no longer require an early arsenic enhancement. All five stars exhibit a high Mo/Ge abundance ratio, independent of Mo/Fe. The trans-Fe elements show an odd-even effect: an odd-Z element abundance is depressed relative to those of adjacent even-Z elements. Its suggested metallicity dependence is supported by previous studies of Sr-Y-Zr. Some theoretical yields show a metallicity-dependent odd-even effect, but none have predicted a constant Mo/Ge abundance ratio. Our work thus highlights the complexity of predicting the production of light trans-Fe elements in metal-poor stars.

Chemical abundance analysis of extremely metal-poor stars in the Sextans dwarf spheroidal galaxy

Context.Metal-poor components of dwarf galaxies around the Milky Way could be remnants of the building blocks of the Galactic halo structure. Low-mass stars that are currently observed as metal-poor stars are expected to have formed in chemically homogeneous clusters in the early phases of galaxy formation. They should have already disintegrated and should exhibit large scatter in abundance ratios of some sets of elements (e.g., Sr/Ba) in the Milky Way field stars. However, chemical abundance ratios are expected to cluster in very metal-poor stars in dwarf galaxies because the number of clusters formed in individual galaxies in the very early phase is expected to be quite limited.Aims.We examine the possible clustering of abundance ratios of Sr and Ba in the Sextans dwarf galaxy to test for the clustering star formation scenario.Methods.We investigate a total of 11 elements (C, Mg, Ca, Sc, Ti, Cr, Mn, Ni, Zn, Sr, Ba) in five stars in the Sextans dwarf galaxy. Previous studies suggest that these have similar abundance ratios. In this study, we focus on the abundance ratio of Sr to Ba. The observations are based on high-resolution spectroscopy (R= 40 000) using the Subaru Telescope High Dispersion Spectrograph.Results.The distribution ofα/Fe abundance ratios of the Sextans dwarf galaxy stars is slightly lower than the average of the values of stars in the Galactic halo. The Sr/Ba abundance ratios for the five metal-poor stars are in good agreement, and this clumping is distinctive compared to the [Sr/Ba] spread seen in the metal-poor halo stars. We find that the probability of such clumping is very small if the Sextans stars have distributions of Sr and Ba abundances similar to halo stars.Conclusions.In the Sextans dwarf galaxy, five out of six of the extremely metal-poor stars for which abundance ratios are well studied so far show clear clustering in abundance ratios including Sr/Ba. These observations tend to support the hypothesis that these stars were formed from a cloud of homogeneous chemical composition.