Ba Abundances Research Articles

Chemical enrichment by collapsars as the origin of the unusually high [Ba/Fe] in a massive star cluster of the dwarf galaxy NGC 1569

Abstract The super star cluster NGC1569-B has recently been observed to have an extremely high [Ba/Fe]. We consider that the observed high [Ba/Fe] (∼1.3) is due to the chemical enrichment of giant molecular clouds by either collapsars, neutron star mergers, or magneto-rotational supernovae, and thereby investigate which of the three polluters can best reproduce the observed [Ba/Fe]. Since it is found that collapsars can best reproduce such an extremely high Ba abundance, we numerically investigate the star cluster formation in NGC1569 using chemodynamical simulations of merging dwarf galaxies with chemical enrichment by collapsars. The principal results are as follows. First, a cluster of the same scale as NGC1569-B was found to match both the observed [Ba/Fe] and [Fe/H] values, the best cluster having [Ba/Fe]=1.3 ± 0.2 and [Fe/H] =−0.7 ± 0.2. This simulation used a core-collapse supernova per collapsar rate of 70, a standard initial mass function and an initial metallicity of [Fe/H]=-1.5. Second, a prediction of the Eu abundance of NGC1569-B is made: [Eu/Fe]=1.9 ± 0.2. These results are shown to be invariant under a change in the orbit parameters used for the merger. The need for a merger to promote the star formation that leads to the synthesis of the Ba and the star cluster formation is confirmed. Collapsars can not only better explain [Ba/Fe] but also be consistent with the observed star formation rate and stellar mass of the dwarf galaxy.

Gaia-Sausage-Enceladus star formation history as revealed by detailed elemental abundances. An archival study using SAGA data

The Gaia-Sausage-Enceladus merger was a major event in the history of the Milky Way. Debris from this merger has been extensively studied with full kinematic data from the Gaia mission. Understanding the star formation history of the progenitor galaxy aids in our understanding of the evolution of the Milky Way and galaxy formation in general. We aimed to constrain the star formation history of the Gaia-Sausage-Enceladus progenitor galaxy using elemental abundances of member stars. Previous studies on Milky Way satellite dwarf galaxies show that key elemental abundance patterns, which probe different nucleosynthetic channels, reflect the host galaxy's star formation history. We gathered Mg, Fe, Ba, and Eu abundance measurements for Gaia-Sausage-Enceladus stars from the SAGA database. Gaia-Sausage-Enceladus members were selected kinematically. Inspired by previous studies, we used Fe/Mg Ba/Mg Eu/Mg and Eu/Ba as a function of Fe/H to constrain the star formation history of Gaia-Sausage-Enceladus. We used the known star formation histories and elemental abundance patterns of the Sculptor and Fornax dwarf spheroidal galaxies as a comparison. The elemental abundance ratios of Fe/Mg Ba/Mg Eu/Mg and Eu/Ba all increase with Fe/H in Gaia-Sausage-Enceladus. The Eu/Mg begins to increase at Fe/H -2.0$ and continues steadily, contrasting with the trend observed in the Sculptor dSph galaxy. The Eu/Ba increases and remains high across the Fe/H range, unlike the pattern seen in the Sculptor dSph galaxy, and deviates from the Fornax dSph galaxy at high Fe/H . The Ba/Mg is higher than those of the Sculptor dSph galaxy at the lowest Fe/H and gradually increases, similar to the Fornax dSph galaxy. We constrained three main properties of the Gaia-Sausage-Enceladus star formation history: 1) star formation started gradually, 2) it extended for over 2 Gyr, and 3) it was quenched around Fe/H of $-0.5$, likely when it fell into the Milky Way. We show that the elemental abundance ratios Fe/Mg Ba/Mg Eu/Mg and Eu/Ba can be used to trace the star formation history of a disrupted galaxy when these measurements are available over an Fe/H range that is representative of the progenitor galaxy's stellar population.

WIYN Open Cluster Study. XC. Barium Surface Abundances of Blue Straggler Stars in the Open Clusters NGC 7789 and M67

We investigate barium (Ba) abundances in blue straggler stars (BSSs) in two open clusters, NGC 7789 (1.6 Gyr) and M67 (4 Gyr), as signatures of asymptotic-giant-branch (AGB) mass transfer. We combine our findings with previous Ba abundance analyses of NGC 6819 (2.5 Gyr) and NGC 188 (7 Gyr). Out of 35 BSSs studied in NGC 7789, NGC 6819, and M67, 15 (43% ± 11%) are Ba enriched; no BSSs in NGC 188 are Ba enriched. The Ba abundances of enriched BSSs show an anticorrelation with cluster age, ranging from an enrichment of [Ba/Fe] ∼ +1.5 dex in NGC 7789 to [Ba/Fe] ∼ +1.0 dex in M67. The Ba-enriched BSSs all lie in the same region of the H-R diagram, irrespective of cluster age or distance from the main-sequence turnoff. Our data suggest a link between AGB donor mass and mass-transfer efficiency in BSSs, in that less massive AGB donors tend to undergo more conservative mass transfer. We find that 40% ± 16% of the Ba-enriched BSSs are in longer-period spectroscopic binaries with orbital periods less than 5000 days. Those Ba-enriched BSSs that do not exhibit radial-velocity variability suggest AGB mass transfer in wide binaries by either wind mass transfer or wind Roche-lobe overflow. Given the preponderance of long orbital periods in the BSSs of M67 and NGC 188 and the frequency of Ba enrichment in NGC 7789, NGC 6819, and M67, it may be that AGB mass transfer is the dominant mechanism of BSS formation in open clusters older than 1 Gyr.

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.

Chemical Analysis of the Brightest Star of the Cetus II Ultrafaint Dwarf Galaxy Candidate* * This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

We present a detailed chemical abundance analysis of the brightest star in the ultrafaint dwarf (UFD) galaxy candidate Cetus II from high-resolution Magellan/MIKE spectra. For this star, DES J011740.53-173053, abundances or upper limits of 18 elements from carbon to europium are derived. Its chemical abundances generally follow those of other UFD galaxy stars, with a slight enhancement of the α-elements (Mg, Si, and Ca) and low neutron-capture element (Sr, Ba, and Eu) abundances supporting the classification of Cetus II as a likely UFD. The star exhibits lower Sc, Ti, and V abundances than Milky Way (MW) halo stars with similar metallicity. This signature is consistent with yields from a supernova originating from a star with a mass of ∼11.2 M ⊙. In addition, the star has a potassium abundance of [K/Fe] = 0.81, which is somewhat higher than the K abundances of MW halo stars with similar metallicity, a signature that is also present in a number of UFD galaxies. A comparison including globular clusters and stellar stream stars suggests that high K is a specific characteristic of some UFD galaxy stars and can thus be used to help classify objects as UFD galaxies.

Non-LTE abundance analysis of A-B stars with low rotational velocities – II. Do A-B stars with normal abundances exist?

ABSTRACT We present chemical composition and fundamental parameters (the effective temperature, surface gravity, and radius) for four sharp-lined A-type stars γ Gem (HD 41705), o Peg (HD 214994), θ Vir (HD 114330), and ν Cap (HD 193432). Our analysis is based on a self-consistent model fitting of high-resolution spectra and spectrophotometric observations over a wide wavelength range. We refined the fundamental parameters of the stars with the sme (Spectroscopy Made Easy) package and verified their accuracy by comparing with the spectral energy distribution and hydrogen line profiles. We found Teff/log g = 9190 ± 130 K/3.56 ± 0.08, 9600 ± 50 K/3.81 ± 0.04, 9600 ± 140 K/3.61 ± 0.12, and 10200 ± 220 K/3.88 ± 0.08 for γ Gem, o Peg, θ Vir, and ν Cap, respectively. Our detailed abundance analysis employs a hybrid technique for spectrum synthesis based on classical model atmospheres calculated in local thermodynamic equilibrium (LTE) assumption together with the non-LTE (NLTE) line formation for 18 of 26 investigated species. Comparison of the abundance patterns observed in A stars of different types (normal A, Am, and Ap) with similar fundamental parameters reveals significant abundance diversity that cannot be explained by the current mechanisms of abundance peculiarity formation in stellar atmospheres. We found a rise of the heavy element (Zn, Sr, Y, Zr, and Ba) abundance excess up to +1 dex with Teff increasing from 7200 to 10 000 K, with a further decrease down to solar value at Teff = 13 000 K, indicating that stars with solar element abundances can be found among late B-type stars.

Chronology of the chemical enrichment of the old Galactic stellar populations

Context. Over its history, the Milky Way has accreted several smaller satellite galaxies. These mergers added stars and gas to the Galaxy and affected the properties of the pre-existing stellar populations. Stellar chemical abundances and ages are needed to establish the chronological order of events that occur before, during, and after such mergers. Aims. We report the precise ages (∼6.5%) and chemical abundances for the TITANS, a sample of old metal-poor dwarfs and subgiants with accurate atmospheric parameters. We also obtain ages with an average precision of 10% for a selected sample of dwarf stars from the GALAH survey. We use these stars, located within ∼1 kiloparsec of the Sun, to analyse the chronology of the chemical evolution of in situ and accreted metal-poor stellar populations. Methods. We determined ages via isochrone fitting. For the TITANS, we determined Mg, Si, Ca, Ti, Ni, Ba, and Eu abundances using spectrum synthesis. The [Mg/Fe] abundances of the GALAH stars were re-scaled to be consistent with the abundances of the TITANS. We separated stellar populations by primarily employing chemical abundances and orbits. Results. We find that star formation in the so-called Gaia-Enceladus or Gaia-Sausage galaxy, the last major system to merge with the Milky Way, lasted at least 3 billion years and got truncated 9.6 ± 0.2 billion years ago. This marks with a very high level of precision the last stage of its merging process. We also identified stars of a heated metal-poor in-situ population with virtually null net rotation, probably disturbed by several of the early Milky Way mergers. We show that this population is more metal-rich than Gaia-Enceladus at any moment in time. Conclusions. The sequence of events uncovered in our analysis supports the hypothesis that Gaia-Enceladus truncated the formation of the high-α disc and caused the gas infall that forms the low-α disc.

The effect of variations in cooling rates on mineral compositions in mid-ocean ridge basalts

We present textural and chemical analyses of minerals from a 150 m thick sequence of submarine mid-ocean ridge basalts from the South China Sea in order to showcase the effect of variations in magma cooling rates on mineral texture and mineral composition. Groundmass plagioclase and clinopyroxene show gradual changes in texture and composition as flows grade from slowly cooled, meter-thick massive flows to more rapidly cooled dm-thick pillow lobes with quenched glassy margins. The corresponding change in cooling-rate is estimated to vary from <1 to ≥100 °C/h. With increasing cooling rate, plagioclase forms elongated, sector-zoned swallow-tail crystals. Sector zoning is associated with increasing FeO (up to 1.5 wt%) and MgO (up to 0.6 wt%) abundances at near-constant anorthite (An), related to a two orders of magnitude increase in growth rate. Sr, Ba, Li and Ti abundances remain constant and appear unaffected by kinetic factors at such growth rates. With increasing cooling rate clinopyroxene becomes dendritic, and its composition is sensitive to changes in crystallization conditions. Increasing cooling rate (<1 to ≥100 °C/h) leads to increasing Al2O3 (average of 3.2 to 4.3 wt%), TiO2 (1.3 to 2.8 wt%) and Na2O (0.37 to 0.44 wt%) and a decrease in SiO2 (50.1 to 46.4 wt%) and Mg#Fetot, i.e., molar MgO/(MgO + FeOtot), from 71.3 to 53.1. Trace element abundances (Y, Zr, Ce, V, Sr) in clinopyroxene increase by up to an order of magnitude at cooling rates ≥100 °C/h and become more heterogeneous spatially. These results support experimental evidence that rapid crystal growth leads to significant departure of mineral compositions from equilibrium, in particular for clinopyroxene. Although plagioclase composition remains relatively insensitive to changes in growth conditions at the studied cooling rates, the sensitivity of clinopyroxene composition to growth rates imply that it should be used with caution as a tool to infer magmatic crystallization conditions.

Melting and melt segregation processes controlling granitic melt composition

Abstract Several important processes in the petrogenesis of granite are still debated due to a poor understanding of complex interactions between minerals during the melting and melt segregation processes. To promote an improved understanding of the mineral-melt relationships, we present a systematic petrographic and geochemical analysis for melanosome and leucosome samples from the Triassic Jindong migmatite, South China. Petrographic observations and zircon U-Pb geochronology indicate that the Jindong migmatite was formed through water-fluxed melting of the Early Paleozoic gneissic granite (437 ± 2 Ma) during the Triassic (238 ± 1 Ma), with the production of melt dominated by the breakdown of K-feldspar, plagioclase, and quartz. The Jindong leucosomes may be divided into lenticular and net-structured types. Muscovite, plagioclase, and K-feldspar in the net-structured leucosome show higher Rb and much lower Ba and Sr contents than those in the lenticular leucosome. This may be attributed to the elevation of Rb and decreasing Ba and Sr abundances in melts during the segregation process due to early fractional crystallization of K-feldspar and plagioclase. These leucosomes show negative correlation between εNd(t) and P2O5, reflecting increasing dissolution of low-εNd(t) apatite during the melting process. The continuous dissolution of apatite caused saturation of monazite and xenotime in melt, resulting in the growth of monazite and xenotime around apatite in the melanosome. This process led to a sharp decrease of Th, Y, and REE with increasing P2O5 in the leucosome samples. This complex interplay of accessory mineral reactions in the source impacts REE geochemistry and Nd isotope ratios of granites. As the granites worldwide exhibit similar compositional and isotopic patterns to the Jindong leucosomes, we suggest that both the melting and melt segregation processes strongly control the granitic melt compositions.

The Barium Odd Isotope Fractions in Seven Ba Stars

Based on the spectra with high resolution and a high signal-to-noise ratio, we investigate the enrichment history of the s-process element in seven barium (Ba) stars by measuring their Ba odd isotope fraction. It is found that the relative contributions of the s-process to their Ba abundance are 91.4±25.7%, 91.4±34.3%, 82.9±28.5%, 77.1±31.4%, and 71.4±37.1% for REJ 0702+129, HD 13611, BD+80°670, HR 5692, and HD 202109, respectively. Our results suggest that these five Ba stars have a prominent s-process signature, which indicates that their heavy elements mainly come from their former AGB companions (now WDs) by mass transfer, while the r-process contribution can naturally be explained by the evolution of the Milky Way. The s-process contribution of BD+80°670 is 51.4±31.4%, which is the lowest among our seven sample stars. Considering its lower values of both [Ba/Nd] and [Ba/Eu], we suspect that BD+68°1027 is likely to be a r-rich Ba star and has similar origins to the CEMP-r/s stars. HD 218356 has an unreasonable s-process contribution over 100%. Combining its stellar atmospheric parameters and the evolutionary stage, we speculate that HD 218356 is a more evolved extrinsic Ba star, and its massive companion should have the largest s-process efficiency in our samples.

Integrative systems analysis identifies genetic and dietary modulators of bile acid homeostasis

SummaryBile acids (BAs) are complex and incompletely understood enterohepatic-derived hormones that control whole-body metabolism. Here, we profiled postprandial BAs in the liver, feces, and plasma of 360 chow- or high-fat-diet-fed BXD male mice and demonstrated that both genetics and diet strongly influence BA abundance, composition, and correlation with metabolic traits. Through an integrated systems approach, we mapped hundreds of quantitative trait loci that modulate BAs and identified both known and unknown regulators of BA homeostasis. In particular, we discovered carboxylesterase 1c (Ces1c) as a genetic determinant of plasma tauroursodeoxycholic acid (TUDCA), a BA species with established disease-preventing actions. The association between Ces1c and plasma TUDCA was validated using data from independent mouse cohorts and a Ces1c knockout mouse model. Collectively, our data are a unique resource to dissect the physiological importance of BAs as determinants of metabolic traits, as underscored by the identification of CES1C as a master regulator of plasma TUDCA levels.

The Pristine survey – XVII. The C-19 stream is dynamically hot and more extended than previously thought

ABSTRACT The C-19 stream is the most metal-poor stellar system ever discovered, with a mean metallicity [Fe/H] = −3.38 ± 0.06. Its low metallicity dispersion (σ[Fe/H] &amp;lt; 0.18 at the 95 per cent confidence level) and variations in sodium abundances strongly suggest a globular cluster origin. In this work, we use Very Large Telescope (VLT)/UV–Visual Echelle Spectrograph (UVES) spectra of seven C-19 stars to derive more precise velocity measurements for member stars, and to identify two new members with radial velocities and metallicities consistent with the stream’s properties. One of these new member stars is located 30° away from the previously identified body of C-19, implying that the stream is significantly more extended than previously known and that more members likely await discovery. In the main part of C-19, we measure a radial velocity dispersion σv = 6.2$^{+2.0}_{-1.4}{\rm \, km\, s^{-1}}$ from nine members, and a stream width of 0.56° ± 0.08°, equivalent to ∼158 pc at a heliocentric distance of 18 kpc. These confirm that C-19 is comparatively hotter, dynamically, than other known globular cluster streams and shares the properties of faint dwarf galaxy streams. On the other hand, the variations in the Na abundances of the three newly observed bright member stars, the variations in Mg and Al for two of them, and the normal Ba abundance of the one star where it can be measured provide further evidence for a globular cluster origin. The tension between the dynamical and chemical properties of C-19 suggests that its progenitor experienced a complex birth environment or disruption history.

MUSE spectroscopic observations of the young massive cluster NGC 1850

Context. The double cluster NGC 1850 in the Large Magellanic Cloud is the nearest young massive cluster of the Local Group with a mass similar to those of Galactic globular clusters. Recent studies have revealed an extended morphology of its main-sequence turn-off (MSTO), which can be interpreted as a spread in either age or internal rotation. In spite of its proximity, an accurate spectroscopic determination of its chemical properties is still missing. Aims. We aim to investigate the general chemistry and the kinematics of this stellar system to test whether possible signs of multiple populations are observable in this cluster. Methods. We analysed the spectra obtained with MUSE in adaptive optics mode of 1167 stars in both components of this cluster (NGC 1850A and NGC 1850B). Thanks to this data set, we were able to measure accurate global metallicities, Ba abundances, and radial velocities for a sample of 38 red supergiants and a guess of the oxygen abundance in the brightest turn-off stars belonging to NGC 1850A. Results. We find an average metallicity of ⟨[M/H]⟩ = −0.31 ± 0.01, a mean Ba abundance of ⟨[Ba/Fe]⟩ = +0.40 ± 0.02, and a systemic radial velocity of ⟨vLOS⟩ = 251.1 ± 0.3 km s−1. The dispersion of the radial velocities suggests a dynamical mass of log(M/M⊙) = 4.84 ± 0.1, while no significant systemic rotation is detected. We detect a significant bimodality in O I line strength among the turn-off stars of NGC 1850A with ∼66% of stars with [O/Fe]∼ − 0.16 and the rest with no detectable line. The majority of O-weak stars preferentially populate the red side of the MSTO and show H lines in emission, suggesting that they are Be stars rotating close to their critical velocity. Among normal MSTO stars, red stars have broader line profiles than blue ones, on average, suggesting a correlation between colour and rotational velocity. Conclusions. The mean metallicity of this cluster lies at the metal-rich side of the metallicity distribution of the Large Magellanic Cloud following its age-metallicity relation. The Ba and O abundances agree with those measured in the bar of this galaxy. The correlation between line broadening and colour suggests that the observed colour spread among turn-off stars can be due to a wide range in rotational velocity covered by these stars.

The influence of black shale weathering on riverine barium isotopes

Barium and its isotopes are receiving growing interest in their capacity to record the variation of the oceanic biological pump through geological timescales. Nonetheless, only little is known about the continental sources and processes that can drive the Ba flux and isotope composition to the ocean. Whereas the role of processes such as secondary phase formation, sorption and biological uptake on Ba isotopes has been recently investigated, the potential of various rock sources in producing a range of Ba isotope compositions remains poorly known. In particular, black shales often exhibit enrichment in Ba that might potentially be associated with various Ba isotope signatures.In this study, we report the isotope composition of Ba in rivers with different relative contribution of black shale rocks in their catchments, located in the Mackenzie Basin (Northwest Canada). Both the dissolved and solid loads of the Mackenzie tributaries are enriched in heavy Ba isotopes with respect to the continental crust, which is consistent with an additional source to the weathering of igneous and siliciclastic rocks. Although the dissolved Ba abundance is partially driven by weathering processes, the river dissolved Ba isotope composition rather reflects a binary mixing between 1) a “classical” siliciclastic source (with a Ba isotope composition close to that of the upper continental crust δ138Ba ∼ 0.15 ± 0.05‰) and 2) an isotopically heavier source especially present in the Mackenzie Mountains (with a δ138Ba ∼ 0.40‰). The positive relationships between dissolved δ138Ba and radiogenic osmium isotope ratios, as well as with sulphuric acid production, indicate that the heavy Ba source is tightly linked to the weathering of black shale. Although the exact reason for such heavy Ba isotope signatures remains elusive, this study emphasises the potential of black shale weathering in shifting the Ba isotope composition of the dissolved flux to the ocean towards heavier values.

TYC5594-576-1: R-PROCESS ENRICHMENT METAL-POOR STAR

Atmospheric parameters and elemental abundances of metal-poor Population II star TYC5594-576-1 ([Fe/H] = –2.8) have been studied, including the elements of neutron (n-) capture processes, as an important part of the enrichment sources of early Galaxy. Na, Mg, Al, Co, Sr, Y, Zr, Mo, Ba, La, Ce, Pr, Sm, Eu, Gd, Dy, Os, and Th abundances were determined using the synthetic spectrum method, taken into account the hyperfine structure (HFS) for the Ba II, La II and Eu II lines. The abundances of Si, Ca, Sc, N were determined based on the equivalent widths of their lines. The carbon abundance was obtained by the molecular synthesis fitting for the CH region of 4300-4330 ÅÅ. For the abundances determinations of C, Na, Mg, Al, Ba, and Th the NLTE corrections have been applied.We have determined the abundances of several n- capture elements for the first time and found that the behaviour of these elements abundances shows a significant trend with increasing atomic number. The elements ratios of [Eu/Fe] = 1.85, [Ba/Eu] = –1.24, [Sr/Ba] = –1.04 confirm the status of TYC5594-576-1 as a r-process enrichment star, with lower strontium [Sr/Fe] = –0.33 and higher thorium [Th/Fe] = 1.28 abundances. The obtained europium and thorium excesses testifies to the early enrichment of the Galaxy by the r-process elements as a result of the merger of neutron stars or black holes. The carbon abundance confirms the effect of canonical additional mixing in this star.

Abundance of barium in the atmospheres of red giants in the Galactic globular cluster NGC 104 (47 Tuc)

Context. While most (if not all) Type I Galactic globular clusters (GGCs) are characterised by spreads in the abundances of light chemical elements (e.g. Li, N, O, Na, Mg, Al), it is not yet well established whether similar spreads may exist in s-process elements as well. Aims. We investigated the possible difference in Ba abundance between the primordial (1P) and polluted (2P) stars in the Galactic globular cluster (GGC) 47 Tuc (NGC 104). For this purpose, we obtained homogeneous abundances of Fe, Na, and Ba in a sample of 261 red giant branch (RGB) stars, which comprises the largest sample used for Na and Ba abundance analysis in any GGC so far. Methods. Abundances of Na and Ba were determined using archival GIRAFFE/VLT spectra and 1D non-local thermodynamic equilibrium (NLTE) abundance analysis methodology. Results. Contrary to the finding of Gratton et al. (2013, A&amp;A, 549, A41), we did not detect any significant Ba–Na correlation or 2P–1P Ba abundance difference in the sample of 261 RGB stars in 47 Tuc. This corroborates the result of D’Orazi et al. (2010, ApJ, 719, L213), who found no statistically significant Ba–Na correlation in 110 RGB stars in this GGC. The average barium-to-iron ratio obtained in the sample of 261 RGB stars, ⟨[Ba/Fe]1D NLTE⟩ = −0.01 ± 0.06, agrees well with those determined in Galactic field stars at this metallicity and may therefore represent the abundance of primordial proto-cluster gas that has not been altered during the subsequent chemical evolution of the cluster.

Sr and Ba abundances: Comparing machine-learning with star-by-star analyses

Context. A new large sample of 895 s-process-rich candidates out of 454 180 giant stars surveyed by LAMOST at a low spectral resolution (R ~ 1800) has been reported by Norfolk et al. (2019, MNRAS, 490, 2219; hereafter N19). Aims. This study is aimed at confirming the s-process enrichment at the higher resolution (R ~ 86 000) offered by the HERMES-Mercator spectrograph for the 15 brightest targets of the N19 sample, which consists of 13 Sr-only stars and two Ba-only stars (designating stars with only the Sr or only Ba lines strengthened). Methods. Abundances were derived for elements Li, C (including the 12C/13C isotopic ratio), N, O, Na, Mg, Fe, Rb, Sr, Y, Zr, Nb, Ba, La, and Ce, using the TURBOSPECTRUM radiative transfer LTE code with MARCS model atmospheres. Binarity has been tested by comparing the Gaia DR2 radial velocity (epoch 2015.5) with the HERMES velocity obtained 1600–1800 days (about 4.5 yr) later. Results. Among the 15 programme stars, 4 show no s-process overabundances ([X/Fe] &lt; 0.2 dex), 8 show mild s-process overabundances (at least three heavy elements with 0.2 &lt; [X∕Fe] &lt; 0.8), and 3 have strong overabundances (at least three heavy elements with [X/Fe] ≥ 0.8). Among the 13 stars classified as Sr-only by the previous investigation, 4 have no s-process overabundances, 8 are mild barium stars, and 1 is a strong barium star. The two Ba-only stars turn out to be both strong barium stars. Especially noteworthy is the fact that these two are actually dwarf barium stars. Two among the three strong barium stars show clear evidence in support of their binary character, as expected for objects produced through mass-transfer. The results for the no s-process and mild barium stars are more surprising; namely, among the no-s stars, there are two binaries out of four, whereas only one out the eight diagnosed mild barium stars shows a clear signature of radial-velocity variations. Conclusions. Blending effects and saturated lines have to be considered very carefully when using machine-learning techniques, especially when applied to low-resolution spectra. Among the Sr-only stars from the previous study sample, about 60% (8/13) of them can be expected to be true mild barium stars and about 8% to be strong barium stars; this fraction is likely close to 100% for the N19 Ba-only stars (2/2). Therefore, we recommend to limit the sample to N19 Ba-only stars when one needs an unpolluted sample of mass-transfer (i.e., extrinsic) objects.

Barium lines in high-quality spectra of two metal-poor giants in the Galactic halo

Context. Theoretical results showed the possibility that neutron capture elements were produced in the early Universe by two different sources: a frequent s-process source hosted by rotating massive stars, and a rare r-process source hosted most likely by neutron star mergers. The two sources produce barium with different isotopic compositions. Aims. We aim to investigate the lines of barium in two halo stars, HD 6268 and HD 4306. The spectra present an exquisite quality, both in terms of resolution (R &gt; 100 000) and signal-to-noise (~400). Due to hyperfine splitting (hfs) effects, barium lines are expected to show slightly different profiles depending on the barium isotopic fraction. Methods. We applied a standard local thermodynamic equilibrium synthesis of the barium lines. We compared the synthetic results assuming an s-process isotopic pattern or an r-process isotopic pattern for the two barium lines for each star that exhibited hfs. We also applied a methodology, less dependent on the accuracy of the theoretical Ba hfs structure, that transforms the lines of HD 4306 into those we would observe if its atmospheric parameter values (i.e. Teff, log g, micro- and macro-turbulence, V sin i, and Ba abundance) were the same as those of HD 6268. Results. With both methods, our results show that the barium lines with hfs effects of HD 4306 are in agreement with an s-process composition and the lines in HD 6268 have a different profile, which is most likely linked to the presence of an r-process isotopic pattern. Conclusions. Two lines of barium of HD 6268 and HD 4306 seem to confirm the theoretical expectation that both r-process events and also s-process contribution by rotating massive stars have polluted the ancient halo of our Galaxy.

Evolution of pyrochlore in carbonatites of the Amba Dongar complex, India.

AbstractPyrochlore-group minerals are common accessory rare-metal bearing minerals in the calcite and ankerite carbonatites of the Amba Dongar complex (India). Pyrochlore from the Amba Dongar carbonatites differs from that in other Indian complexes in Ta, Zr, Ti, rare earth element (REE) and Pb contents, but is similar with respect to Ca, Ba and Sr abundances. The evolution of pyrochlore composition was studied to understand the alteration processes and the formation of late-stage pyrochlores enriched in REE and Pb. The early magmatic pyrochlore are calcio- and niobium-dominant types and were replaced by secondary cation-deficient varieties as a consequence of the action of hydrothermal fluids and supergene weathering. These processes produce changes mainly at theAsite, rarely at theBsite, and the original F is replaced by OH–groups. Calcium and Na can be extracted from the structure at the alteration stage and charge balance is achieved by the introduction of REE, Th, U, Ba or Sr. At the latest supergene stages, marginal and fractured zones of pyrochlore grains are altered to Pb-rich, Si-rich and cation-deficient hydrated varieties. The magmatic pyrochlore was crystallised in a highly alkaline environment at a high activity of Ca and at temperatures near 600°C, the alteration of pyrochlore began in a hydrothermal environment at temperatures below 350°C. The major compositional changes that are associated with the alteration are summarised by the following reactions: Ca2++ Nb5+→REE3++ Ti4+; Nb5++ Fe3+→ Ti4++ Zr4+; and 2Nb5++ Ca2+→ Ti4++ Si4++ U4+.

Evidence for r-process Delay in Very Metal-poor Stars

Abstract The abundances of r-process elements of very metal-poor stars capture the history of the r-process enrichment in the early stage of star formation in a galaxy. Currently, various types of astrophysical sites including neutron star mergers (NSMs), magneto-rotational supernovae, and collapsars, are suggested as the origin of r-process elements. The time delay between the star formation and the production of r-process elements is the key to distinguish these scenarios, with the caveat that the diffusion of r-process elements in the interstellar medium may induce the delay in r-process enrichment because r-process events are rare. Here we study the observed Ba abundance data of very metal-poor stars as the tracer of the early enrichment history of r-process elements. We find that the gradual increase of [Ba/Mg] with [Fe/H], which is remarkably similar among the Milky Way and classical dwarfs, Requires a significant time delay (100 Myr–1 Gyr) of r-process events from star formation rather than the diffusion-induced delay. We stress that this conclusion is robust to the assumption regarding s-process contamination in the Ba abundances because the sources with no delay would overproduce Ba at very low metallicities, even without the contribution from the s-process. Therefore, we conclude that sources with a delay, possibly NSMs, are the origins of r-process elements.