Wako Aoki

Context. We report a detailed abundance analysis of three extremely metal-poor (EMP) stars thanks to spectra obtained with the high-dispersion spectrograph (HDS) at the Subaru telescope. Aims. The aim is to observe a sample of EMP candidate stars selected from the Sloan Digital Sky Survey (SDSS) and determine their detailed chemical composition. Methods. High-resolution spectra of three faint turn-off stars using HDS on the Subaru telescope were acquired. We used standard 1D models to compute the abundances of several elements (Li, C, Mg, Ca, Sr, and Ba). The stars we analysed have a metallicity [Fe/H] of between –3.02 and –3.57 dex. We also measured accurate radial velocities that, coupled with the Gaia astrometric data, allowed us to compute actions and Galactic orbits for the stars. Results. Our analysis confirms that the stars are genuine EMP stars. One of them (SDSS J074238.68+470537.0) can be classified as a CEMP- s star with the highest [Mg/Fe] ratio ever measured in a turn-off CEMP- s star with [Mg/Fe] = 1.85 dex. For the star SDSS J012630.30+073029.3 ([Fe/H]= –3.02 dex), we have a clear detection of lithium with A(Li) = 2.05. Kinematically, SDSS J214340.08−002835.5 is a thick-disc star, while the other two belong to the halo and are candidate members of the Gaia-Sausage-Enceladus.

Kilonova spectra provide information on rapid neutron-capture nucleosynthesis in neutron star mergers. However, it remains challenging to identify individual elements in kilonova spectra, primarily due to incomplete atomic data for heavy elements at near-infrared wavelengths. Recent work demonstrated that spectra of chemically peculiar stars offer excellent laboratory for studying kilonova spectra. In this paper, to expand our understanding of strong transitions of heavy elements at near-infrared wavelengths, we present high-resolution near-infrared spectra of 14 chemically peculiar stars covering a range of effective temperatures, surface gravities, and abundance patterns, obtained with Subaru/IRD. The Sr ii, Ce iii, and Gd iii lines, previously suggested to be among the strongest transitions, are detected in the spectra of most of our targets, confirming their intrinsic importance. In addition, Eu ii and Nd iii lines are detected in several stars. By comparing observed equivalent widths and abundance ratios, we show that under a highly heavy-element-enriched abundance pattern, Eu ii and Nd iii lines may contribute to kilonova spectral features comparably to Sr ii. Importantly, we find no unidentified lines systematically stronger than these transitions despite the larger stellar sample. These results further support the primary importance of the Sr ii and Ce iii transitions and provide new insights into the potential contribution of Eu ii and Nd iii.

Recent large spectroscopic surveys combined with studies on dynamical motions of stars in the Milky Way halo reveal connections between chemical enrichment and galaxy formation. We have determined elemental abundances based on high resolution spectra obtained with the Subaru Telescope High Dispersion Spectrograph for more than 400 very metal poor stars identified by the LAMOST survey. Kinematics of these stars are analyzed combining Gaia astrometry data. This paper provides a brief overview of this program and reports on the results and problems found in the abundance distributions and correlations between elements (e.g., Ti and Sc abundances higher than predictions of standard chemical evolution models), mass and nucleosynthesis of first stars constrained by detailed elemental abundance ratios of extremely metal poor stars, and subgroups of stars identified by the abundance and kinematics data.

Metal-poor stars are crucially important for understanding the early Galaxy, first stars, and the Universe. In this series of papers, we present a homogeneous non–local thermodynamic equilibrium (NLTE) abundance analysis of 12 elements for 103 very metal-poor (VMP)/extremely metal-poor (EMP) stars with metallicity down to −4.3 dex. The sample was selected from the LAMOST survey and observed by the high-resolution spectroscopy of Subaru. In this paper, we present the NLTE abundances and evolution of lithium in these stars. We report different lithium behaviors corresponding to different evolutionary stages and their signatures: (1) The Spite plateau shows a slightly positive slope, indicating increasing lithium abundance with increasing metallicity. Most significantly, it appears to extend to lower metallicities as previously suggested, calling into question the reality of the so-called “meltdown” at low metallicity; (2) we confirm a lithium plateau for lower red giant branch (LRGB) stars with A(Li) = 1.13 dex in our sample, while the lithium abundance drops rapidly to A(Li) < 0.5 as stars continue to evolve to a higher stage. (3) We identify four Li-rich stars in our sample across different evolutionary stages, showing complex and multiple lithium production mechanisms in VMP/EMP stars. These findings suggest that early Galactic lithium enrichment results from a complex interplay between depletion and production processes.

We perform a homogeneous analysis of 82 heavy-element-enhanced metal-poor stars from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) /Subaru sample, including 64 r -process-enhanced stars, two limited- r stars, and 16 stars exhibiting significant or moderate carbon enhancement (comprising nine s -process-enhanced, two r -process-enhanced, two r / s , and three r + s stars). Systematic differences in [X/Eu] between r -I and r -II stars, together with their distinct dynamical properties, suggest that r -II stars likely formed in low-mass dwarf galaxies where contamination from additional nucleosynthetic channels was minimal. We have also discovered a number of peculiar objects. We identify an extremely metal-poor r -II star, J1158+0734 ([Fe/H] = −2.93), whose enhanced Zn abundance ([Zn/Fe] = +0.67) is best explained by yields from a high-energy, massive core-collapse supernova. The surface abundances of s -process-enhanced stars exhibit pronounced dispersion, consistent with enrichment from asymptotic giant branch (AGB) progenitors with diverse properties. Notably, one star, J2256+0215, reveals compelling evidence for additional evolutionary mixing, as indicated by its unusually low carbon abundance ([C/Fe] = +0.08) and extremely low ¹² C/ ¹³ C ratio of 2.33. Additionally, we identify three carbon-enhanced metal-poor (CEMP) r + s stars whose abundance patterns reveal combined contributions from both the r - and s -processes. We further propose a modified diagnostic scheme for the identification of CEMP -r + s stars, defined as −0.2 ≤ [La/Nd] ≤ 0.2 and −0.5 ≤ [Eu/Nd] ≤ 0.0.

We present a non–local thermodynamic equilibrium (NLTE) abundance analysis of the light neutron-capture elements Sr, Y, and Zr in a sample of 103 very metal-poor stars spanning −4.3 ≲ [Fe/H] ≲ −1.7, based on high-resolution Subaru/HDS spectra and atmospheric parameters from Paper I. This is the first NLTE study of Sr−Y−Zr abundances using a large, homogeneous sample optimized for tracing Galactic chemical evolution (GCE). We find a mild positive trend in [Sr, Y, Zr/Fe] with [Fe/H], consistent with GCE models that incorporate s -process, electron-capture supernovae, compact object mergers, and magnetorotational supernovae, without invoking additional light-element primary processes. We report a statistically significant correlation between [Sr/Fe] and [Na/Fe] in our stellar sample. This finding supports the contribution of fast-rotating massive stars (spinstars) to the simultaneous production of sodium through hydrogen burning and light neutron-capture elements via the weak s -process in low-metallicity environments. We identify five Sr-poor stars exhibiting significantly lower Sr, Y, and Zr abundances than other stars at similar [Fe/H], pointing to chemical inhomogeneities in early star-forming environments. These light neutron-capture elements therefore provide valuable tracers of early nucleosynthetic processes and the Milky Way’s assembly history.

We present the measurement of 26 elemental abundances of SMSS J022423.27−573705.1 (SMSS 0224–5737), an extremely metal-poor (EMP) star with a weak r -process signature. We report the measurements of N, O, V, Zn, and Ba, and the upper limits for Mo, Ru, Pd, Ag, and Eu for the first time. SMSS 0224–5737 exhibits low C abundance and high N and O abundances, suggesting that C is converted to N by the enhanced mixing during the evolution. The abundance pattern up to the Fe-peak elements is generally in good agreement with the average abundance of EMP stars, although a notable feature is the high [Zn/Fe] ratio ([Zn/Fe] = + 0.88). We confirm the enhancement of the first-peak neutron-capture elements (Sr, Y, and Zr) and determine a low Ba abundance [Ba/H] = −5.25, that is, [Ba/Fe] = −1.45. The extremely high ratio of [Zr/Ba] = +2.60 makes SMSS 0224–5737 the EMP star with the most pronounced weak r -process signature observed to date. The abundance pattern of the neutron-capture elements is compared with the yields from r -process nucleosynthesis models. The sharp decline in abundances beyond Zr disfavors neutron star merger or electron-capture supernova models, but is reproduced either by protoneutron star wind models or by magneto-rotational supernova models. Considering the high [Zn/Fe] ratio, a magnetorotational supernova is the most plausible origin of SMSS 0224–5737. This study demonstrates that the abundance measurements of both light and neutron-capture elements, even at low abundances, are crucial for unveiling the astrophysical sites of the weak r -process.

We present a potassium (K) abundance analysis in extremely metal-poor (EMP) stars based on high-resolution (R ∼ 60,000) spectra obtained with the High Dispersion Spectrograph on the Subaru Telescope, covering the K i resonance lines at 766 and 769 nm. One-dimensional local thermodynamic equilibrium (LTE) abundances of K and other elements, including Na, Mg, Ca, Ti, Cr, and Ni, were derived using spectral synthesis. Non-LTE (NLTE) corrections were applied to the K abundances by interpolating a precomputed grid of corrections based on stellar parameters and the LTE K abundance. We detected K i lines in seven stars with [Fe/H] < –3.0 and derived upper limits for other stars in the same metallicity regime, making this sample well-suited for investigating the nucleosynthesis origins of K in the early Universe. We found that the [K/Fe] and [K/Ca] ratios of the seven stars are enhanced relative to the solar value, with a scatter of ∼0.1 dex, as small as the typical measurement uncertainty. Under the assumption that each star formed from gas purely enriched by a single or a few massive star’s supernova, the small scatter in [K/Fe] and [K/Ca], contrasted with the ∼0.7 dex scatter in [Na/Mg] ratios (after NLTE correction), suggests that the production of K in massive stars or their supernovae is independent of the processes that drive the Na/Mg variation. These findings demonstrate that K abundances in EMP stars, and their correlations with other elemental abundances, can serve as sensitive tracers of the physical mechanisms governing the final evolutionary stages of massive stars and their supernova explosions.

We present the first detailed chemical abundances for seven GD-1 stream stars from Subaru/High Dispersion Spectrograph spectroscopy. Atmospheric parameters were derived via color calibrations (T _eff) and iterative spectroscopic analysis. LTE abundances for 14 elements (α, odd Z, iron peak, n-capture) were measured. Six stars trace the main orbit; one resides in a “blob.” All exhibit tightly clustered metallicities ([Fe/H] = −2.38; intrinsic dispersion smaller than 0.05 dex; average uncertainty is about 0.13 dex). While one star shows binary mass transfer signatures, the other six display consistent abundance patterns (dispersions < uncertainties). Their iron-peak elements (Sc, Cr, Mn, Ni) match Milky Way halo stars. In contrast, Y and Sr are systematically lower than halo stars of similar [Fe/H]. Significantly, six stars show consistently enhanced [Eu/Fe] ∼ 0.60 (σ = 0.08). A tight Ba–Eu correlation (r = 0.83, p = 0.04) exists, with [Ba/Fe] = −0.03 ± 0.05, indicating a common r-process origin. This extreme chemical homogeneity strongly supports an origin from a single disrupted globular cluster. The lack of light-element anticorrelations may stem from our sample size or the progenitor’s low mass.