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 the Tomo-e Gozen Bright Metal-Poor Star Survey (TeMPS), a wide-area narrow-band photometric survey for bright metal-poor stars in the northern sky. The survey uses the Tomo-e Gozen camera on the 1.05 m Kiso Schmidt telescope with four narrow-band filters centered on the Ca2 H and K lines, the CH G band, Hα, and a reference wavelength region. We review the survey strategy, photometric processing, and calibration of metallicity and carbon abundance estimates derived from narrow-band colors. We further present medium-resolution spectroscopic follow-up with Nayuta/MALLS to validate the photometric selection and identify new metal-poor stars. The current data set covers ≳ 22,000deg^2 in all four bands with a total integration time of ∼ 100 hr. The median limiting magnitudes at S/N = 20 correspond to G∼12.5. By combining narrow-band photometry with archival broad-band photometry and Gaia distances, we estimate T_ eff, log (g), metallicity, and carbon abundance. Calibration against literature abundances derived from high-resolution spectra shows typical scatters of < 0.3 dex in metallicity and < 0.4 dex in carbon abundance. We estimate metallicities for ∼ 1.7 million stars and identify ∼ 16,000 very metal-poor candidates with [M/H]_ NB,fin<-2. We show that Nayuta/MALLS medium-resolution spectra provide metallicities consistent with high-resolution measurements, with a scatter of ∼ 0.27 dex. Among 32 photometrically selected candidates followed up with MALLS, 24 are confirmed to have [M/H]_ MALLS< -2, including one newly identified star with [M/H]_ MALLS≃ -3.4. These results demonstrate that Tomo-e Gozen narrow-band photometry and MALLS medium-resolution spectroscopic follow-up efficiently select bright metal-poor stars for future high-resolution abundance studies. (abbreviated)

Oxygen abundances in very and extremely metal-poor (V/EMP) stars provide critical constraints on early massive stars' nucleosynthesis. An Oxygen abundance analysis is presented for 35 V/EMP stars (-4.0<[Fe/H]< -1.5) using near-infrared H-band OH vibro-rotational lines from high-resolution Subaru/IRD spectra. To examine the reliability of these NIR OH lines, the results are compared with the abundances obtained from the 3D/NLTE-insensitive forbidden [OI] 6300Å line using archival high-resolution optical spectra. After homogeneously rederiving stellar parameters and 1D/NLTE Fe abundances using Gaia photo-astrometry and literature optical Fe equivalent width data, oxygen abundance from OH and [OI] lines is determined through 1D/LTE spectral synthesis. A sensitivity analysis confirms that near-IR OH lines are highly sensitive to the adopted temperature compared to the forbidden line. A temperature-dependent discrepancy between the tracers is identified: in cool red giants (Teff <4600 K), OH-based abundances are systematically lower than [OI]-based abundance by 0.05 to 0.25 dex, while warmer red giants show higher OH-based abundances as expected from 3D effects. Despite this systematic offset, the numerous measurable NIR OH lines yield significantly smaller random abundance errors than that of the single, weak [OI] line. Leveraging this statistical precision, an empirical calibration as a function of Teff, log g, [Fe/H], and [C/Fe] is derived to align the 1D/LTE OH abundances onto the [OI] scale. Applying this correction substantially reduces the scatter and temperature dependence in the [O/Fe] versus [Fe/H] plane and flattens the trend, bringing the results into fairly good agreement with Galactic chemical evolution models.

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 12C/13C 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.

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 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.

Lithium is an ancient element that was first produced by the Big Bang Nucleosynthesis (BBN) a few minutes after the birth of the Universe. Lithium is a sensitive tracer for a number of processes in a variety of astrophysical environments due to its multi-channels of production and fragility. These features also make lithium a complex element at the center of many unsolved problems. The behavior of lithium in the low-mass evolved stars is one such issue. It is known that such stars not only destroy but also produce lithium, while neither the destruction details nor the production mechanisms over different evolutionary phases are clear. In this paper, we summarize the recent results obtained from the studies combining large-scale spectroscopic surveys, asteroseismology, and traditional high-resolution spectroscopy. We present the detailed evolution behaviors from the red giant branch (RGB) to the red clump (RC) phase characterized by the core helium-burning in the stellar interior. We show the new signatures discovered from the recent studies for the lithium-enhanced giants and also discuss various observational and theoretical constraints on lithium production in red clump stars.

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.

Abundances of five elements, Na, Mg, Al, Si, and Sr, are investigated for 44 very metal-poor stars (-4.0 < [Fe/H] < -1.5) in the Galactic halo system based on an Local Thermodinamic Equilibrium (LTE) analysis of high-resolution near-infrared spectra obtained with the Infrared Doppler instrument (IRD) on the Subaru Telescope. Mg and Si abundances are determined for all 44 stars. The Si abundances are determined from up to 29 lines, which provide reliable abundance ratios compared to previous results from a few optical lines. The Mg and Si of these stars are over-abundant, relative to iron, and are well-explained by chemical-evolution models. No significant scatter is found in the abundance ratios of both elements with respect to iron, except for a few outliers. The small scatter of the abundance ratios of these elements provides constraints on the variations of stellar and supernova's yields at very low metallicity. Al abundances are determined for 27 stars from near-infrared lines (e.g., 1312nm), which are expected to be less affected by non-LTE (NLTE) effects than optical resonance lines. The average of the [Al/Fe] ratios is close to the solar value, and no dependence on metallicity is found over -3.0 < [Fe/H] < -2.0. Na abundances are determined for 12 stars; they exhibit Solar abundance ratios and no dependence on metallicity. The Sr abundances determined from the Sr II triplet are significantly higher than those from the optical resonance lines obtained by previous studies for our sample. This discrepancy shows a clear dependence on temperature and surface gravity, supporting models that predict large NLTE effects on the near-infrared lines for metal-poor red giants.

We report the discovery of an actinide-boost, very metal-poor ($\left[\mathrm{Fe/H} \right]=-2.38$), $r$-process-enhanced ($\left[\mathrm{Eu/Fe} \right]=0.80$) star, LAMOST J0804+5740, within the Gaia-Sausage-Enceladus (GSE). Based on the high-resolution ($R\sim36,000\; and \;60,000$) and high signal-to-noise ratio spectra obtained with the High Dispersion Spectrograph on the Subaru Telescope, the abundances of 48 species are determined. Its $\log\epsilon\rm(\mathrm{Th}/\mathrm{Eu}) = -0.22 $ establishes it as the first confirmed actinide-boost star within the GSE. Comparative analysis of its abundance pattern with theoretical $r$-process models reveals that the magnetorotationally driven jet supernova $r$-process model with $\hat{L}v$ = 0.2 provides the best fit and successfully reproduces the actinide-boost signature. Kinematic analysis of actinide-boost stars reveals that approximately two-thirds of them are classified as $\textit{ex-situ}$ stars, suggesting that actinide-boost stars are more likely to originate from accreted dwarf galaxies. As the first actinide-boost star identified within the GSE, J0804+5740 will provide valuable insights into $r$-process nucleosynthesis in accreted dwarf galaxies like the GSE, especially on the production of the heaviest elements.

The observation of the kilonova AT2017gfo and investigations of its light curves and spectra confirmed that neutron star mergers are sites of r -process nucleosynthesis. However, the identification of elements responsible for the spectral features is still challenging, particularly at the near-infrared wavelengths. In this study, we systematically searched for all possible near-infrared transitions of heavy elements using experimentally calibrated energy levels. Our analysis reveals that most candidate elements with strong absorption lines are lanthanides ( Z = 57–71) and actinides ( Z = 89–103). This is due to their complex structures leading to many low-lying energy levels, which results in strong transitions in the near-infrared range. N. Domoto et al. (2022) have shown that La iii and Ce iii can explain the absorption features at λ ∼ 12000–15000 Å. While our results confirm that these two elements show strong infrared features, we additionally identify Gd iii as the next most promising species. Due to its unique atomic structure involving the half-filled 4 f and the outer 5 d orbitals, Gd iii has one of the lowest-lying energy levels, between which relatively strong transitions occur. We also find absorption lines caused by Gd iii in the near-infrared spectrum of a chemically peculiar star HR 465, which supports their emergence in kilonova spectra. By performing radiative transfer simulations, we confirm that Gd iii lines affect the feature at ∼12000 Å previously attributed to La iii . Future space-based time-series observations of kilonova spectra will allow the identification of Gd iii lines.

We present the analysis of four new extremely metal-poor turn-off stars thanks to high-resolution spectra obtained with the Subaru/HDS spectrograph. We determined the abundances and upper limits of Li, C, Mg, Ca, Sr, and Ba. Metallicities range from [Fe/H]=-3.3to-4.4dex. For one of the stars, we measure the lithium abundance log(Li/H)=2.1 +/- 0.2.. Two stars of the sample have low [alpha/Fe] abundances. The most metal-poor star of the sample with [Fe/H]=-4.42 dex has a high [Sr/Fe] abundance ratio[Sr/Fe]= 0.9 dex, a high value also found in HE 1327-2326. This star is the second most iron-poor star observed with Subaru telescope, suggesting that more ultra-metal-poor stars could be discovered using high-resolution spectrographs in the Northern hemisphere.

The r -process production in the early Universe has been well constrained by extensive studies of metal-poor stars. However, the r -process enrichment in the metal-rich regime remains poorly understood. In this study, we examine the abundance ratios of Th and Eu, which represent the actinides and lanthanides, respectively, for a sample of metal-rich disk stars. Our sample covers 89 giant stars in the Kepler field with metallicities −0.7 ≤ [Fe/H] ≤ 0.4 and ages ranging from a few hundred million years to approximately 14 Gyr. Age information for this sample is available from stellar seismology, which is essential for studying the radioactive element Th. We derived Th and Eu abundances through χ 2 fitting of high-resolution archival spectra ( R ≈ 80 000) obtained with the High Dispersion Spectrograph at the Subaru Telescope. We created synthetic spectra for individual stars using a 1D local thermodynamic equilibrium spectral synthesis code, Turbospectrum, adopting MARCS model atmospheres. Our study establishes the use of a less extensively studied Th II line at 5989 Â, carefully taking into account the blends of other spectral lines to derive the Th abundance. We successfully determine the Eu abundance for 89 stars in our sample and the Th abundance for 81 stars. For the remaining eight stars, we estimate the upper limits of the Th abundance. After correcting the Th abundance for decay, we find no correlation between [Th/Eu] and [Fe/H], which indicates that actinide production with respect to lanthanide production does not depend on metallicity. On the other hand, we find a positive correlation of [Th/Eu] with age, with a slope of 0.10 ± 0.04. This may hint at the possibility that the dominant r -process sources are different between the early and late Universe.

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.

Very metal-poor (VMP) stars provide a record of the chemical composition and dynamics of the early Galaxy. Based on the high-resolution and high signal-to-noise ratio spectra from the Subaru Telescope for 103 VMP stars, in this series of papers we homogeneously investigate the nonlocal thermodynamic equilibrium (NLTE) abundances of important astrophysical elements. This sample covers a wide metallicity range from [Fe/H] ∼ −1.7 dex down to −4.3 dex, including 13 objects with [Fe/H] ≤ −3.0 dex. Here, we present a set of homogeneous stellar atmospheric parameters, including the effective temperature, surface gravity, metallicity, and microturbulence velocity with the spectroscopic method, and the NLTE line formation for both Fe i and Fe ii in the classical one-dimensional model atmospheres have been considered. The NLTE effects of the Fe i lines range from ∼0.03 dex to ∼0.3 dex, and increase with decreasing metallicity. In addition, they depend on the surface gravity, which generally increases with decreasing log g . The largest NLTE effects can be found for the giants of log g ∼ 2.5 dex. For dwarfs and subgiants, our final effective temperatures are consistent with those derived from the T IRFM scales of A. Alonso et al. with a mean difference of 7.1 ± 100.2 K, while for giants, our results are slightly lower than those from the T IRFM scales of A. Alonso et al. with a mean difference of −69.5 ± 94.1 K. For dwarfs and subgiants, the spectroscopically derived surface gravity is consistent with that estimated based on the Gaia DR3 parallax; however, for giants, the former leads to a 0.2 dex lower surface gravity.

The thorium and six second-peak r-process element (56

The InfraRed Doppler (IRD) instrument is the Subaru telescope's high-resolution (R > 70,000) spectrograph covering wavelengths from 1000 to 1700 nm. A laser frequency comb (LFC) spectrum simultaneously obtained with an object spectrum calibrates wavelength shifts caused by instrumental instability. We originally developed IRD to carry out precision radial velocity (RV) measurements at near-infrared wavelengths. The wide wavelength coverage of IRD, and the large mirror (8.2 m) of the Subaru Telescope enables IRD to provide the best sensitivities to detect a planet orbiting a cool M-type star. The first science operation of IRD was conducted in 2018 and the large strategic blind survey for planets orbiting cool M-type stars started in 2019. Since then, there have been many observations not only for exoplanet category but also for stellar physics, Galaxy, and high-energy astrophysics. IRD spectroscopy allowed for characterizing exoplanet atmospheres by measuring OH emissions, He absorptions, and spin-orbit obliquities. The IRD survey discovered a super-Earth in orbit near a habitable zone of Ross 508. The IRD RV measurements for many systems that host transiting planets, including TOI-2285 b and Gliese 12 b, helped confirm those and determine or constrain their masses. Using REACH, IRD can be combined with the extreme adaptive optics SCExAO, enabling the use of a single-mode fiber and characterizations of faint sub-stellar companions orbiting bright stars. In this proceeding paper, we review and highlight the scientific results achieved by the IRD observations.

We are developing fabrication methods of a volume binary (VB) grating, trapezoid grating and reflector facet transmission (RFT) grating. The VB grating can achieve a larger angular dispersion and higher diffraction efficiency than conventional surface-relief transmission gratings with step shaped grooves, it can be achieved a wider spectral bandwidth than a volume phase holographic (VPH) grating whose refractive index is sinusoidally modulated. The trapezoid grating can bring the spectral characteristics of s- and p-polarization closer to each other than a VB grating, so it can further improve the peak diffraction efficiency.

Recent discoveries of Earth-sized planets transiting nearby M dwarfs have made it possible to characterize the atmospheres of terrestrial planets via follow-up spectroscopic observations. However, the number of such planets receiving low insolation is still small, limiting our ability to understand the diversity of the atmospheric composition and climates of temperate terrestrial planets. We report the discovery of an Earth-sized planet transiting the nearby (12 pc) inactive M3.0 dwarf Gliese 12 (TOI-6251) with an orbital period ($P_{\rm{orb}}$) of 12.76 days. The planet, Gliese 12b, was initially identified as a candidate with an ambiguous $P_{\rm{orb}}$ from TESS data. We confirmed the transit signal and $P_{\rm{orb}}$ using ground-based photometry with MuSCAT2 and MuSCAT3, and validated the planetary nature of the signal using high-resolution images from Gemini/NIRI and Keck/NIRC2 as well as radial velocity (RV) measurements from the InfraRed Doppler instrument on the Subaru 8.2 m telescope and from CARMENES on the CAHA 3.5 m telescope. X-ray observations with XMM-Newton showed the host star is inactive, with an X-ray-to-bolometric luminosity ratio of $\log L_{\rm X}/L_{\rm bol} \approx -5.7$. Joint analysis of the light curves and RV measurements revealed that Gliese 12b has a radius of 0.96 $\pm$ 0.05 $R_\oplus$, a 3$\sigma$ mass upper limit of 3.9 $M_\oplus$, and an equilibrium temperature of 315 $\pm$ 6 K assuming zero albedo. The transmission spectroscopy metric (TSM) value of Gliese 12b is close to the TSM values of the TRAPPIST-1 planets, adding Gliese 12b to the small list of potentially terrestrial, temperate planets amenable to atmospheric characterization with JWST.