Shinya Wanajo

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.

The material ejected from a binary neutron star merger produces a “kilonova,” a radioactively powered emission at ultraviolet, optical, and infrared wavelengths. The early-phase spectra of the kilonova AT2017gfo, following the gravitational wave event GW170817, exhibit a strong absorption feature around 1 μ m. Helium (He) and strontium (Sr) have been proposed as the candidate elements contributing to this feature. However, due to the lack of consistent modeling including these two elements simultaneously, the exact contributions of each element to this feature remain unclear. In this study, we develop nonlocal thermodynamic equilibrium ionization models for He and Sr that take into account ionization by high-energy electrons, and estimate the abundances of each element required to reproduce the observed feature in the early-phase spectra of AT2017gfo. Our modeling indicates that about 1% of He or 1%–10% of Sr (in mass fraction) are present in the ejecta moving at v ∼ 0.15 c . This Sr mass fraction nicely agrees with the mass fraction in the solar r -process pattern. Based on comparison with nucleosynthesis calculations, our constraints suggest that r -process nucleosynthesis in GW170817 occurs at relatively low electron fraction ( Y _e ≲ 0.35) and low entropy ( s ≲ 30 k _B /nucleon) conditions. Generally, for Y _e ≲ 0.15, the observed feature can be reproduced by He at the mass fraction expected from the α -decay of trans-Pb nuclei, and may therefore serve as an indirect signature for the production of elements beyond the third r -process peak in binary neutron star mergers.

Nebular phase kilonovae (KNe) have significant infrared (IR) emission thought to be mostly forbidden emission lines from rapid neutron capture (r-process) species in neutron star merger ejecta. Lanthanide elements in particular have complex open f-shell atomic structures with many IR transitions. Using non-local thermodynamic equilibrium radiative transfer simulations, we explore the impact of lanthanides on the IR spectra of KNe in the nebular phase, exploring a parameter space of ejecta mass and lanthanide fraction. We find that lanthanide impact is greater at higher densities, corresponding to earlier epochs and greater ejecta masses. The wavelengths most affected are found to be |$\lambda \lesssim 4~\mu$|m, with the species Ce iii and Nd ii being the most important contributors to spectral formation. We also find significant emission from species proposed in observations, notably Te iii at 2.1 |$\mu$|m, and Se iii at 4.5 and 5.7 |$\mu$|m, while W iii is subdominant at 4.5 |$\mu$|m. The Te iii feature at 2.1 |$\mu$|m is always blended, particularly with Zr ii, Ce iii, and Nd ii. We do not reproduce the smooth blackbody-like continua observed in AT2023vfi. Based on our results, we argue that line opacity alone is likely insufficient to produce optically thick continua in the nebular phase, even in the case of lanthanide/actinide-rich ejecta, as our models are optically thin in the IR at these epochs. Given that lanthanide contributions are dominant below 4 |$\mu$|m, we suggest that NIR observations best probe these elements, while mid-IR spectroscopy with James Webb Space Telescope can reliably probe non-lanthanide emission even in relatively lanthanide-rich cases.

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 study the formation of stars with varying amounts of heavy elements synthesized by the rapid neutron-capture process (r-process) based on our detailed cosmological zoom-in simulation of a Milky Way–like galaxy with an N-body/smoothed particle hydrodynamics code, asura. Most stars with no overabundance in r-process elements, as well as the strongly r-process-enhanced (RPE) r-II stars ([Eu/Fe] > +0.7), are formed in dwarf galaxies accreted by the Milky Way within the 6 Gyr after the Big Bang. In contrast, over half of the moderately enhanced r-I stars (+0.3 < [Eu/Fe] ≤ +0.7) are formed in the main in situ disk after 6 Gyr. Our results suggest that the fraction of r-I and r-II stars formed in disrupted dwarf galaxies is larger the higher their [Eu/Fe] is. Accordingly, the most strongly enhanced r-III stars ([Eu/Fe] > +2.0) are formed in accreted components. These results suggest that non-r-process-enhanced stars and r-II stars are mainly formed in low-mass dwarf galaxies that hosted either none or a single neutron star merger, while the r-I stars tend to form in the well-mixed in situ disk. We compare our findings with high-resolution spectroscopic observations of RPE metal-poor stars in the halo and dwarf galaxies, including those collected by the R-Process Alliance. We conclude that observed [Eu/Fe] and [Eu/Mg] ratios can be employed in chemical tagging of the Milky Way’s accretion history.

A resistive magnetohydrodynamics simulation with a dynamo term is performed for modeling the collapsar in full general relativity. As an initial condition, a spinning black hole and infalling stellar matter are modeled based on a stellar evolution result, superimposing a weak toroidal magnetic field. After the growth of a massive torus around the black hole, the magnetic field is amplified in it, developing poloidal fields via dynamo. In an early stage of the torus growth, magnetic fluxes that fall to the vicinity of the central black hole are swallowed by the black hole and global poloidal magnetic fields that can be the source of the Blandford-Znajek mechanism are not developed. However, in a later stage in which the ram pressure of the infalling matter becomes weak, the magnetic field amplified by the black hole spin via the winding becomes large enough to expel the infalling matter by the magnetic pressure, and subsequently, a global poloidal magnetic field that penetrates the black hole is established, launching a jet along the spin axis by the Blandford-Znajek mechanism with the luminosity suitable for explaining typical long gamma-ray bursts. Together with the jet launch, the effectively viscous effect in the inner region of the torus and the magnetocentrifugal effect drive the stellar explosion with the explosion energy comparable to typical or powerful supernovae. We also find large amounts of synthesized Ni 56 and Zn associated with the stellar explosion. In the presence of jet launching, r -process elements are weakly synthesized. The numerical results of the explosion energy, ejecta mass, and Ni 56 mass are in a good agreement with those for observed broad-lined type Ic supernovae. Our result illustrates a self-consistent scenario for the gamma-ray-burst-associated broad-lined type Ic supernovae.

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Kilonova spectra provide us with direct information about rapid neutron-capture nucleosynthesis (r-process) in neutron star (NS) mergers. In this paper, we study the signatures of elements beyond the third r-process peak expected to be produced in neutron-rich ejecta in the photospheric spectra of kilonova. We select Ra ii, Ac iii, and Th iii as our candidates because they have a small number of valence electrons and low-lying energy levels, which tend to result in strong absorption features. We systematically calculate the strength of bound–bound transitions of these candidates by constructing a line list based on the available atomic database. We find that Th iii is the most promising species, showing strong transitions at near-infrared wavelengths. By performing radiative transfer simulations, we find that Th iii produces broad absorption features at ∼18000 Å in the spectra when the mass ratio of actinides to lanthanides is larger than the solar r-process ratio and the mass fraction of lanthanides is ≲6 × 10⁻⁴. Our models demonstrate that the Th feature may be detectable if the bulk of the ejecta in the line-forming region is dominated by relatively light r-process elements with the mixture of a small fraction of very neutron-rich material. Such conditions may be realized in the mergers of unequal-mass NSs or black hole–NS binaries. To detect the Th absorption features, observations from space (such as with the JWST) or high-altitude sites are important as the wavelength region of the Th features overlaps with that affected by strong telluric absorption.

We examine nucleosynthesis in the ejecta of black-hole–neutron-star mergers based on the results of long-term neutrino-radiation-magnetohydrodynamics simulations for the first time. We find that the combination of dynamical and postmerger ejecta reproduces a solarlike r -process pattern. Moreover, the enhancement level of actinides is highly sensitive to the distribution of both the electron fraction and the velocity of the dynamical ejecta. Our result implies that the mean electron fraction of dynamical ejecta should be ≳ 0.05 in order to reconcile the nucleosynthetic abundances with those in r -process-enhanced, actinide-boost stars. Since the tidal ejecta preserve the neutron richness in the inner crust of premerging neutron stars, this result provides an important constraint for nuclear equations of state if black-hole–neutron-star mergers are responsible for actinide-boost stars. Published by the American Physical Society 2024