arXiv subjects

Nanae Domoto

Publications and source records attributed to Nanae Domoto.

13 recordsLinked to original sources

arXiv2026

Heavy element dust explains the late-time spectra of kilonovae

Neutron star mergers are a leading site of $r$-process, producing radioactively powered optical and infrared transients known as kilonovae. Observations of the kilonovae AT2017gfo, associated with the gravitational-wave event GW170817, and AT2023vfi, associated with GRB 230307A, have enabled measurements of the mass of ejected $r$-process material and the identification of heavy elements in the ejecta. However, late-time observations reveal strong infrared emission with temperature below 1000 K, which is difficult to explain by atomic absorption and emission processes alone. In this paper, we show that kilonova ejecta provide conditions favorable for the formation of dust grains composed of refractory $r$-process elements including Zr, W, and Os. We calculate the kinetic formation of dust grains using reaction rate coefficients of W as a proxy, finding that dust forms efficiently, particularly in slow ejecta. This stands in contrast to a previous study that relied on a classical nucleation framework. By performing radiative transfer simulations that incorporate dust formation, we demonstrate that $r$-process dust naturally explains the observed late-time infrared emission. The formation and abundance of $r$-process dust are highly sensitive to the ejecta mass, composition, and expansion velocity. Infrared emission from $r$-process dust can therefore serve a new probe of heavy-element production in neutron star mergers.

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arXiv2025

Theoretical study of Th III energy levels and transitions for applications to kilonova spectra

The neutron star merger is a promising site of heavy element production. By producing heavy elements, the neutron star merger gives rise to a thermal transient called a kilonova. Studying kilonova spectra enables us to quantify the heavy element production. Among the heaviest elements, doubly ionized Thorium (Th, Z=90) is one of the important candidates for producing detectable absorption features in kilonova spectra. This paper investigates the atomic properties of Th III to provide energy level and transition data. The multiconfiguration Dirac-Hartree-Fock and relativistic configuration interaction methods, which are implemented in the general-purpose relativistic atomic structure package GRASP2018, are used to compute energy levels of the $\mathrm{5f6d}$, $\mathrm{6d^2}$, $\mathrm{7s^2}$, $5\mathrm{f^2}$, $\mathrm{6d7s}$, $\mathrm{5f7p}$ and $\mathrm{5f7s}$ configurations and electric dipole transitions between states of these configurations. The accuracy of energy levels is evaluated by comparing it with experimental data and with various theoretical methods. Our calculated energy levels are consistent with the experimental results with a root mean square (RMS) deviation of 436 cm$^{-1}$. The accuracy of transition data is investigated using the quantitative and qualitative evaluation method.By performing radiative transfer simulations for kilonova spectra with our transition data, we show that kilonova including Th with a mass fraction of $(3-10) \times 10^{-5}$ can produce Th III absorption features around 18,000 A.

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arXiv2024

Revisiting Near-Infrared Features of Kilonovae: The Importance of Gadolinium

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. Domoto et al. (2022) have shown that La III and Ce III can explain the absorption features at $λ\sim$ 12,000 - 15,000 A. 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 4f and the outer 5d 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 $\sim$ 12,000 A previously attributed to La III. Future space-based time-series observations of kilonova spectra will allow the identification of Gd III lines.

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arXiv2024

Three dimensional end-to-end simulation for kilonova emission from a black-hole neutron-star merger

We study long-term evolution of the matter ejected in a black-hole neutron-star (BH-NS) merger employing the results of a long-term numerical-relativity simulation and nucleosynthesis calculation, in which both dynamical and post-merger ejecta formation is consistently followed. In particular, we employ the results for the merger of a $1.35\,M_\odot$ NS and a $5.4\,M_\odot$ BH with the dimensionless spin of 0.75. We confirm the finding in the previous studies that thermal pressure induced by radioactive heating in the ejecta significantly modifies the morphology of the ejecta. We then compute the kilonova (KN) light curves employing the ejecta profile obtained by the long-term evolution. We find that our present BH-NS model results in a KN light curve that is fainter yet more enduring than that observed in AT2017gfo. This is due to the fact that the emission is primarily powered by the lanthanide-rich dynamical ejecta, in which a long photon diffusion time scale is realized by the large mass and high opacity. While the peak brightness of the KN emission in both the optical and near-infrared bands is fainter than or comparable to those of binary NS models, the time-scale maintaining the peak brightness is much longer in the near-infrared band for the BH-NS KN model. Our result indicates that a BH-NS merger with massive ejecta can observationally be identified by the long lasting ($>$two weeks) near-infrared emission.

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arXiv2024

Thorium in Kilonova Spectra: Exploring the Heaviest Detectable Element

Kilonova spectra provide us with the direct information of r-process nucleosynthesis in neutron star 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. Ra II, Ac III, and Th III are 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 the line list based on the available atomic database. We find that Th III is the most promising species showing strong transitions at the NIR wavelengths. By performing radiative transfer simulations, we find that Th III produces broad absorption features at ~18000 A 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 $\lesssim 6\times10^{-4}$. 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 neutron stars or black hole-neutron star binaries. To detect the Th absorption features, the observations from the space (such as JWST) or high-altitude sites are important as the wavelength region of the Th features is overlapped with that affected by the strong telluric absorption.

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arXiv2023

Transition probabilities of near-infrared Ce III lines from stellar spectra: applications to kilonovae

Kilonova spectra provide us with information of r-process nucleosynthesis in neutron star mergers. However, it is still challenging to identify individual elements in the spectra mainly due to lack of experimentally accurate atomic data for heavy elements in the near-infrared wavelengths. Recently, Domoto et al. (2022) proposed the absorption features around 14500 A in the observed spectra of GW170817/AT2017gfo as Ce III lines. But they used theoretical transition probabilities (gf-values) whose accuracy is uncertain. In this paper, we derive the astrophysical gf-values of the three Ce III lines, aiming at verification of this identification. We model high resolution H-band spectra of four F-type supergiants showing the clear Ce III absorption features by assuming stellar parameters derived from optical spectra in literatures. We also test the validity of the derived astrophysical gf-values by estimating Ce III abundances in Ap stars. We find that the derived astrophysical gf-values of the Ce III lines are systematically lower by about 0.25 dex than those used in previous work of kilonovae, while they are still compatible within the uncertainty range. By performing radiative transfer simulations of kilonovae with the derived gf-values, we find that the identification of Ce III as a source of the absorption features in the observed kilonova spectra still stands, even considering the uncertainties in the astrophysical gf-values. This supports identification of Ce in the spectra of GW170817/AT2017gfo.

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arXiv2023

Heavy element nucleosynthesis associated with a gamma-ray burst

Kilonovae are a novel class of astrophysical transients, and the only observationally-confirmed site of rapid neutron capture nucleosynthesis (the r-process) in the Universe. To date, only a handful of kilonovae have been detected, with just a single spectroscopically-observed event (AT 2017gfo). Spectra of AT 2017gfo provided evidence for the formation of elements heavier than iron; however, these spectra were collected during the first ~ 10 days, when emission from light r-process elements dominates the observations. Heavier elements, if synthesised, are expected to shape the late-time evolution of the kilonova, beyond the phases for which we have spectral observations. Here we present spectroscopic observations of a rapidly-reddening thermal transient, following the gamma-ray burst, GRB 230307A. Early (2.4 day) optical spectroscopy identifies the presence of a hot (T ~ 6700 K) thermal continuum. By 29 days, this component has expanded and cooled significantly (T ~ 640 K), yet it remains optically thick, indicating the presence of high-opacity ejecta. We show that these properties can only be explained by the merger of compact objects, and further, leads us to infer the production of the heavy lanthanide elements. We identify several spectral features (in both absorption and emission), whose cause can be explained by newly-synthesised heavy elements. This event marks only the second recorded spectroscopic evidence for the synthesis of r-process elements, and the first to be observed at such late times.

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arXiv2023

Kilonovae of binary neutron star mergers leading to short-lived remnant neutron star formation

We study kilonova emission from binary neutron star (BNS) mergers for the case that a remnant massive neutron star (MNS) forms and collapses to a black hole within $20$ ms after the onset of the merger (which we refer to as "a short-lived case") by consistently employing numerical-relativity and nucleosynthesis results. We find that such kilonovae are fainter and last shorter than those for BNSs resulting in the formation of long-lived ($\gg 1\,{\rm s}$) MNSs, in particular in the optical band. The resulting light curves are too faint and last for a too short duration to explain the kilonova observation for the BNS associated with GW170817, indicating that the merger remnant formed in GW170817 is unlikely to have collapsed to a black hole within a short period of time ($\sim 20$ ms) after the onset of the merger. Our present result implies that early observation is necessary to detect kilonovae associated with BNSs leading to short-lived MNS formation in particular for the optical blue band as well as that kilonovae could be hidden by the gamma-ray burst afterglow for nearly face-on observation. We provide a possible approximate scaling law for near-infrared light curves with the given reference time and magnitude when the decline power of the ${\it z}$-band magnitude, $d M_{\it z}/d{\rm log}_{10}t$, reaches $2.5$. This scaling law suggests that the ${\it HK}$-band follow-up observation should be at least $1$ mag deeper than that for the ${\it z}$-band reference magnitude and earlier than 4 times the reference time.

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arXiv2023

Cerium features in kilonova near-infrared spectra: implication from a chemically peculiar star

Observations of the kilonova from a neutron star merger event GW170817 opened a way to directly study r-process nucleosynthesis by neutron star mergers. It is, however, challenging to identify the individual elements in the kilonova spectra due to lack of complete atomic data, in particular, at near-infrared wavelengths. In this paper, we demonstrate that spectra of chemically peculiar stars with enhanced heavy element abundances can provide us with an excellent astrophysical laboratory for kilonova spectra. We show that the photosphere of a late B-type chemically peculiar star HR 465 has similar lanthanide abundances and ionization degrees with those in the line forming region in a kilonova at $\sim 2.5$ days after the merger. The near-infrared spectrum of HR 465 taken with Subaru/IRD indicates that Ce III lines give the strongest absorption features around 16,000 A and there are no other comparably strong transitions around these lines. The Ce III lines nicely match with the broad absorption features at 14,500 A observed in GW170817 with a blueshift of v=0.1c, which supports recent identification of this feature as Ce III by Domoto et al. (2022).

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arXiv2023

Non-LTE analysis for Helium and Strontium lines in the kilonova AT2017gfo

Kilonova spectra imprint valuable information about the elements synthesized in neutron star mergers. In AT2017gfo, the kilonova associated with GW170817, the spectroscopic feature centered around 8000 angstroms has been interpreted as the P-Cygni profile arising from singly ionized Strontium. Recently, Perego et al. (2022) suggested that Helium 10833 line can be an alternative explanation of the feature. Here, we study the line features under non-local thermodynamic equilibrium. We find that the ionization of ejecta by the stopping of radioactive decay product can significantly enhance the ionization states around the line forming region. We compute the kilonova spectrum under the assumption of spherical symmetry and uniform elemental fraction in the line-forming region. We find that 0.2\% (in mass) of Helium in the ejecta can reproduce the P-Cygni feature in the observed spectrum at $1.43$ -- $4.40$ days. Strontium with a mass fraction of $1\%$ is also able to make the absorption feature at $\sim 1.5\,$days, but it gets weaker with time due to ionization by radioactive decay products. The strength of the He line signature depends sensitively on UV strength for the first two epochs. Further modeling of UV line blanketing by $r$-process elements and the optical properties of light $r$-process elements would be crucial to distinguish between Helium and Strontium features. The mass fraction of He is a good indicator for ejecta entropy that allows us to probe the mass ejection mechanism.

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arXiv2022

Lanthanide Features in Near-infrared Spectra of Kilonovae

The observations of GW170817/AT2017gfo have provided us with evidence that binary neutron star mergers are sites of $r$-process nucleosynthesis. However, the observed signatures in the spectra of GW170817/AT2017gfo have not been fully decoded especially in the near-infrared (NIR) wavelengths. In this paper, we investigate the kilonova spectra over the entire wavelength range with the aim of elemental identification. We systematically calculate the strength of bound-bound transitions by constructing a hybrid line list that is accurate for important strong transitions and complete for weak transitions. We find that the elements on the left side of the periodic table, such as Ca, Sr, Y, Zr, Ba, La, and Ce, tend to produce prominent absorption lines in the spectra. This is because such elements have a small number of valence electrons and low-lying energy levels, resulting in strong transitions. By performing self-consistent radiative transfer simulations for the entire ejecta, we find that La III and Ce III appear in the NIR spectra, which can explain the absorption features at $λ\sim 12000$-14000 A in the spectra of GW170817/AT2017gfo. The mass fractions of La and Ce are estimated to be $>2\times 10^{-6}$ and $\sim$ (1-100)$\times 10^{-5}$, respectively. An actinide element Th can also be a source of absorption as the atomic structure is analogous to that of Ce. However, we show that Th III features are less prominent in the spectra because of the denser energy levels of actinides compared to those of lanthanides.

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arXiv2022

Opacity of the highly ionized lanthanides and the effect on the early kilonova

We investigate the effect of the presence of lanthanides (Z = 57- 71) on the kilonova at t~hours after the neutron star merger for the first time. For this purpose, we calculate the atomic structures and the opacities for selected lanthanides: Nd (Z = 60), Sm (Z = 62), and Eu (Z = 63). We consider the ionization degree up to tenth (XI), applicable for the ejecta at t ~ a few hours after the merger, when the temperature is T ~ 10^5 K. We find that the opacities for the highly ionized lanthanides are exceptionally high, reaching k_exp~1000 cm^2/g for Eu, due to the highly dense energy levels. Using the new opacity, we perform radiative transfer simulations to show that the early light curves become fainter by a (maximum) factor of four, in comparison to lanthanide-free ejecta at t~0.1 day. However, the period at which the light curves are affected is relatively brief due to the rapid time evolution of the opacity in the outermost layer of the ejecta. We predict that for a source at a distance of ~100 Mpc, UV brightness for lanthanide-rich ejecta shows a drop to ~21-22 mag at t~0.1 day and the UV peaks around t~0.2 day with a magnitude of ~19 mag. Future detection of such a kilonova by the existing UV satellite like Swift or the upcoming UV satellite ULTRASAT will provide useful constraints on the abundance in the outer ejecta and the corresponding nucleosynthesis conditions in the neutron star mergers.

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arXiv2021

Signatures of r-process elements in kilonova spectra

Binary neutron star (NS) mergers have been expected to synthesize r-process elements and emit radioactively powered radiation, called kilonova. Although r-process nucleosynthesis was confirmed by the observations of GW170817/AT2017gfo, no trace of individual elements has been identified except for strontium. In this paper, we perform systematic calculations of line strength for bound-bound transitions and radiative transfer simulations in NS merger ejecta toward element identification in kilonova spectra. We find that Sr II triplet lines appear in the spectrum of a lanthanide-poor model, which is consistent with the absorption feature observed in GW170817/AT2017gfo. The synthetic spectrum also shows the strong Ca II triplet lines. This is natural because Ca and Sr are co-produced in the material with relatively high electron fraction and their ions have similar atomic structures with only one s-electron in the outermost shell. The line strength, however, highly depends on the abundance distribution and temperature in the ejecta. For our lanthanide-rich model, the spectra show the features of doubly ionized heavy elements, such as Ce, Tb and Th. Our results suggest that the line forming region of GW170817/AT2017gfo was lanthanide-poor. We show that the Sr II and Ca II lines can be used as a probe of physical conditions in NS merger ejecta. Absence of the Ca II line features in GW170817/AT2017gfo implies that the Ca/Sr ratio is < 0.002 in mass fraction, which is consistent with nucleosynthesis for electron fraction >= 0.40 and entropy per nucleon (in units of Boltzmann constant) >= 25.

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