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 ∼ 12000–14000 Å in the spectra of GW170817/AT2017gfo. The mass fractions of La and Ce are estimated to be >2 × 10−6 and ∼(1–100) × 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.

Binary neutron star (NS) mergers have been expected to synthesize r-process elements and emit radioactively powered radiation, called kilonovae. 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 coproduced 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.

We investigate the effect of the presence of lanthanides (Z = 57–71) on the kilonova at t ∼ 1 hr 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 10th (XI), applicable for the ejecta at t ∼ a few hours after the merger, when the temperature is T ∼ 105 K. We find that the opacities for the highly ionized lanthanides are exceptionally high, reaching κ exp ∼ 1000 cm 2 g − 1 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 days. However, the period at which the light curves are affected is relatively brief owing 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 days and the UV peaks around t ∼ 0.2 days with a magnitude of ∼19 mag. Future detection of such a kilonova by an 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.

Observations of the kilonova from the neutron star merger event GW170817 opened a way to study r-process nucleosynthesis directly using neutron star mergers. It is, however, challenging to identify individual elements in kilonova spectra due to a lack of complete atomic data, in particular at near-infrared (NIR) 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 the 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 ∼2.5 days after the merger. The NIR spectrum of HR 465 taken with Subaru/IRD indicates that Ce iii lines give the strongest absorption feature around 16000 Å and there are no other comparably strong transitions around these lines. The Ce iii lines nicely match with the broad absorption feature at 14500 Å observed in GW170817 with a blueshift of v = 0.1 c, which supports recent identification of this feature as Ce iii by Domoto et al.

Binary neutron star (NS) mergers have been expected to synthesize r-process elements and cause electromagnetic radiation called kilonovae. Although r-process nucleosynthesis was confirmed by the observations of GW170817/AT2017gfo, individual elements have not been identified except for strontium. Toward identification of elements in kilonova spectra, we perform radiative transfer simulations in NS merger ejecta. We find that Sr II triplet lines appear in the spectrum, which is consistent with the absorption feature observed in GW170817/AT2017gfo. The synthetic spectrum also shows the strong Ca II triplet lines. 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. Identification of absorption lines in near-infrared wavelengths which have not yet been decoded may lead to clarify the abundances synthesized in NS merger ejecta.