A group of researchers has, for the first time, identified rare earth elements produced by neutron star mergers.

Figure 1: Artist’s conception of a neutron star merger and the resulting kilonova. (Credit: Tohoku University)
Download: [JPG, 5.5 MB]

When two neutron stars spiral inwards and merge, the resulting explosion produces a large amount of the heavy elements that make up our Universe. The first confirmed example of this process was an event in 2017 named GW170817. Yet, even now 5 years later, identifying the specific elements created in neutron star mergers has eluded scientists, except for strontium identified in the optical spectra.

A research group led by Nanae Domoto, a graduate student at the Graduate School of Science at Tohoku University and a research fellow at the Japan Society for the Promotion of Science (JSPS), has systematically studied the spectra from this kilonova—bright emissions caused by the radioactive decay of freshly synthesized nuclei that were ejected during the GW170817 merger. Based on comparisons with detailed simulations of kilonova spectra produced by the supercomputer “ATERUI II” at the National Astronomical Observatory of Japan, the team found that the rare earth elements lanthanum and cerium can reproduce the near-infrared spectral features seen in 2017.

Figure 2: The observed spectra of a kilonova (gray) and model spectra obtained in this study (blue). The numbers on the left show the number of days after the neutron star merger occurred that each spectrum was taken. Dashed lines indicate the absorption line features. The names of the elements that produce these features are shown in the same colors as the dashed lines. The spectra are vertically shifted for visualization. The observed spectra around 1400 nanometer and 1800-1900 nanometer are affected by the earth’s atmosphere. (Credit: Nanae Domoto et al.)
Download: [JPG, 377 KB]

Until now, the existence of rare earth elements has only been hypothesized based on the overall evolution of the brightness of the kilonova, but not confirmed from the spectral features.

“This is the first direct identification of rare elements in the spectra of neutron star mergers, and it advances our understanding of the origin of elements in the Universe,” Dotomo said.

“This study used a simple model of ejected material. Looking ahead, we want to factor in multi-dimensional structures to grasp a bigger picture of what happens when stars collide,” Dotomo added.

These results appeared as Domoto et al. “Lanthanide Features in Near-infrared Spectra of Kilonovae” in The Astrophysical Journal on October 26, 2022.

(October 27, 2022 press release)

[Research Paper]

Title: Lanthanide Features in Near-infrared Spectra of Kilonovae
Authors: Nanae Domoto, Masaomi Tanaka, Daiji Kato, Kyohei Kawaguchi, Kenta Hotokezaka, Shinya Wanajo
Journal: The Astrophysical Journal
DOI: 10.3847/1538-4357/ac8c36

[Supercomputer used in this research]

This research utilized the NAOJ supercomputer ATERUI II (Cray XC50). ATERUI II is operated at NAOJ Mizusawa Campus (Oshu, Iwate) with a theoretical peak performance of 3.087 Pflops. (Image Credit: NAOJ)

[About this research project]

This research was supported by the Grant-in-Aid for JSPS Fellows (22J22810), the Grant-in-Aid for Scientific Research from JSPS (19H00694, 20H00158, 21H04997, 21K13912), MEXT (17H06363), NIFS Collaborative Research Program (NIFS22KIIF005), and Graduate Program on Physics for the Universe (GP-PU) at Tohoku University.

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[Related Links]

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