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

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 GW 170817. Yet, 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 GW 170817 merger. Based on comparisons of detailed kilonovae spectra simulations 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.

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