How did the heavy elements form in our universe? For the rare earth metals lanthanum and cerium, a neutron star collision now provides the answer. Researchers have now detected the signatures of these lanthanides in their spectrum for the first time. This confirms that the neutron capture reactions necessary for such heavy atoms only take place in extreme cosmic events. The actinoid thorium could also have been produced in the neutron star collision, but the evidence is less clear.
Shortly after the Big Bang there was only hydrogen and a little bit of helium and lithium in the universe. Heavier chemical elements formed only through nuclear fusion inside the first stars, red giants and in supernovae. However, these processes are no longer sufficient for atoms from the group of lanthanides and actinides and for heavyweights from bismuth with atomic number 83 onwards. These elements can only be created by high-energy, fast neutron capture, the so-called r-process.
Where this r-process takes place in the cosmos was unclear for a long time. But as astronomers in 2017, for the first time Neutron Star Collision Registered with telescopes and gravitational wave detectors, they were able to detect the spectral signatures of gold, platinum and strontium in the explosion cloud prove – Elements that only arise through fast neutron capture.
Manhunt for cosmic rare earth metals
Now, for the first time, scientists have also identified the signature of rare earth metals in the spectrum of a neutron star collision. Until now it was not clear what the spectral lines of elements from the group of lanthanides and actinides look like in such kilonova explosions and whether they can be observed at all. “In order to obtain element information from the spectra, one first needs spectroscopically precise atomic data,” explain Nanae Domoto from Tohoku University in Japan and his colleagues.
For their study, the researchers therefore first analyzed the structure and energy levels of the electron shells of elements from strontium upwards. Using a model, they then determined which energy states and thus spectral lines would be expected from these atoms under the conditions of a neutron star collision.
What elements would be visible in the spectrum?
The result: “Only a few elements with atomic numbers 38 to 88, including strontium, ytterbium, zirconium, barium, lanthanum and cerium, can produce strong absorption signatures in such spectra,” Domoto and his colleagues report. According to this, elements from the periodic table groups II to IV are particularly visible because they have a relatively small number of outer electrons and low energy levels.
Based on these findings, the researchers next modeled at which wavelength of the spectrum the lines of these elements lie and at which stage of the afterglow of a neutron star collision they would be most visible. This showed that the absorption lines of ytterbium and zirconium, but also of the rare earth metals lanthanum and cerium, must be in the near infrared range.
Clear evidence of lanthanum and cerium
When the researchers searched for these spectral lines in the infrared data of the 2017 neutron star collision, they actually found what they were looking for: Lines appeared in the observed spectra that corresponded well with the spectral signatures of lanthanum and cerium previously simulated in the model. They were also able to identify possible traces of thorium, ytterbium and zircon. “This is the first direct detection of rare earth metals in the spectrum of neutron star collisions,” says Domoto.
The team even managed to roughly estimate how much lanthanum and cerium were formed in the neutron star collision GW17081. Accordingly, the mass fraction of lanthanum is a good two millionths, that of cerium at one to 100 hundred thousandths. “This is the first spectroscopic estimate of lanthanide abundances in the ejecta of a neutron star collision,” Domoto and his colleagues say.
The researchers hope that their method will make it possible to detect other heavy elements in neutron star collisions and other extreme cosmic events in the future. “This helps us understand how elements are formed in the universe,” says Domoto. (The Astrophysical Journal, 2022; doi: 10.3847/1538-4357/ac8c36)
Source: Center for Computational Astrophysics