Advanced three-dimensional computer simulations closely mirror actual observations of the light emitted by neutron star mergers, deepening our understanding of the origins of heavy elements. New advanced three-dimensional computer simulations of the light emitted by the merger of two neutron stars produce a similar sequence of spectral signatures to those observed in kilonovae.
Two neutron stars merging. Recent advances in three-dimensional computer simulations have provided a deeper understanding of the light emitted by merging neutron stars. These simulations are crucial to understanding the origins of elements heavier than iron. Source: DanaBerrySkyWorksDigital,Inc.
"The unprecedented agreement between our simulations and the observed kilonova AT2017gfo shows that we understand roughly what happened in and after the explosion," said GSI/FAIR scientist Luke J. Shingles, lead author of the paper in The Astrophysical Journal Letters. "Recent observations combining gravitational waves and visible light suggest that neutron star mergers are the primary site for the production of this element."
The mechanics behind radiative transfer simulations
The interactions between electrons, ions and photons in the material ejected when neutron stars merge determine the light we see through telescopes. Both these processes and the emitted light can be modeled using computer simulations of radiative transfer. Researchers have recently produced for the first time a three-dimensional simulation that can self-consistently track the energy deposited by neutron star mergers, neutron capture nucleosynthesis, radioactive decay, and the radiative transfer of tens of millions of atomic transitions in heavy elements.
The epicenter serves as a three-dimensional model, and the observed light rays can be predicted in any viewing direction. When the observation direction is almost perpendicular to the orbital plane of two neutron stars (such as the kilonova AT2017gfo shown by observational evidence), the spectral distribution sequence predicted by the model is very similar to the observed AT2017gfo. "Research in this area will help us understand the origins of elements heavier than iron, such as platinum and gold, which are primarily produced by rapid neutron capture processes in neutron star mergers," Shingles said.
About half of the elements heavier than iron were created in environments with extreme temperatures and neutron densities, like when two neutron stars merge with each other. When the two neutron stars eventually precess each other and condense together, the resulting explosion causes material to be ejected, which, under the right conditions, produces unstable neutron-rich heavy nuclei through a cascade of neutron capture and beta decay. These nuclei decay into a stable state, releasing energy that powers the explosive "kilonova" transient, a bright light emission that rapidly fades within about a week.
The three-dimensional simulations combine several areas of physics, including the behavior of matter at high densities, the properties of unstable heavy nuclei, and the atom-light interactions of heavy elements. Further challenges remain, such as calculating the rate of change of the spectral distribution and characterizing late-stage ejected material.
Future advances in this area will improve the accuracy with which we predict and understand spectral features and will further deepen our understanding of the conditions for the synthesis of heavy elements. An essential element of these models is high-quality atomic and nuclear experimental data, which the FAIR facility will provide.