COSAM News Articles 2024 02 SJB Executive Summary

SJB Executive Summary

Published: 02/09/2024

EXECUTIVE SUMMARY

Most of the heavy elements on the periodic table (things like strontium, platinum, neodymium, etc) were predicted to be produced from mergers of neutron stars in the mid 20^th century. In 2017, gravitational wave detectors finally caught a glimpse of a neutron star merger. Once the position on the sky was localized, the light from the aftermath was captured across the entire electromagnetic spectrum by many telescopes across Earth and space.

The light from the aftermath, called a ‘kilonova’, encodes the signatures of the heavy elements that get produced by the rapid neutron capture process that takes place during and shortly after the neutron star merger. Within the first few days to a week, the expanding remnant of this event is hot and very dense. To explain the light that is seen during this period, models need to know the allowed energy levels, and possible radiative decays, of the many elements that are expected to exist in the remnant, and how they are excited by the blackbody radiation emitted by this hot environment. This could be called an ‘electronic structure problem’, as models would need to include the electronic structures (energy levels and transitions, or wavelengths of light emitted by the decays of these levels) of these very heavy elements. The distribution of energy levels, and resulting emissions of light, can be reasonably predicted at these dense conditions by assuming ‘Local Thermodynamic Equilibrium (LTE)’. Many research groups are actively addressing this problem.

After ~1 week, the radiation field decreases in intensity, and modeling the light emitted by the remnant becomes much more difficult. The conditions for LTE are no longer valid. Instead, modeling of which ionization states are present, and which wavelengths of light are emitted, requires an understanding of how the very heavy elements are ionized, excited, and recombining with electrons in the plasma. At the moment, the rates for these processes are not known, and modelers must utilize empirical (and less accurate than one would prefer) expressions that have been derived decades ago.

At Auburn, our expertise is in these ionization and recombination processes, particularly for heavy species, as we can draw on a knowledge bank gained while working on tungsten for fusion research projects on campus. Our NSF-funded project aims to calculate these ionization and recombination rates for three very important elements in kilonova, with each element belonging to one of the ‘abundance peaks’ produced by the rapid neutron capture process. We have chosen Strontium (atomic number 38), Tellurium (atomic number 52), and Osmium (atomic number 76). At Auburn, we will be computing the ionization of these elements with the Flexible Atomic Code. We are collaborating with University of Georgia, who will compute the recombination (the inverse of ionization) with AUTOSTRUCTURE, a code that has been in use at Auburn for many years. Combined, we are hoping to provide the astrophysical community with improved rates for these important elements. We will be collaborating with Kenta Hotokezaka, an established kilonova modeler at the University of Tokyo, to make these data available to the astrophysics community in convenient and useful formats. We hope that these improved data can lead to more reliable models of kilonova, and enable a refinement of the yields of elements produced by neutron star mergers.