# Superheavy Elements in Kilonovae

**Authors:** Erika M. Holmbeck, Jennifer Barnes, Kelsey A. Lund, Trevor M. Sprouse, G. C. McLaughlin, Matthew R. Mumpower  
**arXiv:** [2304.02125](https://arxiv.org/abs/2304.02125)  
**Date:** 2023-04-04  
**Summary page:** https://www.summarizepaper.com/en/paper/2304.02125v1/

## Key points

- The LIGO-Virgo-KAGRA collaboration has started its fourth observing run
- The first "kilonova" associated with a binary neutron star binary (NSM) revealed that these sites produce heavy elements through the rapid neutron capture process
- It is still unknown how robust elemental production can be in mergers
- Recent studies have investigated the detailed elemental compositions of the ejecta and fission properties of heavy nuclei, which were found to be important predictors of kilonova light curves
- Composition inference and the claim that r-process elements were synthesized in the GW170817 event are based on the effect of high-opacity lanthanides on the light curve evolution
- Recent works have identified individual nuclei that can power late-time light curves, such as 254Cf with a half-life of about 60 days, providing observational proof that actinides are synthesized in mergers—not just lanthanides
- Researchers explore a subset of nuclear models that can produce superheavy elements in an NSM environment and study their effect on kilonova light curves
- Favorable NSM conditions yield a mass fraction of superheavy elements at 7.5 hours post merger, which could appear similar to those arising from lanthanide poor ejecta.
- A superheavy element signature in kilonovae represents a route to establishing a lower limit on heavy element production in NSMs as well as possibly being the first evidence of superheavy element synthesis in nature.

## Summary

The LIGO-Virgo-KAGRA collaboration has entered its fourth observing run, providing a new opportunity to search for electromagnetic counterparts of compact object mergers. In particular, the first "kilonova" associated with a binary neutron star binary (NSM) revealed that these sites are hosts of the rapid neutron capture ("$r$") process, which produces heavy elements. However, it is still unknown just how robust elemental production can be in mergers. Identifying signposts of the production of particular nuclei is critical for fully understanding merger-driven heavy-element synthesis. Recent studies have investigated the detailed elemental compositions of the ejecta and fission properties of heavy nuclei, which were found to be important predictors of kilonova light curves. In general, composition inference and the claim that r-process elements were synthesized in the GW170817 event are based on the effect of high-opacity lanthanides (57 ≤ Z ≤ 71) on the light curve evolution. It cannot currently be definitively claimed that anything beyond the lanthanides were created in that event. Recent works have also identified individual nuclei that can power late-time light curves, such as 254Cf with a half-life of about 60 days. The energy released by its spontaneous fission can prolong the evolution of light in JHK bands, leaving a measurable excess beginning as early as 25 days post-merger. Such a late-time detection would provide observational proof that actinides (89 ≤ Z ≤ 103) are synthesized in mergers—not just lanthanides. An earlier signal could possibly arise from even heavier nuclei: superheavy elements with Z ≥ 104. Whether these elements are even produced by nature is a topic of debate and depends sensitively on nuclear physics at high proton and neutron numbers. In this study, researchers explore a subset of nuclear models that can produce superheavy elements in an NSM environment and study their effect on kilonova light curves. They choose conditions that favorably produce these elements, such as the low entropy and high neutron richness of dynamical ejecta from NSMs. The results show that favorable NSM conditions yield a mass fraction of superheavy elements at 7.5 hours post merger, which could appear similar to those arising from lanthanide poor ejecta. A superheavy element signature in kilonovae represents a route to establishing a lower limit on heavy element production in NSMs as well as possibly being the first evidence of superheavy element synthesis in nature. However, photometric characterizations of superheavy element rich kilonovae may possibly misidentify them as lanthanide poor events.