Thorium in Kilonova Spectra: Exploring the Heaviest Detectable Element

Abstract Kilonova spectra provide us with direct information about rapid neutron-capture nucleosynthesis ( r -process) in neutron star (NS) mergers. In this paper, we study the signatures of elements beyond the third r -process peak expected to be produced in neutron-rich ejecta in the photospheric spectra of kilonova. We select Ra ii , Ac iii , and Th iii as our candidates because they have a small number of valence electrons and low-lying energy levels, which tend to result in strong absorption features. We systematically calculate the strength of bound–bound transitions of these candidates by constructing a line list based on the available atomic database. We find that Th iii is the most promising species, showing strong transitions at near-infrared wavelengths. By performing radiative transfer simulations, we find that Th iii produces broad absorption features at ∼18000 Å in the spectra when the mass ratio of actinides to lanthanides is larger than the solar r -process ratio and the mass fraction of lanthanides is ≲6 × 10 −4 . Our models demonstrate that the Th feature may be detectable if the bulk of the ejecta in the line-forming region is dominated by relatively light r -process elements with the mixture of a small fraction of very neutron-rich material. Such conditions may be realized in the mergers of unequal-mass NSs or black hole–NS binaries. To detect the Th absorption features, observations from space (such as with the JWST) or high-altitude sites are important as the wavelength region of the Th features overlaps with that affected by strong telluric absorption.