Actinide signatures in low electron fraction kilonova ejecta

TL;DR

This study explores how actinide signatures in kilonova spectra can reveal neutron star merger conditions, emphasizing the impact of nuclear physics uncertainties and identifying potential spectral indicators like doubly ionized Ac and Th.

Contribution

It introduces models of neutron star merger ejecta with varying actinide content and analyzes their spectral signatures using NLTE radiative transfer, highlighting key indicators of actinide presence.

Findings

Spectral and lightcurve differences depend on actinide content.

Uncertainties in nuclear physics significantly affect model outputs.

Doubly ionized Ac and Th are potential spectral signatures.

Abstract

Neutron star (NS) mergers are known to produce heavy elements through rapid neutron capture (r-process) nucleosynthesis. Actinides are expected to be created solely by the r-process in the most neutron rich environments. Confirming if NS mergers provide the requisite conditions for actinide creation is therefore central to determining their origin in the Universe. Actinide signatures in kilonova (KN) spectra may yield an answer, provided adequate models are available in order to interpret observational data. In this study, we investigate actinide signatures in neutron rich merger ejecta. We use three ejecta models with different compositions and radioactive power, generated by nucleosynthesis calculations using the same initial electron fraction ($Y_e = 0.15$) but with different nuclear physics inputs and thermodynamic expansion history. These are evolved from 10 - 100 days after merger using the SUMO non-local thermodynamic equilibrium (NLTE) radiative transfer code. We highlight how uncertainties in nuclear properties, as well as choices in thermodynamic trajectory, may yield entirely different outputs for equal values of $Y_e$. We consider an actinide-free model and two actinide-rich models, and find that the emergent spectra and lightcurve evolution are significantly different depending on the amount of actinides present, and the overall decay properties of the models. We also present potential key actinide spectral signatures, of which doubly ionized $_{89}$Ac and $_{90}$Th may be particularly interesting as spectral indicators of actinide presence in KN ejecta.