Actinide opacities for modelling the spectra and light curves of kilonovae

We extend previous ab initio calculations of lanthanide opacities to include a complete set of actinide opacities for use in the modelling of kilonova (KN) light curves and spectra. Detailed, fine-structure line features are generated using the configuration-interaction approach. These actinide opacities display similar trends to those observed for lanthanide opacities, such as the lighter actinides producing higher opacity than the heavier ones for relevant conditions in the dynamical ejecta. A line-binned treatment is employed to pre-compute opacity tables for 14 actinide elements (89 <= Z <= 102) over a grid of relevant temperatures and densities. These tabular opacities will be made publicly available for general usage in KN modelling. We demonstrate the usefulness of these opacities in KN simulations by exploring the sensitivity of light curves and spectra to different actinide abundance distributions that are predicted by different nuclear theories, as well as to different choices of ejecta mass and velocity. We find very little sensitivity to the two considered distributions, indicating that opacities for actinides with Z >= 99 do not contribute strongly. On the other hand, a single actinide element, protactinium, is found to produce faint spectral features in the far-infrared at late times (5-7 d post merger). More generally, we find that the choice of ejecta mass and velocity have the most significant effect on KN emission for this study.

Automated summary

This paper extends previous calculations of lanthanide opacities to include actinide opacities, and demonstrates their usefulness in kilonova simulations. The opacities exhibit similar trends to lanthanide opacities, but produce different results for different actinide elements. Opacity tables for 14 actinide elements are provided for general usage. The study also explores the impact of different actinide abundance distributions and choices of ejecta mass and velocity on light curves and spectra. The results indicate that actinides with Z >= 99 do not significantly contribute to the opacity, while protactinium produces faint spectral features in the far-infrared.