Actinide opacities for modeling the spectra and light curves of kilonovae

TL;DR

This paper extends actinide opacity calculations for kilonova modeling, providing detailed tables and analyzing their impact on light curves and spectra, revealing limited sensitivity to actinide distributions but notable effects from ejecta parameters.

Contribution

It introduces comprehensive actinide opacities for kilonova simulations, including publicly available tables and analysis of their influence on observable features.

Findings

Actinide opacities show similar trends to lanthanides, with lighter actinides having higher opacity.

Spectral features from actinides are faint and mainly in the far infrared at late times.

Ejecta mass and velocity significantly affect kilonova emission, more than actinide distribution variations.

Abstract

We extend previous ab initio calculations of lanthanide opacities (Fontes et al., 2020, MNRAS, 493, 4143) to include a complete set of actinide opacities for use in the modeling of kilonova 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 $(89 \le Z \le 102)$ over a grid of relevant temperatures and densities. These tabular opacities will be made publicly available for general usage in kilonova modeling. We demonstrate the usefulness of these opacities in kilonova 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 $Z \ge 99$ 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 days 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.