Systematic opacity calculations for kilonovae -- II. Improved atomic data for singly ionized lanthanides

TL;DR

This paper improves atomic data calculations for singly ionized lanthanides, leading to more accurate opacities crucial for modeling kilonova emissions from neutron star mergers.

Contribution

It introduces enhanced atomic calculation strategies for lanthanides, resulting in higher and more reliable opacity estimates compared to previous models.

Findings

New atomic data yield opacities 1.5 to 10 times higher than prior results.

Good agreement found between improved calculations and ab-initio methods.

Accurate energy level distributions are essential for reliable lanthanide opacity modeling.

Abstract

Lanthanides play most important roles in the opacities for kilonova, ultraviolet-optical-infrared emission from neutron star mergers. Although several efforts have been made to construct atomic data, the accuracy in the opacity is not fully assessed and understood. In this paper, we perform atomic calculations for singly ionized lanthanides with improved strategies, aiming at understanding the physics of the lanthanide opacities in kilonova ejecta and necessary accuracy in atomic data. Our results show systematically lower energy level distributions as compared with our previous study (Paper I). As a result, the opacities evaluated with our new results are higher by a factor of up to 3 - 10, depending on the element and wavelength range. For a lanthanide-rich element mixture, our results give a higher opacity than that in Paper I by a factor of about 1.5. We also present opacities by using the results of ab-initio atomic calculations by using Grasp2K code. In general, our new opacities show good agreements with those with ab-initio calculations. We identify that structure of the lanthanide opacities are controlled by transition arrays among several configurations, for which derivation of accurate energy level distribution is important to obtain reliable opacities.