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

ABSTRACT Lanthanides play the 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 of 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 is controlled by transition arrays among several configurations, for which derivation of accurate energy level distribution is important to obtain reliable opacities.