# Extended calculations of energy levels and transition rates of Nd II-IV ions for application to neutron star mergers

## TL;DR

This study performs advanced atomic calculations for neodymium ions relevant to kilonovae, improving data accuracy to better model their light curves and spectra, which are crucial for understanding heavy element synthesis in neutron star mergers.

## Contribution

The paper introduces extended atomic calculations of Nd II-IV ions using multiconfiguration Dirac-Hartree-Fock methods, enhancing the accuracy of energy levels and transition data for astrophysical applications.

## Key findings

- New energy level data agree better with experimental values.
- Atomic calculation accuracy impacts wavelength-dependent opacity features.
- Opacity impact on kilonova timescales is up to 20%.

## Abstract

Coalescence of binary neutron star give rise to electromagnetic emission, kilonova, powered by radioactive decays of r-process nuclei. Observations of kilonova associated with GW170817 provided unique opportunity to study the heavy element synthesis in the Universe. However, atomic data of r-process elements to decipher the light curves and spectral features of kilonova are not fully constructed yet. In this paper, we perform extended atomic calculations of neodymium (Nd, Z=60) to study the impact of accuracies in atomic calculations to the astrophysical opacities. By employing multiconfiguration Dirac-Hartree-Fock and relativistic configuration interaction methods, we calculate energy levels and transition data of electric dipole transitions for Nd II, Nd III, and Nd IV ions. Compared with previous calculations, our new results provide better agreement with the experimental data. The accuracy of energy levels was achieved in the present work 10 %, 3 % and 11 % for Nd II, Nd III and Nd IV, respectively, comparing with the NIST database. We confirm that the overall properties of the opacity are not significantly affected by the accuracies of the atomic calculations. The impact to the Planck mean opacity is up to a factor of 1.5, which affects the timescale of kilonova at most 20 %. However, we find that the wavelength dependent features in the opacity are affected by the accuracies of the calculations. We emphasize that accurate atomic calculations, in particular for low-lying energy levels, are important to provide predictions of kilonova light curves and spectra.