Extended Calculations of Energy Levels and Transition Rates of Nd ii-iv Ions for Application to Neutron Star Mergers

The coalescence of a binary neutron star gives rise to electromagnetic emission, known as a kilonova, that is powered by radioactive decays of r-process nuclei. Observations of a kilonova associated with GW170817 provide a unique opportunity to study heavy element synthesis in the universe. However, the atomic data of r-process elements are not yet complete enough to decipher the light curves and spectral features of kilonovae. In this paper, we perform extended atomic calculations of neodymium (Nd, Z = 60) to study the impact of the accuracy in atomic calculations on astrophysical opacities. By employing multiconfiguration Dirac-Hartree-Fock and relativistic configuration interaction methods, we calculate the 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 energy level accuracies achieved in the present work are 10%, 3%, and 11% for Nd ii, Nd iii, and Nd iv, respectively, compared to 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 on the Planck mean opacity is up to a factor of 1.5, which affects the timescale of kilonovae by 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.