Opacity of the Highly Ionized Lanthanides and the Effect on the Early Kilonova

We investigate the effect of the presence of lanthanides (Z = 57-71) on the kilonova at t ~ 1 hr after the neutron star merger for the first time. For this purpose, we calculate the atomic structures and the opacities for selected lanthanides: Nd (Z = 60), Sm (Z = 62), and Eu (Z = 63). We consider the ionization degree up to 10th (XI), applicable for the ejecta at t ~ a few hours after the merger, when the temperature is T ~ 10⁵ K. We find that the opacities for the highly ionized lanthanides are exceptionally high, reaching ${\kappa }_{\exp }\sim 1000\,{\mathrm{cm}}^{2}\,{{\rm{g}}}^{-1}$ for Eu, due to the highly dense energy levels. Using the new opacity, we perform radiative transfer simulations to show that the early light curves become fainter by a (maximum) factor of four, in comparison to lanthanide-free ejecta at t ~ 0.1 days. However, the period at which the light curves are affected is relatively brief owing to the rapid time evolution of the opacity in the outermost layer of the ejecta. We predict that for a source at a distance of ~100 Mpc, UV brightness for lanthanide-rich ejecta shows a drop to ~21-22 mag at t ~ 0.1 days and the UV peaks around t ~ 0.2 days with a magnitude of ~19 mag. Future detection of such a kilonova by an existing UV satellite like Swift or the upcoming UV satellite ULTRASAT will provide useful constraints on the abundance in the outer ejecta and the corresponding nucleosynthesis conditions in the neutron star mergers.