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We investigate the effect of the presence of lanthanides (Z = 57- 71) on thekilonova at t~hours after the neutron star merger for the first time. For thispurpose, we calculate the atomic structures and the opacities for selectedlanthanides: Nd (Z = 60), Sm (Z = 62), and Eu (Z = 63). We consider theionization degree up to tenth (XI), applicable for the ejecta at t ~ a fewhours after the merger, when the temperature is T ~ 10^5 K. We find that theopacities for the highly ionized lanthanides are exceptionally high, reachingk_exp~1000 cm^2/g for Eu, due to the highly dense energy levels. Using the newopacity, we perform radiative transfer simulations to show that the early lightcurves become fainter by a (maximum) factor of four, in comparison tolanthanide-free ejecta at t~0.1 day. However, the period at which the lightcurves are affected is relatively brief due to the rapid time evolution of theopacity in the outermost layer of the ejecta. We predict that for a source at adistance of ~100 Mpc, UV brightness for lanthanide-rich ejecta shows a drop to~21-22 mag at t~0.1 day and the UV peaks around t~0.2 day with a magnitude of~19 mag. Future detection of such a kilonova by the existing UV satellite likeSwift or the upcoming UV satellite ULTRASAT will provide useful constraints onthe abundance in the outer ejecta and the corresponding nucleosynthesisconditions in the neutron star mergers.

Opacity of the highly ionized lanthanides and the effect on the early kilonova