  We investigate the effect of the presence of lanthanides (Z = 57- 71) on the
kilonova at t~hours 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 tenth (XI), applicable for the ejecta at t ~ a few
hours after the merger, when the temperature is T ~ 10^5 K. We find that the
opacities for the highly ionized lanthanides are exceptionally high, reaching
k_exp~1000 cm^2/g 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 day. However, the period at which the light
curves are affected is relatively brief due 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 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 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.

High Energy Astrophysical Phenomena;Solar and Stellar Astrophysics;Atomic Physics