Observations of “kilonova" from neutron star (NS) merger GW170817 provided the first evidence that the NS merger is a production site of r-process elements. Although the observed spectra of the kilonova have multiple features especially in near-infrared (NIR) wavelengths, most of them are not yet decoded. The lines of doubly ionized Ce (Z=58), one of the lanthanides, are promising candidates for the absorption features at NIR wavelengths. This identification is, however, still not yet conclusive, because atomic transition data in NIR region are poorly understood by experiments. To make this identification conclusive, we propose high-resolution NIR spectroscopic observations of chemically peculiar (CP) stars with various effective temperatures and abundance patterns. Because atmospheric condition of CP stars can remarkably mimic that of photosphere of kilonovae, CP stars can be the best astrophysical laboratory to study strong transitions in kilonova spectra. By this observation, we systematically study the strength of transitions of the proposed Ce III lines and search if any unknown lines contribute absorption features in kilonovae. This enables the firm element identification in kilonova spectra, which opens a new way to directly measure the abundances of lanthanides produced by NS mergers.
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