Transition Probabilities of Near-infrared Ce iii Lines from Stellar Spectra: Applications to Kilonovae

Kilonova spectra provide us with information of r -process nucleosynthesis in neutron star mergers. However, it is still challenging to identify individual elements in the spectra mainly due to the lack of experimentally accurate atomic data for heavy elements at near-infrared wavelengths. Recently, Domoto et al. proposed that the absorption features around 14500 Å in the observed spectra of GW170817/AT2017gfo are Ce iii lines. But they used theoretical transition probabilities ( gf -values) whose accuracy is uncertain. In this paper, we derive the astrophysical gf -values of three Ce iii lines, aiming at verifying this identification. We model high-resolution H -band spectra of four F-type supergiants showing clear Ce iii absorption features by assuming stellar parameters derived from optical spectra in the literature. We also test the validity of the derived astrophysical gf -values by estimating the Ce iii abundances of Ap stars. We find that the derived astrophysical gf -values of the Ce iii lines are systematically lower by about 0.25 dex than those used in previous work of kilonovae, while they are still compatible within the uncertainty ranges. By performing radiative transfer simulations of kilonovae with the derived gf -values, we find that the identification of Ce iii as a source of absorption features in the observed kilonova spectra still stands, even considering the uncertainties in the astrophysical gf -values. This supports the identification of Ce in the spectra of GW170817/AT2017gfo.