Transition probabilities of near-infrared Ce III lines from stellar spectra: applications to kilonovae

TL;DR

This study derives more accurate astrophysical transition probabilities for Ce III lines in near-infrared spectra, confirming their role in kilonova spectra and improving element identification in neutron star merger events.

Contribution

The paper provides empirically derived gf-values for Ce III lines, enhancing the accuracy of spectral analysis in kilonovae and confirming the presence of cerium in GW170817/AT2017gfo.

Findings

Astrophysical gf-values for Ce III lines are about 0.25 dex lower than previous theoretical estimates.

Derived gf-values are consistent within uncertainties when modeling kilonova spectra.

Identification of Ce III lines in kilonova spectra remains valid with new gf-values.

Abstract

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 lack of experimentally accurate atomic data for heavy elements in the near-infrared wavelengths. Recently, Domoto et al. (2022) proposed the absorption features around 14500 A in the observed spectra of GW170817/AT2017gfo as 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 the three Ce III lines, aiming at verification of this identification. We model high resolution H-band spectra of four F-type supergiants showing the clear Ce III absorption features by assuming stellar parameters derived from optical spectra in literatures. We also test the validity of the derived astrophysical gf-values by estimating Ce III abundances in 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 range. By performing radiative transfer simulations of kilonovae with the derived gf-values, we find that the identification of Ce III as a source of the absorption features in the observed kilonova spectra still stands, even considering the uncertainties in the astrophysical gf-values. This supports identification of Ce in the spectra of GW170817/AT2017gfo.