Cerium features in kilonova near-infrared spectra: implication from a chemically peculiar star

TL;DR

This study uses chemically peculiar stars as astrophysical laboratories to identify cerium features in kilonova spectra, aiding the understanding of heavy element formation in neutron star mergers.

Contribution

It demonstrates that spectra of chemically peculiar stars can serve as proxies to identify elements in kilonova spectra, especially for near-infrared lanthanide features.

Findings

Ce III lines match broad absorption features in GW170817 spectra.

Chemically peculiar star HR 465 has similar lanthanide abundances to kilonova regions.

Supports identification of Ce III as a key element in kilonova spectra.

Abstract

Observations of the kilonova from a neutron star merger event GW170817 opened a way to directly study r-process nucleosynthesis by neutron star mergers. It is, however, challenging to identify the individual elements in the kilonova spectra due to lack of complete atomic data, in particular, at near-infrared wavelengths. In this paper, we demonstrate that spectra of chemically peculiar stars with enhanced heavy element abundances can provide us with an excellent astrophysical laboratory for kilonova spectra. We show that the photosphere of a late B-type chemically peculiar star HR 465 has similar lanthanide abundances and ionization degrees with those in the line forming region in a kilonova at $\sim 2.5$ days after the merger. The near-infrared spectrum of HR 465 taken with Subaru/IRD indicates that Ce III lines give the strongest absorption features around 16,000 A and there are no other comparably strong transitions around these lines. The Ce III lines nicely match with the broad absorption features at 14,500 A observed in GW170817 with a blueshift of v=0.1c, which supports recent identification of this feature as Ce III by Domoto et al. (2022).