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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).