The spectral features observed in kilonovae (KNe) reveal the elemental composition and the velocity structures of matter ejected from neutron star mergers. In the spectra of the kilonova AT2017gfo, a P Cygni line at about 1μm has been linked to Sr II, providing the first direct evidence of freshly synthesised r-process material. An alternative explanation to Sr II was proposed - He I λ1083.3nm under certain non-local-thermodynamic-equilibrium (NLTE) conditions. A key way to robustly discriminate between these identifications, and indeed other proposed identifications, is to analyse the temporal emergence and evolution of the feature. In this analysis we trace the earliest appearance of the observed feature and detail its spectro-temporal evolution, which we compare with a collisional-radiative model of helium. We show that the 1μm P Cygni line is inconsistent with a He I interpretation both in emergence time and in subsequent spectral evolution. Self-consistent helium masses cannot reproduce the observed feature, due to the diminishing strength of radiative pathways leaving triplet helium.
@article{arxiv.2407.12907,
title = {Helium features are inconsistent with the spectral evolution of the kilonova AT2017gfo},
author = {Albert Sneppen and Rasmus Damgaard and Darach Watson and Christine E. Collins and Luke Shingles and Stuart A. Sim},
journal= {arXiv preprint arXiv:2407.12907},
year = {2026}
}