Helium features are inconsistent with the spectral evolution of the kilonova AT2017gfo

The spectral features observed in kilonovae have revealed 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 1 μm has been linked to Sr II, providing the first direct evidence of freshly synthesised r-process material. An alternative interpretation of this feature has been proposed – He Iλ1083.3 nm under certain non-local thermodynamic equilibrium 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 1 μm 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 out of triplet helium.