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

TL;DR

This study investigates the spectral feature at 1μm in kilonova AT2017gfo, demonstrating it is inconsistent with helium interpretation and supporting its identification as Sr II, thus clarifying the elemental composition of the ejecta.

Contribution

The paper provides a detailed temporal and spectral analysis that rules out helium as the source of the 1μm feature, confirming its association with Sr II in kilonova spectra.

Findings

The 1μm feature does not match helium spectral evolution.

Helium models cannot reproduce the observed feature.

The feature is consistent with Sr II identification.

Abstract

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 1$\mu$m 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 $\lambda 1083.3$nm 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 1$\mu$m 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.