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Kilonova spectra imprint valuable information about the elements synthesizedin neutron star mergers. In AT2017gfo, the kilonova associated with GW170817,the spectroscopic feature centered around 8000 angstroms has been interpretedas the P-Cygni profile arising from singly ionized Strontium. Recently, Peregoet al. (2022) suggested that Helium 10833 line can be an alternativeexplanation of the feature. Here, we study the line features under non-localthermodynamic equilibrium. We find that the ionization of ejecta by thestopping of radioactive decay product can significantly enhance the ionizationstates around the line forming region. We compute the kilonova spectrum underthe assumption of spherical symmetry and uniform elemental fraction in theline-forming region. We find that 0.2\% (in mass) of Helium in the ejecta canreproduce the P-Cygni feature in the observed spectrum at $1.43$ -- $4.40$days. Strontium with a mass fraction of $1\%$ is also able to make theabsorption feature at $\sim 1.5\,$days, but it gets weaker with time due toionization by radioactive decay products. The strength of the He line signaturedepends sensitively on UV strength for the first two epochs. Further modelingof UV line blanketing by $r$-process elements and the optical properties oflight $r$-process elements would be crucial to distinguish between Helium andStrontium features. The mass fraction of He is a good indicator for ejectaentropy that allows us to probe the mass ejection mechanism.

Non-LTE analysis for Helium and Strontium lines in the kilonova AT2017gfo