Tellurium emission line in kilonova AT 2017gfo

The late-time spectra of the kilonova AT 2017gfo associated with GW170817 exhibit a strong emission line feature at 2.1 mu m. The line structure develops with time and there is no blueshifted absorption feature in the spectra, suggesting that this emission line feature is produced by electron collision excitation. We attribute the emission line to a fine structure line of Tellurium (Te) III, which is one of the most abundant elements in the second r-process peak. By using a synthetic spectral modelling including fine structure emission lines with the solar r-process abundance pattern beyond the first r-process peak, i.e. atomic mass numbers A greater than or similar to 88, we demonstrate that [Te III] 2.10 mu m is indeed expected to be the strongest emission line in the near infrared region. We estimate that the required mass of Te III is similar to 10(-3) M-circle dot, corresponding to the merger ejecta of 0.05 M-circle dot, which is in agreement with the mass estimated from the kilonova light curve.