Modelling the spectra of the kilonova AT2017gfo -- II: Beyond the photospheric epochs

Binary neutron star mergers are the first confirmed site of element nucleosynthesis by the rapid neutron-capture process (r-process). The kilonova AT2017gfo is the only electromagnetic counterpart of a neutron star merger spectroscopically observed. We analyse the entire spectral sequence of AT2017gfo (from merger to +10.4 days) and identify seven emission-like features. We confirm that the prominent 1.08 um feature can be explained by the Sr II near-infrared triplet evolving from a P-Cygni profile through to pure emission. We calculate the expected strength of the [Sr II] doublet and show that its absence requires highly clumped ejecta. Near-infrared features at 1.58 and 2.07 um emerge after three days and become more prominent as the spectra evolve. We model these as optically thick P-Cygni profiles and alternatively as pure emission features (with FWHM = 35600 +/- 6600 km/s), and favour the latter interpretation. The profile of the strong 2.07 um emission feature is best reproduced with two lines, centred at 2.059 and 2.135 um. We search for candidate ions for all prominent features in the spectra. Strong, permitted transitions of La III, Ce III, Gd III, Ra II and Ac I are plausible candidates for the emission features. If any of these features are produced by intrinsically weak, forbidden transitions, we highlight candidate ions spanning the three r-process peaks. The second r-process peak elements Te and I have plausible matches to multiple features. We highlight the need for more detailed and quantitative atomic line transition data.