Modelling the spectra of the kilonova AT2017gfo – II. Beyond the photospheric epochs

Abstract

ABSTRACT 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 d) and identify seven emission-like features. We confirm that the prominent 1.08 $\mu{\text{m}}$ 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 $\mu {\text{m}}$ 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 ≃ 35 600 ± 6600 km s−1) and favour the latter interpretation. The profile of the strong 2.07 $\mu {\text{m}}$ emission feature is best reproduced with two lines, centred at 2.059 and 2.135 $\mu {\text{m}}$. 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.