NLTE spectra of kilonovae

Abstract

ABSTRACT The electromagnetic transient following a binary neutron star merger is known as a kilonova (KN). Owing to rapid expansion velocities and small ejecta masses, KNe rapidly transition into the non-local thermodynamic equilibrium (NLTE) regime. In this study, we present synthetic NLTE spectra of KNe from 5 to 20 d after merger using the sumo spectral synthesis code. We study three homogeneous composition, 1D multizone models with characteristic electron fractions of Ye ∼ 0.35, 0.25, and 0.15. We find that emission features in the spectra tend to emerge in windows of reduced line blocking, as the ejecta are still only partially transparent even at 20 d. For the Ye ∼ 0.35 (lanthanide-free) ejecta, we find that the neutral and singly ionized species of Rb, Sr, Y, and Zr dominate the spectra, all with good potential for identification. We directly test and confirm an impact of Sr on the 10 000 Å spectral region in lanthanide-free ejecta, but also see that its signatures may be complex. We suggest the Rb i$\rm {5p^{1}}$–$\rm {5s^{1}}$ 7900 Å transition as a candidate for the λ0 ∼ 7500–7900 Å P-Cygni feature in AT2017gfo. For the Ye ∼ 0.25 and 0.15 compositions, lanthanides are dominant in the spectral formation, in particular Nd, Sm, and Dy. We identify key processes in KN spectral formation, notably that scattering and fluorescence play important roles even up to 20 d after merger, implying that the KN ejecta are not yet optically thin at this time.