Modelling of early-stage kilonova ejecta opacity in the multi-messenger era: the selenium case

Matteo Bezmalinovich (INAF – Osservatorio Astronomico d’Abruzzo, IT): In the multi-messenger era, kilonovae (KNe) represent key sites of rapid (r-) neutron capture process nucleosynthesis. KNe are quasi-thermal electromagnetic transients powered by the radioactive decay of heavy neutron-rich isotopes synthesized in the aftermath of mergers involving either two neutron stars or a neutron star and a black hole. Essential information about KN ejecta composition can be achieved thanks to models that incorporate radiative transfer simulations together with accurate numerical opacity calculations for spectral characterisation. In this context, the present work focuses on a detailed study of selenium, including atomic data calculations, expansion opacity estimates, and spectral analysis. Selenium is chosen as it represents one of the major abundant element produced by the r-process in the early (0.5-1.5 d) phases of KN. Using the GRASP2018 code, the selenium atomic data are calculated from Se I to Se X. These data are then used to compute the opacity (in the expansion opacity approximation) in conditions relevant for the early phases of a KN. Finally, those energy-dependent opacities were included in a radiative transfer code (POSSIS) to quantify the impact of improved Se atomic data on the expected KN spectrum, and the resulting synthetic spectra are compared with the X-shooter observations of the KN AT2017gfo.