New atomic data in heavy elements for the determination of kilonova ejecta opacity

The production of elements heavier than iron in the Universe still remains an unsolved mystery. About half of them are thought to be produced by the astrophysical r-process (rapid neutron-capture process), for which one of the most promising production sites are neutron star mergers (NSMs). In August 2017, gravitational waves generated by a NSM were detected for the first time by the LIGO detectors (event GW170817), and the observation of its electromagnetic counterpart, the kilonova (KN) AT2017gfo, suggested the presence of heavy elements in the KN ejecta. The luminosity and spectra of such KN emission depend significantly on the ejecta opacity, which is thought to be dominated by millions of lines from the heavy elements produced by the r-process, in particular f-shell elements, i.e. lanthanides and actinides. Atomic data and opacities for these elements are thus sorely needed to model and interpret KN light curves and spectra.

In this context, the present work focusses on large-scale atomic data and opacity computations for all heavy elements with Z ≥ 20, with a special effort on lanthanides and actinides, for a grid of typical KN ejecta conditions (temperature, density and time post-merger) between one day and one week after the merger (corresponding to the LTE photosphere phase of the KN ejecta). In order to do so, we used the pseudo-relativistic Hartree-Fock (HFR) method as implemented in the Cowan’s codes, in which the choice of the interaction configuration model is of crucial importance. In this contribution, HFR atomic data and opacities for all lanthanides and actinides will be presented. Besides, we will also discuss the contribution of every single element with Z ≥ 20 to the total KN ejecta opacity for several NSM models, depending on their Planck mean opacities and elemental abundances deduced from NSM simulations. The impact of considering such atomic-physics based opacity data instead of typical crude approximation formulae for the determination of the total KN ejecta opacity used in KN light curve modeling will also be evaluated.