The origin of elements heavier than iron remains a major open question in astrophysics. Roughly half of them are believed to have formed through the rapid neutron capture process, with neutron star mergers (NSMs) among the most promising sites. The 2017 detection of gravitational waves from an NSM (GW170817) and the observation of its electromagnetic counterpart—the kilonova (KN) AT2017gfo—provided the first direct evidence for heavy element production in NSMs. The luminosity and spectra of KNe depend critically on the ejecta opacity, which is dominated by millions of lines from r-process elements, especially lanthanides and actinides. Reliable atomic data and opacities for these elements are thus essential to model KNe and interpret these observations.
This work presents a large-scale computation of atomic data and opacities for all heavy elements (Z ≥ 20), with special attention paid to lanthanides and actinides, across a grid of typical KN ejecta conditions within one week after the merger (LTE photospheric phase) using the pseudo-relativistic Hartree–Fock (HFR) method as implemented in Cowan’s code. All resulting atomic and opacity data are publicly available (https://zenodo.org/records/14017952).
Beyond a week post-merger, the KN ejecta enters the nebular phase, where NLTE effects become important, so that every radiative and collisional process must be considered to derive level populations and, hence, the opacity of each element within the KN ejecta, making the spectral modelling highly complex. Due to the scarcity of atomic data for collisional processes in heavy elements, radiative transfer simulations rely on crude empirical formulae to model nebular-phase KN spectra. To improve the situation both in terms of completeness and accuracy, we benchmarked an efficient and scalable approach based on the Plane-Wave Born approximation within the HFR framework. The results are compared with more sophisticated R-Matrix calculations and with existing empirical prescriptions, demonstrating a good balance between computational feasibility and accuracy.
