Opacities of Singly and Doubly Ionised Neodymium and Uranium for Kilonova Emission Modeling

Abstract

In 2017, the electromagnetic counterpart AT2017gfo to the binary neutron star merger GW170817 was observed by all major telescopes on Earth. While it was immediately clear that the transient following the merger event, is powered by the radioactive decay of r-process nuclei, only few tentative identifications of light r-process elements have been made so far. One of the major limitations for the identification of heavy nuclei based on light curves or spectral features is incomplete or missing atomic data. While some progress has been made on lanthanide atomic data over the last few years, for actinides very little atomic data is available. We perform atomic structure calculations of neodymium (𝑍 = 60) as well as the corresponding actinide uranium (𝑍 = 92). Using two different codes (FAC and HFR) for the calculation of the atomic data, we investigate the accuracy of the calculated data (energy levels and electric dipole transitions) and their effect on kilonova opacities. For the FAC calculations, we optimise the local central potential and the number of included configurations and use a dedicated calibration technique to improve the agreement between theoretical and available experimental atomic energy levels (AELs). For ions with vast amounts of experimental data available, the presented opacities agree quite well with previous estimations. On the other hand, the optimisation and calibration method cannot be used for ions with only few available AELs. For these cases, where no experimental nor benchmarked calculations are available, a large spread in the opacities estimated from the atomic data obtained with the various atomic structure codes is observed, most-likely due to the uncertainty in the density of low-lying levels predicted by theory. We find that the opacity of uranium is almost double the neodymium opacity.

Opacity of the Highly Ionized Lanthanides and the Effect on the Early Kilonova

“…In the past decade, calculations of the atomic opacity have been the intense focus of research (e.g., Kasen et al 2013;Tanaka & Hotokezaka 2013;Fontes et al 2015;Wollaeger et al 2017;Tanaka et al 2018;Gaigalas et al 2019;Tanaka et al 2020;Fontes et al 2020;Rynkun et al 2022;Fontes et al 2023;Flörs et al 2023), although all of these studies focus on the relatively lower ionized elements, maximum up to third (or IV in spectroscopic notation; hereafter used to describe the ionization), which are important for the kilonova at late times (t > 1 day).…”

On the sensitivity of uranium opacity with respect to the atomic properties in the context of kilonova emission modeling

“…The Hamiltonian eigenvalues and eigenstates obtained are then used to compute the radiative parameters such as wavelengths and oscillator strengths for all transitions. The multiconfiguration models considered for U II and U III are based on one of our recent works [26], where the importance of the multiconfiguration model choice is notably highlighted. The configurations included in the models, which are listed in Table 1, were built by considering single electron excitations from the ground configuration to n = 6, 7, 8, 9 shells as well as several double electron excitations toward selected n = 6 and n = 7 subshells.…”

“…Several studies have focused on atomic data computations for opacity determination in weakly and moderately charged lanthanides [16][17][18][19][20][21][22][23][24][25][26]. However, only a few and very recent studies concerned actinide opacities for kilonova modeling purpose [25][26][27].…”

“…In this paper, we focus on atomic data and opacity determination for singly-and doubly ionized uranium (U II and U III), since they are the dominant uranium species predicted to be present in the kilonova ejecta for typical conditions as observed in AT2017gfo 1 day after the merger (i.e., a temperature T = 5000 K and a density ρ = 10 −13 g/cm 3 ) [20,26]. In order to do this, we used the pseudo-relativistic Hartree-Fock (HFR) method as implemented in Cowan's code [28].…”

“…The main purpose of this study is to assess the necessity of taking some extra physical effects into account in our HFR computations, which were not considered in our previous work [26], such as the core polarization effects [29], as well as to evaluate the added value of performing a calibration of our HFR results with respect to experimental data or data available in the literature (which exist only for a very limited number of energy levels). 9f, 5f 4 9g, 5f 3 6d 2 , 5f 3 6d7s, 5f 3 6d7p, 5f 3 6d7d, 5f 3 7s7p, 5f 3 7s7d U III 5f 4 , 5f 3 6d, 5f 3 6f, 5f 3 6g,5f 3 7s, 5f 3 7p, 5f 3 7d, 5f 3 7f, 5f 3 7g, 5f 3 8s, 5f 3 8p, 5f 3 8d, 5f 3 8f, 5f 3 8g,5f 3 9s, 5f 3 9p, 5f 3 9d, 5f 3 9f, 5f 3 9g, 5f 2 6d 2 , 5f 2 6d7s, 5f 2 6d7p, 5f 2 6d7d,5f 2 7s 2 , 5f 2 7s7p, 5f 2 7s7d…”