Statistical RTA simulations of atomic data for astrophysical opacity modeling in the context of kilonova emission

Abstract

When considering some complex lanthanide ions characterized by half-filled 4f
subshell, the atomic structure Hamiltonian matrix sizes are so big that their diagonalization
is challenging and therefore the atomic data of these ions are hardly provided for computing
the expansion opacity of a kilonova. In order to avoid this problem, we propose a statistical
simulation method for computing kilonova expansion opacities based on the Resolved
Transition Array (RTA) method of Bauche et al (1991, Phys. Rev. A 44, 5707). The
atomic structure HFR method has been employed to compute the radial integrals necessary
to our statistical RTA simulations where the atomic data are randomly drawn using their
corresponding statistical distributions and to determine the exact expansion opacities where
the atomic data are obtained by the diagonalization of the Hamiltonian matrix. The statistical
RTA simulations carried out in two specific ions, i.e. Sm VIII and Eu VI, for which it is
still possible to diagonalize the Hamiltonian matrix, reproduce well the expansion opacities
computed using HFR atomic data. Based on these good agreements, the statistical RTA method
has been used to compute the expansion opacity of Dy VIII which is hardly determined through
diagonalization. The proposed statistical RTA simulation method allows to compute reliable
astrophysical expansion opacities which are of paramount importance for kilonova light curve
modeling and spectral analysis.