Theoretical study of Th III energy levels and transitions for applications to kilonova spectra

TL;DR

This study provides detailed atomic data for Th III, crucial for modeling kilonova spectra, and demonstrates that thorium can produce detectable spectral features in neutron star merger events.

Contribution

The paper offers new energy level and transition data for Th III using advanced atomic structure calculations, aiding kilonova spectral analysis.

Findings

Th III energy levels agree with experimental data within 436 cm$^{-1}$ RMS deviation.

Simulations show Th can produce detectable absorption features at 18,000 Å in kilonova spectra.

Th mass fraction of (3-10) x 10$^{-5}$ can generate observable spectral signatures.

Abstract

The neutron star merger is a promising site of heavy element production. By producing heavy elements, the neutron star merger gives rise to a thermal transient called a kilonova. Studying kilonova spectra enables us to quantify the heavy element production. Among the heaviest elements, doubly ionized Thorium (Th, Z=90) is one of the important candidates for producing detectable absorption features in kilonova spectra. This paper investigates the atomic properties of Th III to provide energy level and transition data. The multiconfiguration Dirac-Hartree-Fock and relativistic configuration interaction methods, which are implemented in the general-purpose relativistic atomic structure package GRASP2018, are used to compute energy levels of the $\mathrm{5f6d}$, $\mathrm{6d^2}$, $\mathrm{7s^2}$, $5\mathrm{f^2}$, $\mathrm{6d7s}$, $\mathrm{5f7p}$ and $\mathrm{5f7s}$ configurations and electric dipole transitions between states of these configurations. The accuracy of energy levels is evaluated by comparing it with experimental data and with various theoretical methods. Our calculated energy levels are consistent with the experimental results with a root mean square (RMS) deviation of 436 cm$^{-1}$. The accuracy of transition data is investigated using the quantitative and qualitative evaluation method.By performing radiative transfer simulations for kilonova spectra with our transition data, we show that kilonova including Th with a mass fraction of $(3-10) \times 10^{-5}$ can produce Th III absorption features around 18,000 A.