Measurements of Optical Emission from Singly Ionized Er Ions at LHD for Laboratory Assessment of Atomic Data Relevant to Opacity of Kilonovae

The atomic data of heavy elements, especially rare-earth metals, plays a crucial role in enhancing our understanding and interpreting kilonova spectra and underlying astrophysical processes. Among these elements, Erbium (Er) is particularly intriguing because it is important for opacities of the kilonova observed in 2017 (GW170817). In order to assess the atomic data, optical spectra of Er ions were precisely measured in 385-400\,nm at Large Helical Device (LHD). In the present experiment, Er was injected into the core plasma of LHD through carbon pellets containing Er powders. The electron density and temperature of the Er-contained C pellet ablation cloud were obtained to be 1.6 × 10²²\,m⁻³ and 1.4 eV using the Stark broadening of a C II line and the Boltzmann plot of Er II lines, respectively. Transition probabilities of observed Er II lines were assessed using the Boltzmann plot analysis. Recent measurements with laser-induced breakdown spectroscopy (LIBS) of an Er II line at 393.86 nm were confirmed by the present work.