Formation of Eu II spectral features in magnetic chemically peculiar stars

The advancing resolution of modern spectrometers uncovers increasingly detailed spectral features in astrophysical observations which can be attributed to properties of the energy structure of the corresponding atomic systems such as fine, hyperfine and Zeeman splitting. At the same time, increasing computational power enables us to include these quantum mechanical interactions on different levels of sophistication in our theoretical atomic structure calculations. This project aims at investigating the hyperfine and Zeeman splitting in five spectral lines of singly-ionized Europium which are relevant for astrophysical studies. To that end, we perform ab initio atomic structure calculations with the latest versions of the code packages Grasp2018 and Rhyze in which we treat the two interactions with a) first-order perturbation theory and b) a generalised full-matrix (`all-order') approach. For both sets of atomic data, we synthesize stellar Stokes I and V spectra for a model atmosphere and stellar magnetic field regime typical for magnetic, chemically peculiar Ap stars. We confirm the overall importance of accounting for hyperfine and Zeeman interactions in the atomic data and find significant differences between the two approaches in the synthesized spectra for the Zeeman interaction. The established computational machinery represents a systematic and largely generalised approach to synthesize spectra of magnetic stars from purely ab initio atomic calculations, including hyperfine and Zeeman interaction simultaneously for any atomic species in the periodic table.