Interpretation of Anomalous Ultraviolet Transitions of Fe II Observed in Laboratory Fourier Transform Spectra and Stellar HST and IUE Spectra

Abstract

We discuss LS-forbidden ΔS = 2 transitions of Fe II that appear as strong lines in ultraviolet stellar and laboratory spectra. The lines occur because of indirect level mixing-two close energy levels, W²P_3/2 and x⁶P_3/2, are mixed through a third level acting as a catalyst. In transition multiplets, that involve either the w²P_3/2 or the x⁶P_3/2 level, the normal component is accompanied by a close, parasite component. These components are well resolved in laboratory Fourier transform spectra, from which accurate intensity ratios can be measured. Spectra of the HgMn star χ Lupi recorded with the Hubble Space Telescope, show the normal and the parasite components resolved for the first time in stellar spectra. These have been used together with laboratory spectra to derive oscillator strengths for four multiplets and the mixing coefficients for the two interacting energy levels. We also provide improved spectroscopic data for all Fe ii multiplets that include the w²P_3/2 and x⁶P_3/2 levels.

The level mixing involves the UV 191 multiplet of Fe II around 1780 Å, which is shown to have one parasite component. This multiplet is known to be a prominent emission feature in spectra of various objects, e.g., cool star chromospheres and symbiotic stars. As an example, we show IUE spectra of the eclipsing binary 32 Cyg, which provides perfect conditions for fluorescence experiments, and discuss possible excitation mechanisms of UV 191.

Based on second-order perturbation theory and multiconfiguration Hartree-Fock calculations, an atomic three-level model is constructed. This model explains the contradiction found when applying first-order perturbation theory, viz., two close energy levels perturbing each other without causing a repulsion.