The origin of the anomalous intensity ratio between very bright UV FeII lines and their satellites in gaseous condensations close to the star eta Carinae

Abstract

We present a model that explains the anomalous intensity ratios between each of two very bright UV lines of Fe II at 2507 Å and 2509 Å and its corresponding satellite line, observed in HST spectra of a compact gaseous condensation (blob B) in the vicinity of the star protect eta Car.The model is based on the assumption that the FeII transitions have a substantial optical thickness, which varies as a result of the photodepletion of the long-lived lower states, c⁴F_7/2,9/2, by the intense Lyalpha radiation. In conditions where the photodepletion rates of these two states differ, the resonance scattering lengths of the spectral lines become different as well. In the presence of a weak, nonresonant (continuous) absorption, this gives rise to a difference in attenuation between the spectral lines (the Hummer effect (Hummer 1968)). It is the combination of these two effects that explains the anomalous (as compared with laboratory data) intensity ratios observed in the two pairs of UV FeII lines. Ionization channels of stimulated photodepletion by the Lyalpha radiation, including the possible role of autoionization levels, are considered.