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 betweeneach of two very bright UV lines of Fe II at 2507 Å and 2509Å and its corresponding satellite line, observed in HST spectra ofa compact gaseous condensation (blob B) in the vicinity of the starprotect eta Car.The model is based on the assumption that the FeIItransitions have a substantial optical thickness, which varies as aresult of the photodepletion of the long-lived lower states,c<SUP>4</SUP>F<SUB>7/2,9/2</SUB>, by the intense Lyalpha radiation. Inconditions where the photodepletion rates of these two states differ,the resonance scattering lengths of the spectral lines become differentas well. In the presence of a weak, nonresonant (continuous) absorption,this gives rise to a difference in attenuation between the spectrallines (the Hummer effect (Hummer 1968)). It is the combination of thesetwo effects that explains the anomalous (as compared with laboratorydata) intensity ratios observed in the two pairs of UV FeII lines.Ionization channels of stimulated photodepletion by the Lyalpharadiation, including the possible role of autoionization levels, areconsidered.