A model for the origin of the anomalous and very bright UV lines of Fe II in gaseous condensations of the star η Carinae

Abstract

We present a qualitative physical model of the origin of two very bright UV lines of Fe II at 2507-2509 Å, present in high-resolution spectra of gaseous condensations (blobs) close to the central star of eta Carinae. The spectra have been obtained with the Hubble Space Telescope (HST). The model is based on a type of Bowen mechanism with selective photoexcitation of Fe II by a broad HLyalpha profile, generated in the HII region and diffusely transferred to the optically thick HI region. The frequency distribution by large Doppler diffusion into the HI region provides a substantial broadening of HLyalpha , which makes it possible to selectively photo-excite short-lived (~1 ns) states at 11.2 eV in Fe II. Within the frame of the present model the intense HLyalpha radiation induces a depletion of the lower, long-lived (~1 ms) level of the strong UV lines by photoionization. The subsequent recombination of Fe III explains the appearance of the ``forest'' of narrow Fe II lines. The possibility of a cyclic process is also discussed, where HLyalpha induces depletion to bound states, which have fast decays in the far-UV to the lower state of the Bowen pumping channel.