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

Abstract:
We present a qualitative physical model of the origin of two very brightUV lines of Fe II at 2507-2509 Å, present in high-resolutionspectra of gaseous condensations (blobs) close to the central star ofeta Carinae. The spectra have been obtained with the Hubble SpaceTelescope (HST). The model is based on a type of Bowen mechanism withselective photoexcitation of Fe II by a broad HLyalpha profile,generated in the HII region and diffusely transferred to the opticallythick HI region. The frequency distribution by large Doppler diffusioninto the HI region provides a substantial broadening of HLyalpha , whichmakes it possible to selectively photo-excite short-lived (~1 ns) statesat 11.2 eV in Fe II. Within the frame of the present model the intenseHLyalpha radiation induces a depletion of the lower, long-lived (~1 ms)level of the strong UV lines by photoionization. The subsequentrecombination of Fe III explains the appearance of the ``forest'' ofnarrow Fe II lines. The possibility of a cyclic process is alsodiscussed, where HLyalpha induces depletion to bound states, which havefast decays in the far-UV to the lower state of the Bowen pumpingchannel.