EDP Sciences
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Volume 378, Number 1, October IV 2001
Page(s) 266 - 278
Section Physical and chemical processes
DOI http://dx.doi.org/10.1051/0004-6361:20011062

A&A 378, 266-278 (2001)
DOI: 10.1051/0004-6361:20011062

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

S. Johansson1 and V. S. Letokhov2, 1

1  Department of Physics, Lund University, PO Box 118, 22100 Lund, Sweden
    e-mail: Vladilen.Letokhov@fysik.lu.se
2  Institute of Spectroscopy, Russian Academy of Sciences, Troitsk, Moscow region, 142190, Russia

(Received 13 March 2001 / Accepted 19 July 2001 )

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 HLy$\alpha$ 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 HLy$\alpha$, which makes it possible to selectively photo-excite short-lived ($\approx$1 ns) states at 11.2 eV in Fe II. Within the frame of the present model the intense HLy$\alpha$ radiation induces a depletion of the lower, long-lived ($\approx$1 ms) level of the strong UV lines by photoionization. The subsequent recombination of Fe III explains the appearance of the "forest" of narrow Fe II lines. The possibility of a cyclic process is also discussed, where HLy$\alpha$ induces depletion to bound states, which have fast decays in the far-UV to the lower state of the Bowen pumping channel.

Key words: atomic processes -- line: formation -- radiation mechansims: non-thermal -- stars: individual: $\eta$ Carinae

Offprint request: S. Johansson, sveneric.johansson@fysik.lu.se

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