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Issue A&A
Volume 428, Number 2, December III 2004
Page(s) 497 - 509
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20040379



A&A 428, 497-509 (2004)
DOI: 10.1051/0004-6361:20040379

Astrophysical lasers operating in optical Fe II lines in stellar ejecta of $\mathsf{\eta}$ Carinae

S. Johansson1 and V. S. Letokhov2, 1

1  Lund Observatory, Lund University, PO Box 43, 22100 Lund, Sweden
    e-mail: [sveneric.johansson;Vladilen.Letokhov]@astro.lu.se
2  Institute of Spectroscopy, Russian Academy of Sciences, Troitsk, Moscow region 142190, Russia

(Received 3 March 2004 / Accepted 25 August 2004 )

Abstract
After the discovery of space masers based on OH radicals (Weaver et al. 1965) and H 2O (Cheung et al. 1969) such microwave lasers have been found to work in more than 100 molecular species (Elitzur 1992; Townes 1997), as well as in highly excited H atoms (Strelnitski et al. 1996). In the IR region (10  $\mu$m), the effect of stimulated emission of radiation in the CO 2 molecule has been discovered in the Martian and Venus' atmospheres (Betz et al. 1976; Mumma et al. 1981). We report here on the discovery of laser action in the range 0.9-2  $\mu$m in several spectral lines of Fe II, which are associated with transitions from "pseudo-metastable" states populated by spontaneous transitions from Ly $\alpha$ pumped Fe II levels. The intense Ly $\alpha$ radiation is formed in the HII region of gas condensations close to the star $\eta$ Car. The laser transitions form together with spontaneous transitions closed radiative cycles, one of which includes the extremely bright 2507/09 Å lines. Closed radiative cycles, together with an accidental mixing of energy levels, may provide an explanation of the abnormal intensities of these UV non-lasing lines. Using the complicated energy level diagram of Fe II we present those peculiar features, which are essential for the inverted population and laser effect: the pumping, the level mixing, and the "bottle neck" for spontaneous decay. The laser action is a new indicator of non-equilibrium and spatially non-homogeneous physical conditions as well as a high brightness temperature of Ly $\alpha$ in ejecta from eruptive stars. Such conditions are very difficult to probe by existing methods, and we propose some future experiments. The fact, that the lasing near-IR lines appear in the spectrum with about the same intensity as non-lasing lines is discussed and compared with the situation in masers.


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

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