EDP Sciences
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Volume 385, Number 1, April I 2002
Page(s) 313 - 327
Section Physical and chemical processes
DOI http://dx.doi.org/10.1051/0004-6361:20020059

A&A 385, 313-327 (2002)
DOI: 10.1051/0004-6361:20020059

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

V. Klimov1, S. Johansson2 and V. S. Letokhov2, 3

1  P.N. Lebedev Physical Institute, Russian Academy of Sciences, 53 Leninsky Prospect, Moscow 117294, Russia
2  Atomic Astrophysics, Lund Observatory, Lund University, PO Box 43, Lund 221 00, Sweden
3  Institute of Spectroscopy, Russian Academy of Sciences, Troitsk 142190, Moscow region, Russia
    e-mail: vladilen.letokhov@astro.lu.se

(Received 12 July 2001 / Accepted 14 January 2002)

We present a model that explains the anomalous intensity ratios between each of two very bright UV lines of Fe II at 2507 Å and 2509 Å and its corresponding satellite line, observed in HST spectra of a compact gaseous condensation (blob B) in the vicinity of the star $\eta$ Car.The model is based on the assumption that the FeII transitions have a substantial optical thickness, which varies as a result of the photodepletion of the long-lived lower states, c 4F 7/2,9/2, by the intense Ly $\alpha$ radiation. In conditions where the photodepletion rates of these two states differ, the resonance scattering lengths of the spectral lines become different as well. In the presence of a weak, nonresonant (continuous) absorption, this gives rise to a difference in attenuation between the spectral lines (the Hummer effect (Hummer 1968)). It is the combination of these two effects that explains the anomalous (as compared with laboratory data) intensity ratios observed in the two pairs of UV FeII lines. Ionization channels of stimulated photodepletion by the Ly $\alpha$ radiation, including the possible role of autoionization levels, are considered.

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

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

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