Radiative cycle with stimulated emission from atoms and ions in an astrophysical plasma

We propose that a radiative cycle operates in atoms and ions located in a rarefied gas in the vicinity of a hot star. Besides spontaneous transitions the cycle includes a stimulated transition in one very weak intermediate channel. This radiative "bottleneck" creates a population inversion, which for an appropriate column density results in amplification and stimulated radiation in the weak transition. The stimulated emission opens a fast decay channel leading to a fast radiative cycle in the atom (or ion). We apply this model by explaining two unusually bright Fe II lines at 250.7 and 250.9 nm in the UV spectrum of gas blobs close to eta Carinae, one of the most massive and luminous stars in the Galaxy.