Radiative Cycle with Stimulated Emission from Atoms and Ions in an Astrophysical Plasma

Abstract:
We propose that a radiative cycle operates in atoms and ions located ina rarefied gas in the vicinity of a hot star. Besides spontaneoustransitions the cycle includes a stimulated transition in one very weakintermediate channel. This radiative bottleneck creates apopulation inversion, which for an appropriate column density results inamplification and stimulated radiation in the weak transition. Thestimulated emission opens a fast decay channel leading to a fastradiative cycle in the atom (or ion). We apply this model by explainingtwo unusually bright FeII lines at 250.7 and 250.9nm in the UV spectrumof gas blobs close to eta Carinae, one of the most massive andluminous stars in the Galaxy.