Stimulated Amplification under Spatially Nonhomogeneous Conditions: An Astrophysical Fe II Laser in the Vicinity of Eta Carinae

Abstract

We consider the conditions necessary for the development of an inverted population among the energy levels of free atoms in a medium exposed to radiation in spectral non-equilibrium. In particular, we focus on monochromatic radiation occurring as a result of an accidental wavelength coincidence between a pumping emission line and a pumped absorption line in two different elements. We examine especially the influence of `pseudo-metastable' states (at medium excitation) in the complex spectrum of Fe II, which cause a `bottleneck' in the radiative decay leading to an inverted level population. We also discuss the effect of `spectral compression' of the Lyman continuum into HLyα radiation at a high effective temperature. In conclusion we describe those differences between microwave astrophysical masers (APM) and optical astrophysical lasers (APL) that make the latter a more rare phenomenon in Nature.