Abstract
We present a study of the emission spectra and the ionization structure of FeI--IV in gaseous nebulae. This work includes: (I) new atomic data; (II) a study of excitation mechanisms of [FeII], [FeIII], and [FeIV] lines, and spectroscopic analysis of observed spectra; (III) a study of the physical structure and kinematics of nebulae and their ionization fronts. Spectral analysis of the Orion nebula is carried out as a test case. It is shown that the [FeII] emission from Orion is predominantly excited via electron collisions in high density partially ionized zones; radiative fluorescence is relatively less effective. Further evidence for high density zones is derived from measured velocities of the flow in the nebula. Photoionization models also indicate that the optical [OI] and [FeII] emission originates in high density regions within ionization fronts.
