Ionization Structure and Spectra of Iron in Gaseous Nebulae

The emission spectra and the ionization structure of the low ionizationstages of iron, Fe I--IV, in gaseous nebulae are studied. This work includes:(i) new atomic data: photoionization cross sections, total e-ion recombinationrates, excitation collision strengths, and transition probabilities; (ii)detailed study of excitation mechanisms for the [Fe II], [Fe III], and [Fe IV]emission, and spectroscopic analysis of the observed IR, optical, and UVspectra; (iii) study of the physical structure and kinematics of the nebulaeand their ionization fronts. Spectral analysis of the well observed Orionnebula is carried out as a test case, using extensive collisional-radiative andphotoionization models. It is shown that the [Fe II] emission from the Orionnebula is predominantly excited via electron collisions in high densitypartially ionized zones; radiative fluorescence is relatively less effective.Further evidence for high density zones is derived from the [O I] and [Ni II]spectral lines, as well as from the kinematic measurements of ionic species inthe nebula. The ionization structure of iron in Orion is modeled using thenewly calculated atomic data, showing some significant differences fromprevious models. The new model suggests a fully ionized H II region atdensities on the order of $10^3$ cm$^{-3}$, and a dynamic partially ionized HII/H I region at densities of $10^5-10^7$ \cm3. Photoionization models alsoindicate that the optical [O I] and [Fe II] emission originates in high densitypartially ionized regions within ionization fronts. The gas phase ironabundance in Orion is estimated from observed spectra.Comment: AAS LaTex, 60 pages 18 figures. Astrophysical Journal. in pres