The emission spectra and the ionization structure of the low ionization stages of iron, Fe I–IV, in gaseous nebulae are studied. This work includes: (i) new atomic data: photoionization cross sections, total e-ion recombination rates, excitation collision strengths, and transition probabilities calculated under the Iron Project by the Ohio State atomic-astrophysics group; (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 UV spectra; (iii) study of the physical structure and kinematics of the nebulae and their ionization fronts. Spectral analysis of the well observed Orion nebula is carried out as a test case, using extensive collisional-radiative and photoionization models. It is shown that the [Fe II] emission from the Orion nebula 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 the [O I] and [Ni II] spectral lines, as well as from the kinematic measurements of ionic species in the nebula. The ionization structure of iron in Orion is modeled using the newly calculated atomic data, showing some significant differences from previous models. The new model suggests a fully ionized H II region at densities on the order of 10 3 cm −3, and a dynamic partially ionized H II/H I region at densities of 105 − 107 cm−3. Photoionization models also indicate that the optical [O I] and [Fe II] emission originates in high density partially ionized regions within ionization fronts, thereby confirming the general Fe II/O I correlation in H II regions determined in earlier studies. The gas phase iron abundance in Orion is estimated from observed spectra, including recently observed [Fe IV] lines. – 3 – Subject headings: H II regions – ISM:individual (Orion Nebula) – ISM:abundances – ISM:structure – atomic processes – star:formation – 4 – 1
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