IONIZATION STRUCTURE AND SPECTRA OF IRON IN GASEOUS NEBULAE

The emission spectra and the ionization structure of the low ionizatio n stages of iron, Fe I-Fe IV, in gaseous nebulae are studied. This wor k includes (i) new atomic data for photoionization cross sections, tot al e-ion recombination rates, excitation collision strengths, and tran sition probabilities calculated under the Iron Project by the Ohio Sta te atomic astrophysics group; (ii) detailed study of excitation mechan isms for the [Fe II], [Fe III], and [Fe IV] emission, and spectroscopi c analysis of the observed IR, optical, and UV spectra; (iii) study of the physical structure and kinematics of the nebulae and their ioniza tion fronts. Spectral analysis of the well-observed Orion Nebula is ca rried out as a test case, using extensive collisional-radiative and ph otoionization models. It is shown that the [Fe II] emission from the O rion Nebula is predominantly excited via electron collisions in high-d ensity partially ionized zones; radiative fluorescence is relatively l ess effective. Further evidence for high-density zones is derived from the [O I] and [Ni II] spectral lines, as well as from the kinematic m easurements of ionic species in the nebula. The ionization structure o f iron in Orion is modeled using the newly calculated atomic data and shows 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 10(5)-10(7) cm(-3). Photoionization models also indicate that the optical [O I] and [Fe II] emission originates in high-density partiall y ionized regions within ionization fronts, thereby confirming the gen eral Fe II/O I correlation in H II regions that was determined in earl ier studies. The gas-phase iron abundance in Orion is estimated from o bserved spectra, including recently observed [Fe IV] lines.