2017ApJ...841....3L

Recent, state-of-the-art calculations of A-values and electron impact excitation rates for Fe III are used in conjunction with the Cloudy modeling code to derive emission-line intensity ratios for optical transitions among the fine-structure levels of the 3d⁶ configuration. A comparison of these with high-resolution, high signal-to-noise spectra of gaseous nebulae reveals that previous discrepancies found between theory and observation are not fully resolved by the latest atomic data. Blending is ruled out as a likely cause of the discrepancies, because temperature- and density-independent ratios (arising from lines with common upper levels) match well with those predicted by theory. For a typical nebular plasma with electron temperature T_e=9000 K and electron density N_e=10⁴  cm^–3, cascading of electrons from the levels ³G₅, ³G₄ and ³G₃ plays an important role in determining the populations of lower levels, such as ³F₄, which provide the density diagnostic emission lines of Fe III, such as ⁵D₄ - ³F₄ at 4658 A. Hence, further work on the A-values for these transitions is recommended, ideally including measurements if possible. However, some Fe III ratios do provide reliable N_e-diagnostics, such as 4986/4658. The Fe III cooling function, calculated with Cloudy using the most recent atomic data, is found to be significantly greater at T_e ≃30,000 K than predicted with the existing Cloudy model. This is due to the presence of additional emission lines with the new data, particularly in the 1000-4000 A wavelength region.