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 Te = 9000 K and electron density Ne = 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 Å. 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 Ne-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 Te ≃ 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 Å wavelength region.
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France. S.L., C.A.R., and F.P.K. are grateful to STFC for financial support via grant ST/L000709/1. G.J.F. acknowledges financial support from the Leverhulme Trust via Visiting Professorship grant VP1-2012-025, and also support by the NSF (1108928, 1109061, and 1412155), NASA (10-ATP10-0053, 10-ADAP10-0073, NNX12AH73G, and ATP13-0153) and STSciI (HST-AR-13245, GO-12560, HST-GO-12309, GO-13310.002-A, HST-AR-13914, and HST-AR-14286.001).
Laha, Sibasish; Tyndall, Niall B.; Keenan, Francis P.; Ballance, Connor P.; Ramsbottom, Catherine A.; Ferland, Gary J.; and Hibbert, Alan, "Emission Line Ratios of FE III as Astrophysical Plasma Diagnostics" (2017). Physics and Astronomy Faculty Publications. 489.