Theoretical emission line ratios for [Fe III] and [Fe VII] applicable to the optical and infrared spectra of gaseous nebulae

Abstract

Recent calculations of electron impact excitation rates and EinsteinA-coefficients for transitions among the 3d 6 levels of Fe III and among the 3d 2 levels of Fe VII are used to derive theoretical emission line ratios applicable to the optical and infrared spectra of gaseous nebulae. Results for [Fe III] are generated for electron temperatures Te ‫؍‬ 7,000 -20,000 K and densities Ne ‫؍‬ 10 2 -10 8 cm ؊3 , whereas those for [Fe VII] are provided for Te ‫؍‬ 10,000 -30,000 K and N e ‫؍‬ 10 2 -10 8 cm ؊3 . The theoretical line ratios are significantly different in some instances from earlier calculations and resolve discrepancies between theory and observation found for the planetary nebulae IC 4997 and NGC 7027. (7) produced updated theoretical line ratios for these ions by using electron impact excitation rates for Fe III (8) and Fe VII (9) calculated with the R-matrix code (10).However, since the publication of the above papers, there have been significant improvements in the quality of the available atomic data for Fe III and Fe VII. Specifically, Nahar and Pradhan (11,12) and Berrington et al. (13) have calculated improved Einstein A-coefficients and R-matrix electron impact excitation rates, which show large differences (of up to an order of magnitude or more) with the data adopted by Keenan et al. (6) and Keenan and Norrington (7). In this paper, we use the most recent atomic physics calculations for Fe III and Fe VII to derive emission line ratios for these ions over a wide range of electron temperatures and densities, for application to the spectra of gaseous nebulae.Adopted Atomic Data. The model ion for Fe III consisted of the five LS states in the 3d 6 ground configuration, namely 5 D, 3 P, 3 H, 3 F, and 3 G, making a total of 17 fine-structure levels. For Fe VII, a model ion including the five LS states in the 3d 2 ground configuration was adopted, i.e.,

The Chemical Composition of the 30 Doradus Nebula Derived from Very Large Telescope Echelle Spectrophotometry

“…Figure 1 presents the region near the Balmer limit, where the Balmer continuum can be easily appreciated. The [Fe iii] density was derived from the I(4986)/I(4658) ratio and the computations by Keenan et al (2001). In Table 4 I compare the observed I( nm )/I(4658) ratios for a set of [Fe iii] lines with the predicted ratios by Keenan et al (2001) Table 4 it can be seen that the observed and predicted line ratios are in excellent agreement since the differences between them are of the order of their estimated errors.…”

The ionized gas in the central region of NGC 5253

“…our knot 1), the estimated T e ([O ii]) can be up to ∼17% smaller. For n e ([Fe iii]), we used the line ratios tabulated by Keenan et al (2001), assuming T e = 12 000 K. Note that for the emission lines utilized, the dependence of the derived n e on the assumed temperature is negligible. Finally, n e ([Ar iv]) was derived using directly temden and assuming Notes.…”

Emission Line Ratios of FE III as Astrophysical Plasma Diagnostics

“…These Fe III transitions are also important tracers of Fe abundance in the case of H II regions and lowly ionized planetary nebulae, where they are often the only ionization state of Fe detected in the optical band. Garstang et al (1978) first noted the diagnostic potential of optical Fe III lines; subsequently, several authors have generated theoretical electron temperature (T e ) and density (N e ) dependent emission-line intensity ratios for this ion, and used these to determine plasma parameters for nebular sources (for example, Keenan et al 1993Keenan et al , 2001Bautista et al 2010, and references therein.). However, there are longstanding inconsistencies between electron densities derived from different Fe III ratios, as well as discrepancies with values of N e determined from other species such as OII, SII, and ClIII with similar ionization potentials to Fe III, which hence should originate in nearby regions of the nebular plasma.…”