Emission line ratios of Fe III as astrophysical plasma diagnostics

Abstract

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 3d${}^6$ 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 $T_{\mathrm{e}}=9000$ K and electron density ${\mathrm{N}}_{\mathrm{e}}=10^4\mathrm{c}{\mathrm{m}}^{-3}$, cascading of electrons from the levels ${}^3{\mathrm{G}}_5$, ${}^3{\mathrm{G}}_4$ and ${}^3{\mathrm{G}}_3$ plays an important role in determining the populations of lower levels, such as ${}^3{\mathrm{F}}_4$, which provide the density diagnostic emission lines of Fe III, such as ${}^5{\mathrm{D}}_4$ - ${}^3{\mathrm{F}}_4$ 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 $N_{\mathrm{e}}$-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 $T_e$ $\simeq$ 30000 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.

Accepted for publication in the ApJ

References in corpus (2)

• CHIANTI - An atomic database for Emission Lines. Version 8
• Properties of the ionized gas in HH202. II: Results from echelle spectrophotometry with UVES

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