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 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}}={10}4 {{cm}}-3, cascading of electrons from the levels {}3{{{G}}}5, {}3{{{G}}}4 and {}3{{{G}}}3 plays an important role in determining the populations of lower levels, such as {}3{{{F}}}4, which provide the density diagnostic emission lines of Fe III, such as {}5{{{D}}}4 - {}3{{{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}{{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 Å wavelength region.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- May 2017
- DOI:
- 10.3847/1538-4357/aa7071
- arXiv:
- arXiv:1705.00207
- Bibcode:
- 2017ApJ...841....3L
- Keywords:
-
- atomic data;
- H II regions;
- planetary nebulae: general;
- planetary nebulae: individual: NGC 7009;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- Accepted for publication in the ApJ