[Fe [CSC]iv[/CSC]] in the Orion Nebula

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Abstract
Using the Goddard High-Resolution Spectrograph on the Hubble Space Telescope, we measured the flux of [Fe IV] (3d54P5/2 → 3d56S5/2) λvac = 2836.56 Å in the Orion Nebula, the first detection of an [Fe IV] line in an H II region. A useful upper limit is set on the sum of fluxes of [Fe IV] (3d54D5/2, 3/2 → 3d56S5/2) λvac = 2568.4, 2568.2 Å. By comparing these observations with predicted fluxes from simply "retrofitting" our two previous photoionization models, we are able to derive (or set an upper limit on) the Fe/H abundance ratio: 70, 200 times lower than solar from the 2837 Å line, and ≥38, ≥120 times lower than solar from the 2568 Å line limit. If collisional excitation from the ground state were indeed the dominant mechanism for populating the respective upper levels of these lines, then the inferred Fe/H from the 2837 Å line and limit from the 2568 Å line would be ~3.0 and ~3.4 times larger than above. All these ratios are much lower than several recent determinations of gas-phase Fe/H ~ 3 × 10-6 in Orion, which themselves are a factor ~10 depleted relative to solar. Because the inferred Fe/H should be at least as high in the Fe+3 zone as in the Fe+ and Fe+2 zones, a reexamination of the Fe+3 atomic data and improved modeling would be valuable.
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