[Fe IV] in the Orion Nebula

Using the Goddard High-Resolution Spectrograph on the Hubble Space Telescope, we measured the flux of [Fe IV] (3d^{5 4}P_5/2 → 3d^{5 6}S_5/2) λ_vac = 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] (3d^{5 4}D_{5/2, 3/2} → 3d^{5 6}S_5/2) λ_vac = 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.