Using the Goddard High-Resolution Spectrograph on the Hubble Space Telescope, we measured the flux of [Fe IV] (3d5 4P5/2 --> 3d5 6S5/2) 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] (3d5 4D5/2, 3/2 --> 3d5 6S5/2) 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. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at STScI, which is operated by AURA, Inc., under NASA contract NAS5-26555.
Digital Object Identifier (DOI)
Rubin, R. H.; Dufour, R. J.; Ferland, Gary J.; Martin, P. G.; O'Dell, C. R.; Baldwin, J. A.; Hester, J. J.; Walter, D. K.; and Wen, Z., "[Fe IV] in the Orion Nebula" (1997). Physics and Astronomy Faculty Publications. 125.