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.
|Issue number||2 PART II|
|State||Published - 1997|
Bibliographical noteFunding Information:
It is a pleasure to thank Don Osterbrock for piquing our interest in [Fe IV] and for helpful discussions. We also thank Jeff Fuhr and John Pelan for providing information, and Abby Wong for assistance. Support for this work was provided by NASA through grants GO-4385, GO-5748, and GO-6056 from the STScI. R. J. D. and G. J. F. acknowledge NASA/Ames Research Center Interchange grants NCC2-5008 and NCC2-5028, respectively.
- H II regions
- ISM: abundances
- ISM: atoms
- ISM: individual (Orion Nebula)
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science