Midcourse Space Experiment Spectra of the Orion Nebula and the Implications for Abundances in the Interstellar Medium

Spectra of the Orion Nebula were obtained with the Midcourse Space Experiment Spirit III interferometer from 370 to 2000 cm-1 with 2 cm-1 resolution in a 6' × 9' field of view (FOV) in 1996 November. Lines were detected of [S III] 534.4 cm-1, [Ne III] 642.9 cm-1, [Ne II] 780.4 cm-1, [S IV] 951.4 cm-1, [Ar III] 1112.2 cm-1, [Ar II] 1431.6 cm-1, H (7-6) 808.3 cm-1, H (8-6) 1332.9 cm-1, H (6-5) 1340.5 cm-1, H2(S1) 587.0 cm-1, H2(S2) 814.4 cm-1, H2(S3) 1034.7 cm-1, H2(S4) 1246.1 cm-1, and H2(S5) 1447.3 cm-1. The following abundances were determined from these lines: Ne/H = 9.9 ± 1.1 × 10-5, S/H = 8.1 ± 1.1 × 10-6, and Ar/H = 2.5 ± 0.2 × 10-6. These abundances are all less than solar and confirm that the Sun is overabundant in heavy elements without the need for correction for the composition of interstellar dust. The low sulfur abundance compared with solar is an indication that a significant amount of the sulfur in Orion is in dust grains. The FOV-averaged molecular hydrogen column density is ~1.6 × 1020 cm-2 for an excitation temperature of ~670 K and an extinction correction corresponding to an optical depth of 1.5 at 9.7 μm. The unidentified infrared emission features at 6.2, 7.7, 8.6, 11.3, and 12.7 μm, attributable to polycyclic aromatic hydrocarbons, were also detected. A prominent, broad silicate feature centered near 18 μm and additional weak features were detected and are discussed.

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