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 Experime nt Spirit III interferometer from 370 to 2000 cm(-1) with 2 cm(-1) resoluti on in a 6' x 9' held 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), H-2(S1) 587.0 cm(-1), H-2(S2) 814.4 cm(-1), H-2(S3) 1034.7 cm(-1), H-2(S4) 1246.1 cm(-1), and H- 2(S5) 1447.3 cm(-1). The following abundances were determined from these li nes: Ne/H = 9.9 +/- 1.1 x 10(-5), S/H = 8.1 +/- 1.1 x 10(-6), and Ar/H = 2. 5 +/- 0.2 x 10(-6). These abundances are all less than solar and confirm th at the Sun is overabundant in heavy elements without the need for correctio n for the composition of interstellar dust. The low sulfur abundance compar ed with solar is an indication that a significant amount of the sulfur in O rion is in dust grains. The FOV-averaged molecular hydrogen column density is similar to 1.6 x 10(20) cm(-2) for an excitation temperature of similar to 670 K and an extinction correction corresponding to an optical depth of 1.5 at 9.7 mu m. The unidentified infrared emission features at 6.2, 7.7, 8 .6, 11.3, and 12.7 mu m, attributable to polycyclic aromatic hydrocarbons, were also detected. A prominent, broad silicate feature centered near 18 mu m and additional weak features were detected and are discussed.