FURTHER OBSERVATIONS OF ROTATIONALLY EXCITED FAR-INFRARED OH16 AND OH18 EMISSION IN ORION-KL - TIGHTER CONSTRAINTS ON THE NATURE OF THE EMITTING REGION

We have observed the region within 1 arcmin of Orion-KL and report the first detections of the 16OH 2Π1/2 → 2Π3/2 J = 3/2- → 3/2+ rotational cross-ladder transition (53.351 μm) and the 18OH 2Π3/2 J = 5/ 2+ → 3/2- rotational ground-state transition (120.1719 μm). We find that both of these lines exhibit a P-Cygni profile and unambiguously show that the OH gas is expanding out from the central BN/KL infrared cluster. In addition, we have velocity resolved the 16OH 2Π3/2 J = 5/2- → 3/2+ rotational ground-state transition (119.234 μm) and find that its intrinsic full-width at half-maximum (FWHM) is 75 km s-1. We model both the line fluxes and line profiles, along with the previously measured 16OH 2Π3/2 84 μm and 2Π1/2 163 μm rotational transitions, and find that no single temperature and density component can reproduce the data. Rather, the best overall fit to the data requires emission from three main components of the gas: (1) postshocked gas with the profiles of temperature, density, and OH abundance like that predicted by Draine and Roberge (1982) for a 38 km s-1 C-type shock; (2) a higher density (n[H2] ≃ 2 × 107 cm-3) component to the cool postshocked region than given by Draine and Roberge; and (3) the plateau region. All three components require a significant radiative background in order to fit the data.