Far-Infrared Excited Hydroxyl Lines from Orion KL Outflows

Abstract

As part of the first far-IR line survey toward Orion KL, we present the detection of seven new rotationally excited OH Λ-doublets (at ~48, ~65, ~71, ~79, ~98, and ~115 μm). Observations were performed with the Long Wavelength Spectrometer Fabry-Pérot on board the Infrared Space Observatory. In total, more than 20 resolved OH rotational lines, with upper energy levels up to ~620 K, have been detected at angular and velocity resolutions of ~80'' and ~33 km s-1, respectively. The OH line profiles show a complex behavior evolving from pure absorption, P Cygni type, to pure emission. We also present a large-scale, 6' declination raster in the OH 2Π3/2 J = 5/2+-3/2- and 2Π3/2 J = 7/2--5/2+ lines (at 119.441 and 84.597 μm) revealing a decrease of excitation outside the core of the cloud. From the observed profiles, mean intrinsic line widths, and velocity offsets between emission and absorption line peaks, we conclude that most of the excited OH arises from Orion outflow(s), that is, the "plateau" spectral component. We determine an averaged OH abundance relative to H2 of χ(OH) = (0.5-1.0) × 10-6, a kinetic temperature of ≳100 K, and a density of n(H2) ≃ 5 × 105 cm-3. Even with these conditions, the OH excitation is heavily coupled with the strong dust continuum emission from the inner "hot core" regions and from the expanding flow itself.