Far-Infrared Excited Hydroxyl Lines from Orion KL Outflows

As part of the first far-IR line survey towards Orion KL, we present thedetection of seven new rotationally excited OH Lambda-doublets (at 48, 65, 71,79, 98 and 115 um). Observations were performed with the Long WavelengthSpectrometer (LWS) Fabry-Perots on board the Infrared Space Observatory (ISO).In total, more than 20 resolved OH rotational lines, with upper energy levelsup to 620 K, have been detected at an angular and velocity resolutions of 80$''and 33 km s^-1 respectively. OH line profiles show a complex behavior evolvingfrom pure absorption, P-Cygni type to pure emission. We also present a largescale 6' declination raster in the OH ^2\Pi_3/2 J=5/2^+-3/2^- and ^2\Pi_3/2J=7/2^-5/2^+ lines (at 119.441 and 84.597 um) revealing the decrease ofexcitation outside the core of the cloud. From the observed profiles, meanintrinsic line widths and velocity offsets between emission and absorption linepeaks we conclude that most of the excited OH arises from Orion outflow(s),i.e. the ``plateau'' component. We determine an averaged OH abundance relativeto H_2 of X(OH)=(0.5-1.0)x10^-6, a kinetic temperature of 100 K and a densityof n(H_2)=5x10^5 cm^-3. Even with these conditions, the OH excitation isheavily coupled with the strong dust continuum emission from the inner hot coreregions and from the expanding flow itself.Comment: Pre-print version(5 pages, 3 figures). Accepted in ApJ letters, 2006 March