ESA Science & Technology - Far-Infrared Excited Hydroxyl Lines from Orion KL Outflows

Publication date: 16 April 2006

Authors: Goicoechea, J.R. et al.

Journal: The Astrophysical Journal
Volume: 641
Issue: 1
Page: L49-L52
Year: 2006

Copyright: American Astronomical Society

As part of the first far-IR line survey toward Orion KL, we present the detection of seven new rotationally excited OH Lambda-doublets (at ~48, ~65, ~71, ~79, ~98, and ~115 mum). 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, 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 ²Pi_3/2 J=5/2⁺-3/2^- and ²Pi_3/2 J=7/2^--5/2⁺ lines (at 119.441 and 84.597 mum) 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 H₂of chi(OH)=(0.5-1.0)×10^-6, a kinetic temperature of more than ~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. Based on observations with ISO, an ESA project with instruments funded by ESA member states (especially the PI countries: France, Germany, the Netherlands, and the UK) and with the participation of ISAS and NASA.