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Using the Long Wavelength Spectrometer (LWS) onboard the Infrared SpaceObservatory (ISO), we have observed thermal water vapor emission from a roughlycircular field of view approximately 75 arc seconds in diameter centered on theOrion BN-KL region. The Fabry-Perot line strengths, line widths, and spectralline shifts observed in eight transitions between 71 and 125 microns show goodagreement with models of thermal emission arising from a molecular cloudsubjected to a magnetohydrodynamic C-type shock. Both the breadth and therelative strengths of the observed lines argue for emission from a shock ratherthan from warm quiescent gas in the Orion core. Though one of the eighttransitions appears anomalously strong, and may be subject to the effects ofradiative pumping, the other seven indicate an H2O/H2 abundance ratio of order5E-4, and a corresponding gas-phase oxygen-to-hydrogen abundance ratio of order4E-4. Given current estimates of the interstellar, gas-phase, oxygen and carbonabundances in the solar vicinity, this value is consistent with theoreticalshock models that predict the conversion into water of all the gas-phase oxygenthat is not bound as CO. The overall cooling provided by rotational transitionsof H2O in this region appears to be comparable to the cooling throughrotational lines of CO, but is an order of magnitude lower than cooling throughH2 emission. However, the model that best fits our observations shows coolingby H2O and CO dominant in that portion of the post-shock region wheretemperatures are below ~ 800 K and neither vibrational nor rotational radiativecooling by H2 is appreciable.Comment: 13 pages, 1 postscript figure, LaTeX, uses aaspp4.sty, accepted for  publication in ApJ Letter

Thermal Water Vapor Emission from Shocked Regions in Orion