  Using the Long Wavelength Spectrometer (LWS) onboard the Infrared Space
Observatory (ISO), we have observed thermal water vapor emission from a roughly
circular field of view approximately 75 arc seconds in diameter centered on the
Orion BN-KL region. The Fabry-Perot line strengths, line widths, and spectral
line shifts observed in eight transitions between 71 and 125 microns show good
agreement with models of thermal emission arising from a molecular cloud
subjected to a magnetohydrodynamic C-type shock. Both the breadth and the
relative strengths of the observed lines argue for emission from a shock rather
than from warm quiescent gas in the Orion core. Though one of the eight
transitions appears anomalously strong, and may be subject to the effects of
radiative pumping, the other seven indicate an H2O/H2 abundance ratio of order
5E-4, and a corresponding gas-phase oxygen-to-hydrogen abundance ratio of order
4E-4. Given current estimates of the interstellar, gas-phase, oxygen and carbon
abundances in the solar vicinity, this value is consistent with theoretical
shock models that predict the conversion into water of all the gas-phase oxygen
that is not bound as CO. The overall cooling provided by rotational transitions
of H2O in this region appears to be comparable to the cooling through
rotational lines of CO, but is an order of magnitude lower than cooling through
H2 emission. However, the model that best fits our observations shows cooling
by H2O and CO dominant in that portion of the post-shock region where
temperatures are below ~ 800 K and neither vibrational nor rotational radiative
cooling by H2 is appreciable.
