The water vapor abundance in Orion KL outflows

We present the detection and modeling of more than 70 far-IR pure rotational lines of water vapor, including the O-18 and O-17 isotopologues, toward Orion KL. Observations were performed with the Long Wavelength Spectrometer in Fabry-Perot mode (lambda/Delta lambda similar to 6800-9700) on board the Infrared Space Observatory between similar to 43 and 197 mm. The water line profiles evolve from P Cygni - type profiles (even for the (H2O)-O-18 lines) to pure emission at wavelengths above similar to 100 mu m. We find that most of the water emission/ absorption arises from an extended flow of gas expanding at 25 km +/- 5 km s(-1). Nonlocal radiative transfer models show that much of the water excitation and line profile formation is driven by the dust continuum emission. The derived beam- averaged water abundance is (2-3) x 10(-5). The inferred gas temperature T-k = 80 - 100 suggests that (1) water could have been formed in the plateau by gas-phase neutral-neutral reactions with activation barriers if the gas was previously heated ( e. g., by shocks) to >= 500 K, and/or (2) H2O formation in the outflow is dominated by in situ evaporation of grain water-ice mantles, and/or (3) H2O was formed in the innermost and warmer regions (e. g., the hot core) and was swept up in approximate to 1000 yr, the dynamical timescale of the outflow.