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 18O and 17O isotopologues, toward Orion KL. Observations were performed with the Long Wavelength Spectrometer in Fabry-Pérot mode (λ/Δλ ~ 6800-9700) on board the Infrared Space Observatory between ~43 and 197 μm. The water line profiles evolve from P Cygni-type profiles (even for the HO lines) to pure emission at wavelengths above ~100 μm. We find that most of the water emission/absorption arises from an extended flow of gas expanding at 25 ± 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) × 10-5. The inferred gas temperature Tk = 80-100 K 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 ≈1000 yr, the dynamical timescale of the outflow.