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The Water Vapor Abundance in Orion KL Outflows

AuthorsCernicharo, José ; Goicoechea, Javier R. ; Daniel, Fabien ; Lerate, M. R.; Barlow, Michael J.; Swinyard, Bruce M.; Dishoeck, E. van; Lim, Tanya L.; Viti, S.; Yates, J.
ISM: lines and bands
ISM: individual (Orion)
ISM: molecule
Radiative transfer
Issue Date16-Aug-2006
CitationThe Astrophysical Journal Letters, 649:L33–L36, 2006 September 20
AbstractWe present the detection and modeling of more than 70 far-IR pure rotational lines of water vapor, including the 18O and 17O isotopologues, towards Orion KL. Observations were performed with the Long Wavelength Spectrometer Fabry-Perot (LWS/FP; R~6800-9700) on board the Infrared Space Observatory (ISO) between ~43 and ~197 μm. The water line profiles evolve from P-Cygni type profiles (even for the H2O18 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 kms^-1. Non-local 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 2-3x10^-5. The inferred gas temperature Tk=80-100 K suggests that: (i) 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 (ii) H2O formation in the outflow is dominated by in-situ evaporation of grain water-ice mantles and/or (iii) 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.
DescriptionAccepted 2006 August 7; To be published in ApJ letters
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