2010MNRAS.406.2445L

We present chemical and radiative transfer models for the many far-infrared ortho- and para-H₂ O lines that were observed from the Orion-KL region in high resolution Fabry-Pérot (FP) mode by the Long Wavelength Spectrometer (LWS) on board the Infrared Space Observatory (ISO). The chemistry of the region was first studied by simulating the conditions in the different known components of Orion-KL: chemical models for a hot core, a plateau and a ridge were coupled with an accelerated Λ-iteration radiative transfer model to predict H₂ O line fluxes and profiles. Our models include the first 45 energy levels of ortho- and para-H₂O. We find that lines arising from energy levels below 560 K were best reproduced by a gas of density 3 x10⁵/cm3 at a temperature of 70-90 K, expanding at a velocity of 30 km/s and with a H₂O/H₂ abundance ratio of the order of 2-3x10^–5, similar to the abundance derived by Cernicharo et al. However, the model that best reproduces the fluxes and profiles of H₂ O lines arising from energy levels above 560 K has a significantly higher H₂O/H₂abundance, 1-5x10^–4, originating from gas of similar density, in the Plateau region, that has been heated to 300 K, relaxing to 90-100 K. We conclude that the observed water lines do not originate from high-temperature shocks.