Chemical and radiative transfer modelling of the ISO-LWS Fabry-Perot spectra of Orion-KL water lines

Abstract

We present chemical and radiative transfer models for the many far-IR ortho- and para-H2O lines that were observed from the Orion-KL region in high resolution Fabry-Perot (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 Lambda$-iteration (ALI) radiative transfer model to predict H2O 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 3x10^5 cm^-3 at a temperature of 70-90 K, expanding at a velocity of 30 km s^-1 and with a H2O/H2 abundance ratio of the order of 2 - 3 x 10^-5, similar to the abundance derived by Cernicharo et al. (2006). However, the model that best reproduces the fluxes and profiles of H2O lines arising from energy levels above 560 K has a significantly higher H2O/H2 abundance, 1 - 5 x 10^-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.