2008MNRAS.387.1660L

As part of the first high-resolution far-infrared (far-IR) spectral survey of the Orion Kleinmann-Low (KL) region, we observed 20 CO emission lines with J_up= 16 to 39 (upper levels from ~752 to 4294 K above the ground state). Observations were taken using the Long Wavelength Spectrometer on board the Infrared Space Observatory (ISO), in its high-resolution Fabry-Pérot (FP) mode (~33 km/s). We present here an analysis of the final calibrated CO data, performed with a more sophisticated modelling technique than hitherto, including a detailed analysis of the chemistry, and discuss similarities and differences with previous results. The inclusion of chemical modelling implies that atomic and molecular abundances are time predicted by the chemistry. This provides one of the main differences with previous studies in which chemical abundances needed to be assumed as initial condition. The chemistry of the region is studied by simulating the conditions of the different known components of the KL region: chemical models for a hot core, a plateau and a ridge are coupled with an accelerated Λ-iteration (ALI) radiative transfer model to predict line fluxes and profiles. We conclude that the CO transitions with 18 < J_up< 25 mainly arise from a hot core of diameter 0.02 pc and a density of 10⁷/cm3 rather from the plateau as previous studies had indicated. The rest of the transitions originate from shocked gas in a region of diameter ~0.06 pc with densities ranging from 3x10⁵ to 1x10⁶/cm3. The resulting CO fractional abundances are in the range X(CO) = (7.0-4.7)x10^–5. A high-temperature post-shock region at more than 1000 K is necessary to reach transitions with J_up > 32, whilst transitions with J_up< 18 probably originate from the extended warm component. Finally, we discuss the spatial origin of the CO emission compared with that of the next most abundant species detected by the far-IR survey towards Orion KL: H₂ O and OH.