2002A&A...386.1074G

ISO/SWS observations of Orion Peak 1 and Peak 2 show strong emission in the ro-vibrational lines of CO v=1-0 at 4.45-4.95µm and of H₂O ν₂=1-0 at 6.3-7.0µm. Toward Peak 1 the total flux in both bands is, assuming isotropic emission, ≃2.4 and ≃0.53L_☉, respectively. This corresponds to ≃14 and ≃3% of the total H₂ luminosity in the same beam. Two temperature components are found to contribute to the CO emission from Peak 1/2: a warm component, with T_k=200-400 K, and a hot component with T_k∼3x10³ K. At Peak 2 the CO flux from the warm component is similar to that observed at Peak 1, but the hot component is a factor of ≃2 weaker. The H₂O band is ≃25% stronger toward Peak 2, and seems to arise only in the warm component. The P-branch emission of both bands from the warm component is significantly stronger than the R-branch, indicating that the line emission is optically thick. Neither thermal collisions with H₂ nor with H I seem capable of explaining the strong emission from the warm component. Although the emission arises in the postshock gas, radiation from the most prominent mid-infrared sources in Orion BN/KL is most likely pumping the excited vibrational states of CO and H₂O. CO column densities along the line of sight of N(CO)=5-10x10¹⁸cm^–2 are required to explain the band shape, the flux, and the P-R-asymmetry, and beam-filling is invoked to reconcile this high N(CO) with the upper limit inferred from the H₂ emission. CO is more abundant than H₂O by a factor of at least 2. The density of the warm component is estimated from the H₂O emission to be ∼2x10⁷cm^–3. The CO emission from the hot component is neither satisfactorily explained in terms of non-thermal (streaming) collisions, nor by resonant scattering. Vibrational excitation through collisions with H₂ for densities of ∼3x10⁸cm^–3 or, alternatively, with atomic hydrogen, with a density of at least 10⁷ cm^–3, are invoked to explain simultaneously the emission from the hot component and that from the high excitation H₂ lines in the same beam. A jump shock is most probably responsible for this emission. The emission from the warm component could in principle be explained in terms of a C-shock. The underabundance of H₂O relative to CO could be the consequence of H₂O photodissociation, but may also indicate some contribution from a jump shock to the CO warm emission.