2003A&A...399.1047B

The infrared spectra toward Orion-IRc2, Peak 1 and Peak 2 in the 13.5-15.5µm wavelength range are presented, obtained with the Short Wavelength Spectrometer on board the Infrared Space Observatory. The spectra show absorption and emission features of the vibration-rotation bands of gas-phase CO₂, HCN, and C₂H₂, respectively. Toward the deeply embedded massive young stellar object IRc2 all three bands appear in absorption, while toward the shocked region Peak 2 CO₂, HCN, and C₂H₂ are seen in emission. Toward Peak 1 only CO₂ has been detected in emission. Analysis of these bands shows that the absorption features toward IRc2 are characterized by excitation temperatures of ∼175-275K, which can be explained by an origin in the shocked plateau gas. HCN and C₂H₂ are only seen in absorption in the direction of IRc2, whereas the CO₂ absorption is probably more widespread. The CO₂ emission toward Peak 1 and 2 is best explained with excitation by infrared radiation from dust mixed with the gas in the warm component of the shock. The similarity of the CO₂ emission and absorption line shapes toward IRc2, Peak 1 and Peak 2 suggests that the CO₂ is located in the warm component of the shock (T∼200K) toward all three positions. The CO₂ abundances of ∼10^–8 for Peak 1 and 2, and of a few times 10^–7 toward IRc2 can be explained by grain mantle evaporation and/or reformation in the gas-phase after destruction by the shock. The HCN and C₂H₂ emission detected toward Peak 2 is narrower (T∼50-150K) and originates either in the warm component of the shock or in the extended ridge. In the case of an origin in the warm component of the shock, the low HCN and C₂H₂ abundances of ∼10^–9 suggest that they are destroyed by the shock or have only been in the warm gas for a short time (t≲10⁴yr). In the case of an origin in the extended ridge, the inferred abundances are much higher and do not agree with predictions from current chemical models at low temperatures.