2000A&A...355..699L

Observations of gas-phase C₂H₂ and HCN along the line of sight toward a large sample of deeply embedded massive young stellar objects (YSOs) have been performed using the Short Wavelength Spectrometer on board the Infrared Space Observatory. The ν₅ vibration-rotation band of C₂H₂ around 13.7µm and the ν₂ band of HCN around 14.0µm have been detected for most lines of sight. These wavelength regions are heavily affected by instrumental fringing and a detailed discussion of the data reduction techniques is given. Comparison with model spectra allows the excitation temperatures and the abundances of the molecules to be determined. The inferred excitation temperatures range from <100 to 1000 K, and correlate well with each other, indicating that the two molecules probe the same warm gas component. The C₂H₂ and HCN column densities increase by more than an order of magnitude with increasing excitation temperature, and with the amount of heating of the ices. The corresponding abundances of C₂H₂ and HCN in the warm gas increase from ∼10^–8 to ∼10^–6 with increasing temperatures. The enhanced abundances are compared with a variety of chemical models. The observed gas-phase C₂H₂ most likely results from direct evaporation of interstellar ices, where C₂H₂ must be present at an abundance of ∼0.1-0.5% with respect to H₂O ice. This abundance is consistent with the measured amount of C₂H₂ in cometary ices. The observed gas-phase HCN abundance shows a stronger increase with temperature and results from a combination of evaporation of ices and high-temperature gas-phase chemistry in the hot core.