ISO-SWS spectroscopy of gas-phase C2H2 and HCN toward massive young stellar objects

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 nu(5) vibration-rotation band of C2H2 around 13.7 mu m and the nu(2) band of HCN around 14.0 mu m 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 similar to 10(-8) to similar 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 similar to 0.1 - 0.5 % with respect to H2O ice. This abundance is consistent with the measured amount of C2H2 in cometary ices. The observed gasphase 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.