The ISO/SWS spectrum of IRC+10216:: The vibrational bands of C2H2 and HCN

We present the infrared Space Observatory/Short-Wavelength Spectrometer full grating resolution spectrum of IRC +10216, which is dominated by strong absorption/emission bands of C2H2 and HCN. All C2H2 bands and the strong near-infrared stretching bands of HCN are observed in absorption, whereas the fundamental, hot, and combination bands of HCN involving the nu(2) bending mode around 14 mu m are observed in emission. Particularly strong is the HCN nu(2) = 2(0) -> nu(2) = 1(1) vibrational transition at 14.3 mu m. The most plausible mechanism for such emission is the radiative pumping of molecules from the ground to the nu(2) = 2(0) state (7.1 mu m) followed by radiative decay: nu(2) = 2(0) -> v(2) = 1(1). We present detailed models for HCN that verify the efficiency of the mentioned effect. The HCN abundance inferred from these models is (1.5-3) x 10(-5).