The [ITAL]ISO[/ITAL]/SWS Spectrum of IRC +10216: The Vibrational Bands of C[TINF]2[/TINF]H[TINF]2[/TINF] and HCN

Abstract

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 C$_{2}$H$_{2}$ and HCN. All C$_{2}$H$_{2}$ 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 {$ν$}$_{2}$ bending mode around 14 {$μ$}m are observed in emission. Particularly strong is the HCN {$ν$}$_{2}$=2$^{0}$--$\gt${$ν$}$_{2}$=1$^{1}$ vibrational transition at 14.3 {$μ$}m. The most plausible mechanism for such emission is the radiative pumping of molecules from the ground to the {$ν$}$_{2}$=2$^{0}$ state (7.1 {$μ$}m) followed by radiative decay: {$ν$}$_{2}$=2$^{0}$--$\gt${$ν$}$_{2}$=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){\times}10$^{-5}$. Based on observations with ISO, an ESA project with instruments funded by ESA member states (especially the PI countries: France Germany, the Netherlands, and the United Kingdom) and with the participation of ISAS and NASA.