The ISO/SWS Spectrum of IRC +10216: The Vibrational Bands of C₂H₂ 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 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 ν₂ bending mode around 14 μm are observed in emission. Particularly strong is the HCN ν₂=2⁰-->ν₂=1¹ vibrational transition at 14.3 μm. The most plausible mechanism for such emission is the radiative pumping of molecules from the ground to the ν₂=2⁰ state (7.1 μm) followed by radiative decay: ν₂=2⁰-->ν₂=1¹. 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)×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.