Methylpolyynes and Small Hydrocarbons in CRL 618

Abstract

We report on the detection with the Infrared Space Observatory of strong infrared absorption from NH₃ and C₂H₄ in CRL 618. The observed NH₃ and C₂H₄ bands arise from a region with kinetic temperatures ~=200 K, i.e., the dense gas in the photodissociation region associated to the dense torus surrounding the central star, as was the case for the polyynes and cyanopolyynes (see the companion Letter). Several absorption bands, probably arising from small gas-phase hydrocarbons, are observed between 5.5 and 11 μm. Two of these species have been identified with the 30 m IRAM telescope as the methylpolyynes CH₃C₂H and CH₃C₄H. However, the absorption around 6.2 μm is particularly broad and could arise from the combination of these small hydrocarbons and from the aromatic CC stretching of polycyclic aromatic hydrocarbons of moderate size. These bands and those associated to the polyynes, cyanopolyynes, methylpolyynes, and benzene are not present in the infrared spectrum of the asymptotic giant branch star IRC +10216.