Methylpolyynes and small hydrocarbons in CRL 618

We report on the detection with the Infrared Space Observatory of strong in frared absorption from NH3 and C2H4 in CRL 618. The observed NH3 and C2H4 b ands arise from a region with kinetic temperatures similar or equal to 200 K, i.e., the dense gas in the photodissociation region associated to the de nse torus surrounding the central star, as was the case for the polyynes an d cyanopolyynes (see the companion Letter). Several absorption bands, proba bly arising from small gas-phase hydrocarbons, are observed between 5.5 and 11 mum. Two of these species have been identified with the 30 m IRAM teles cope as the methylpolyynes CH3C2H and CH3C4H. However, the absorption aroun d 6.2 mum is particularly broad and could arise from the combination of the se small hydrocarbons and from the aromatic C=C stretching of polycyclic ar omatic hydrocarbons of moderate size. These bands and those associated to t he polyynes, cyanopolyynes, methylpolyynes, and benzene are not present in the infrared spectrum of the asymptotic giant branch star IRC +10216.