ITAL]Infrared Space Observatory's[/ITAL] Discovery of C[TINF]4[/TINF]H[TINF]2[/TINF], C[TINF]6[/TINF]H[TINF]2[/TINF], and Benzene in CRL 618

Abstract

We report on the detection with the Infrared Space Observatory (ISO), for the first time in the circumstellar medium, of the polyacetylenic chains C4H2 and C6H2 and of benzene (C6H6) in the direction of the proto-planetary nebula CRL 618. Surprisingly, the abundances of di- and triacetylene are only a factor of 2-4 lower than that of C2H2. Benzene is 40 times less abundant than acetylene. We suggest that the chemistry in CRL 618 has been strongly modified by the UV photons coming from the hot central star and by the shocks associated with its high-velocity winds. All the infrared bands arise from a region with kinetic temperatures between 200 and 250 K, probably the photodissociation region associated with the dense torus that surrounds the central star. C4H2 and C6H2 have also been detected in CRL 2688, so it seems that C-rich proto-planetary nebulae are the best organic chemistry factories in space.