Infrared Space Observatory's discovery of C-4H2, C-6H2, and benzene in CRL618

We report on the detection with the Infrared Space Observatory (ISO), for t he first time in the circumstellar medium, of the polyacetylenic chains C4H 2 and C6H2 and of benzene (C6H6) in the direction of the proto-planetary ne bula CRL 618. Surprisingly, the abundances of di- and triacetylene are only a factor of 2-4 lower than that of C2H2. Benzene is similar or equal to 40 times less abundant than acetylene. We suggest that the chemistry in CRL 6 18 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 in frared bands arise from a region with kinetic temperatures between 200 and 250 K, probably the photodissociation region associated with the dense toru s 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 org anic chemistry factories in space.