2007ApJ...671L.153W

Polycyclic aromatic hydrocarbons (PAHs) have been proposed as potential carriers of the unidentified infrared bands (UIRs) and the diffuse interstellar bands (DIBs). PAHs are not likely to form by gas-phase or solid-state interstellar chemistry, but rather might be produced in the outflows of carbon-rich evolved stars. PAHs could form from acetylene addition to the phenyl radical (C₆H₅), which is closely chemically related to benzene (C₆H₆) and ortho-benzyne (o-C₆H₄). To date, circumstellar chemical models have been limited to only a partial treatment of benzene-related chemistry, and so the expected abundances of these species are uncertain. A detection of benzene has been reported in the envelope of the proto-planetary nebula (PPN) CRL 618, but no other benzene-related species has been detected in this or any other source. The spectrum of o-C₆H₄ is significantly simpler and stronger than that of C₆H₅, and so we conducted deep Ku-, K-, and Q-band searches for o-C₆H₄ with the Green Bank Telescope. No transitions were detected, but an upper limit on the column density of 8.4x10¹³/cm2 has been determined. This limit can be used to constrain chemical models of PPNe, and this study illustrates the need for complete revision of these models to include the full set of benzene-related chemistry.