A search for ortho-benzyne (o-C6H4) in CRL 618

TL;DR

This study searched for ortho-benzyne in the circumstellar envelope of CRL 618 using radio observations but did not detect it, providing upper limits that inform and challenge existing chemical models of such environments.

Contribution

First deep radio search for ortho-benzyne in CRL 618, setting upper limits and highlighting the need for comprehensive benzene-related chemistry in models.

Findings

No ortho-benzyne detected in CRL 618.

Established an upper limit on o-C6H4 column density.

Results constrain and suggest revisions to chemical models.

Abstract

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 unclear. 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 8.4x10^13 cm^-2 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.