Detection of C₃O in IRC +10216: Oxygen-Carbon Chain Chemistry in the Outer Envelope

The oxygen-bearing species C₃O has been identified in the circumstellar envelope of the carbon star IRC +10216. The J = 8 → 7, 9 → 8, 10 → 9, 14 → 13, and 15 → 14 transitions were detected at 2 and 3 mm using the Arizona Radio Observatory's 12 m telescope. Measurements of the J = 9 → 8, 10 → 9, and 12 → 11 lines were simultaneously conducted at the IRAM 30 m telescope. The line profiles of C₃O are roughly U-shaped, indicating an extended shell distribution for this molecule in IRC +10216. The total column density derived for C₃O is 1.2 × 10¹² cm^-2, at least an order of magnitude higher than that predicted by current chemical models. However, a revised model that includes reactions of atomic oxygen with carbon-chain radicals, such as l-C₃H and C₄, can reproduce the observed abundance. This model also predicts that C₃O arises from a shell source with an outer radius near r ~ 30'', consistent with the observations. These results suggest that gas phase neutral-neutral chemistry may be producing the oxygen-bearing molecules present in the outer envelope of IRC +10216.