Knowledge Bank

We report the detection of C$3$O in the circumstellar envelope of IRC+10216. Five rotational lines of this molecule were observed at 2 and 3 mm using the Kitt Peak 12 m telescope of the Arizona Radio Observatory (ARO). The line profiles were U-shaped, indicating that the molecule is present in the outer shell with a source size $\ge$ 81.6''. The derived C$3$O column density of 1x10$\mathrm{<mrow\; data-mjx-texclass="ORD">12</mrow>}$ cm$\mathrm{<mrow\; data-mjx-texclass="ORD">-2</mrow>}$ is two orders of magnitude larger than the value predicted by the most recent chemical model of IRC+10216 (Millar, Herbst & Bettens 2000). C$3$O is the sixth oxygen-bearing molecule that has been identified in IRC+10216, and our observations represent the second astronomical detection of C$3$O.