Detection of CCN (X²∏_r) in IRC+ 10216: Constraining carbon-chain chemistry

The CCN radical (X²∏_r) has been detected for the first time in the interstellar medium. The molecule was observed at the 1-2 mK level toward the circumstellar envelope of IRC + 10216, using the facilities of the Arizona Radio Observatory (ARO). Lambda doublets of the J = 4.5 → 3.5 and the J = 6.5 → 5.5 transitions at 106 GHz and 154 GHz in the Ω = 1/2 ladder were measured with the ARO 12 mtelescope, as well as the J = 9.5 → 8.5 lines near 225 GHz, using the ARO Sub-Millimeter Telescope (SMT). In the two lower frequency transitions, the e component of the doublet was partially blended with other lines, while emission from the f component was uncontaminated. The lambda doublet of the 1 mm transition is collapsed and appears adjacent to an unidentified line. Modeling of the spectra, using the new radiative transfer code ESCAPADE, indicates that CCN arises from a shell in the outer envelope centered at r ∼ 3 × 10¹⁶ cm, with a peak fractional abundance of f₀ ∼ 6 × 10^-8. The column density of the radical is ∼4 × 10¹³ cm^-2, in agreement with that derived from a rotational diagram analysis. Modeling was also conducted for spectra of CN, C₃N, and HC₃N, obtained with the SMT in the ARO 1 mm survey. CCN, C₃N, and HC₃N appear to originate from adjacent shells in the envelope, suggesting they are chemically related. The [CN]/[CCN]/[C₃N] abundance ratio was found to be ∼500:1:50.