DETECTION OF CCN ( X 2 Π r ) IN IRC+10216: CONSTRAINING CARBON-CHAIN CHEMISTRY

Abstract

The CCN radical (X 2Πr) 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 m telescope, 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 f 0 ~ 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.