DETECTION OF CCN (X2Πr) IN IRC+10216: CONSTRAINING CARBON-CHAIN CHEMISTRY

The CCN radical (X-2 Pi(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 O = 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 similar to 3 x 10(16) cm, with a peak fractional abundance of f(0) similar to 6 x 10(-8). The column density of the radical is similar to 4 x 10(13) 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 similar to 500:1:50.