2003ApJ...596..556A

We report the results of the presently lowest frequency spectral line survey of the asymptotic giant branch carbon star IRC +10°216. The entire Arecibo C band (4-6 GHz) was observed with a noise level of ∼0.3 mJy (1 σ). We report the detection of HC₅N (J=2-1), which is the lowest frequency transition of the HC₅N molecule found in the envelope of IRC +10°216. The HC₅N J=2 column density derived from our observations confirms the presence of two temperature components in the molecular envelope of IRC +10°216. The HC₅N J=2-1 transition traces the cold molecular component characterized by an excitation temperature of ∼13.8 K. We derive upper limits for several transitions of molecules that had previously been detected in IRC +10°216 at higher frequencies. We also report the frequency ranges in the Arecibo C band that were affected by radio frequency interference.