Abstract
We report the detection of the linear radical HC4N in the C-rich envelope of IRC +10216. After HCCN, HC4N is the second member of the allenic chain family HC2nN observed in space. The column density of HC4N is found to be 1.5 × 1012 cm-2. The abundance ratio HC2N/HC4N is 9, a factor of 2 larger than the decrement observed for the cyanopolyynes (HC2n+1N/HC2n+3N). Linear HC4N has a 3Σ electronic ground state and is one of the three low-energy isomeric forms of this molecule. We have searched for the bent and ringed HC4N isomers but could only derive an upper limit to their column densities: ≤3 × 1012 cm-2 (at 3 σ).
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