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Astronomical detection of the free radical SiCN

We report the detection of the SiCN radical in an astronomical source, the envelope of the C star IRC +10216/CW Leo. The microwave spectrum of SiCN was recently studied by four of us in the laboratory and the rotational transition frequencies were accurately measured. The ground fine structure state, Π1/2, has three rotational transitions, each with λ-doubling in the 80–116 GHz atmospheric window (J = 7.5 → 6.5, 8.5 → 7.5 and 9.5 → 8.5, at 83.0, 94.0, and 105.1 GHz). The three λ-doublets (six components) are detected at a level of 5 mK with the IRAM 30-m telescope. Judging from the cusped shape of the line profiles, SiCN is largely confined to the outer molecular envelope, like most other radicals. Its abundance relative to H2 is estimated to be 4×10, a factor of 20 lower than that of MgNC. The isoelectronic radical SiCCH was not detected. We confirm our previous tentative detections of the carbon chain H2C6 and of NP in IRC+10216.