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 wa s recently studied by four of us in the laboratory and the rotational trans ition frequencies were accurately measured. The ground fine structure state , (2)Pi (1/2), has three rotational transitions, each with lambda -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 lambda -doublets (six co mponents) are detected at a level of 5 mK with the IRAM 30-m telescope. Jud ging from the cusped shape of the line profiles, SiCN is largely confined t o the outer molecular envelope, like most other radicals. Its abundance rel ative to H-2 is estimated to be 4 x 10(-9), a factor of 20 lower than that of MgNC. The isoelectronic radical SiCCH was not detected. We confirm our previous t entative detections of the carbon chain H2C6 and of NP in IRC+10216.