Seven rotational transitions of SiC3, a planar, highly polar ring molecule, have been detected in the millimeter-wave band in the expanding envelope of the evolved carbon star IRC +10216. On the assumption that SiC3 has the same 40'' diameter shell as SiCC, the mean column density of SiC3 is 4.3×1012 cm−2. The rotational excitation of SiC3 is similar to that of SiCC, with a low rotational temperature within the K-stacks of 10-20 K and a high rotational temperature across the K-stacks of roughly 50 K, which is probably close to the kinetic temperature of the shell.
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Apponi et al. (1999) studied this question.
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