Astronomical detection of rhomboidal SiC3

Seven rotational transitions of SIG,, a planar, highly polar ring molecule, have been detected in the millimeterwave 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 x 10(12) 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 hi gh rotational temperature across the K-stacks of roughly 50 K, which is pro bably close to the kinetic temperature of the shell.