Identification of the SiCC radical toward IC +10216 - The first molecular ring in an astronomical source

Abstract

Nine unassigned lines in the millimeter-wave spectrum of IRC +10216 have been identified as the SiCC radical, following a recent laser experiment in a supersonic molecular beam which showed that this molecule is a compact symmetric (C2[;) ring, not linear as previously thought. The fines attributed to SiCC are identical in shape and width as required, and their relative intensities are those expected for a prolate asymmetric top like SiCC under colfisional excitation: the rotational temperature Trm within the ^-ladders is low, about 10 K, comparable to that of linear polar molecules in this source, while Tmi across the ladders is high, 140 K, because cross-ladder radiative transitions are only weakly permitted. If collisions determine the relative populations of the Â'-ladders, the kinetic temperature must be at least 140 K. The mean column density of SiCC over a 90" beam derived from the rotational analysis is fairly large with respect to molecules of comparable size: 1.5 X 1014 cm-2.