Full synthesis maps of circumstellar SiC2 in IRC + 10216

The molecule SiC2 in the circumstellar shell around the evolved star IRC + 10216 has been mapped using the Berkeley-Indiana- Maryland Array (BIMA) millimeter-wave interferometer. In this paper we present results of a full synthesis mapping of the 422-321 transition of SiC2. This is an improvement over earlier interferometry of SiC2 because we have used single-element observations to fill in the central 'hole' in the Fourier transform plane; data taken with the National Radio Astronomy Observatory (NRAO) 12 m telescope were combined with the interferometry to fully sample the u-v plane. A comparison of the full synthesis maps in the present work is made with previous interferometry of SiC2. Full synthesis maps show a clear shell-like structure with a bipolar symmetry. Our results indicate that SiC2 is distributed in a shell with inner and outer radii approximately 2 x 1016 cm and approximately 6 x 1016 cm, respectively. We estimate a fractional abundance, (SiC2)/(H2), of 1.4 x 10-6. The fractional abundance of SiC2 in the inner envelope (less than or approximately = 5 x 10-8) is at least an order of magnitude less than the calculations of thermal equilibrium chemistry will allow. The most likely reason for the discrepancy is that silicon is rapidly incorporated into grains in the inner envelope; grain formation is not considered in the thermal equilibrium models. Implications for the chemistry and evolution of the circumstellar envelope are discussed.