The distribution of molecules in the circumstellar envelope of IRC +10216: HC₃N, C₃N, and SiS

We present aperture synthesis images made with the BIMA mm interferometer of molecular emission lines of HC₃N, C₃N, and SiS at ∼109 GHz toward the carbon star IRC +10216. The images have angular resolutions of 5″-7″ and velocity resolution of ∼1 km/s. The data are compared with model calculations using a newly-developed statistical equilibrium code which predicts the brightness distribution and spectrum for linear molecules in a spherical expanding circumstellar envelope. Excitation by IR photons and collisions is included. The model calculations are used to derive the abundance of HC₃N and SiS as a function of distance from the star. For C₃N, an LTE calculation is used to derive the abundance distribution. The data clearly show that HC₃N and C₃N are distributed in concentric shells, with little or no emission from the inner envelope. The abundances derived from our model calculations are much larger than published ion-molecule chemical reaction networks predict, but are in reasonably good agreement with chemical models which include neutral molecule-radical reactions. Statistical equilibrium models for SiS indicate that the gas phase abundance of SiS must decrease substantially as the gas flows from the photosphere to a distance of ∼2×10¹⁵ cm, probably as a result of grain adsorption processes. Photodissociation appears to cut off the SiS distribution between 2-3×10¹⁶ cm. We find that IR line overlaps of SiS with HCN or C₂H₂ are probably not important in the excitation of SiS, but that the excitation temperatures of some rotational transitions of SiS are very sensitive to the IR flux from the central star and dust shell. This strong dependence of excitation on IR flux may account for observed spectral variability of SiS lines.