Silicon Chemistry in the Outer Envelope of IRC + 10216: The Detailed Distribution of SiC2

Abstract The distribution of SiC2 in the envelope of the carbon-rich star IRC +10216 was observed with the Nobeyama Millimeter Array using the JKa,Kc = 404 → 303 rotational transition (93.06358 GHz). The beam size was about 9″ × 6″ (HPFW). The SiC2 distribution shows a clear shell-like structure with a diameter of about 30″. In addition, the shell is not uniform, but clumpy and nonspherical. The SiC2 line is relatively stronger in intensity in the northeast and southwest parts of the shell. This may reflect anisotropic physical conditions in the envelope. Thermodynamical equilibrium reactions and ion-molecule reactions were investigated as the production reactions of SiC2. Calculations of the thermodynamical equilibrium reaction model were carried out; the results show that a significant amount of SiC2 is generated only close to the star (2 to 4 stellar radii). On the other hand, the ion-molecule reactions have been considered to be dominant in the shell region where SiC2 was observed, based on the model calculations so far reported. These results indicate that SiC2 is produced mainly in the outer envelope by ion-molecule reactions.