Gas phase formation of c-SiC₃ molecules in the circumstellar envelope of carbon stars.

Abstract

Complex organosilicon molecules are ubiquitous in the circumstellar envelope of the asymptotic giant branch (AGB) star IRC+10216, but their formation mechanisms have remained largely elusive until now. These processes are of fundamental importance in initiating a chain of chemical reactions leading eventually to the formation of organosilicon molecules-among them key precursors to silicon carbide grains-in the circumstellar shell contributing critically to the galactic carbon and silicon budgets with up to 80% of the ejected materials infused into the interstellar medium. Here we demonstrate via a combined experimental, computational, and modeling study that distinct chemistries in the inner and outer envelope of a carbon star can lead to the synthesis of circumstellar silicon tricarbide (c-SiC₃) as observed in the circumstellar envelope of IRC+10216. Bimolecular reactions of electronically excited silicon atoms (Si(¹D)) with allene (H₂CCCH₂) and methylacetylene (CH₃CCH) initiate the formation of SiC₃H₂ molecules in the inner envelope. Driven by the stellar wind to the outer envelope, subsequent photodissociation of the SiC₃H₂ parent operates the synthesis of the c-SiC₃ daughter species via dehydrogenation. The facile route to silicon tricarbide via a single neutral-neutral reaction to a hydrogenated parent molecule followed by photochemical processing of this transient to a bare silicon-carbon molecule presents evidence for a shift in currently accepted views of the circumstellar organosilicon chemistry, and provides an explanation for the previously elusive origin of circumstellar organosilicon molecules that can be synthesized in carbon-rich, circumstellar environments.