Photochemical origin of SiC2 in the circumstellar envelope of carbon-rich AGB stars revealed by ALMA

The fact that whether SiC2 is a parent species, formed in the photosphere or as a by-product of high-temperature dust formation, or a daughter species, formed in chemistry driven by the photodestruction of the parent species in the outer envelope, has been debated for a long time. In this study, we analyze the Atacama Large Millimeter Array (ALMA) observations of four SiC2 transitions in the circumstellar envelopes (CSEs) of three C-rich asymptotic giant branch (AGB) stars (AI Vol, II Lup, and RAFGL 4211) and find that SiC2 exhibits an annular, shell-like distribution in these targets, suggesting that SiC2 can be a daughter species in the CSEs of carbon-rich AGB stars. The results may provide important references for future chemical models.