Photochemical Origin of SiC_2 in the Circumstellar Envelope of Carbon-Rich AGB Stars Revealed by ALMA

The fact that whether SiC 2 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 SiC 2 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 SiC 2 exhibits an annular, shell-like distribution in these targets, suggesting that SiC 2 can be a daughter species in the CSEs of carbon-rich AGB stars. The results may provide important references for future chemical models.