Photochemical origin of SiC$_2$ in the circumstellar envelope of carbon-rich AGB stars revealed by ALMA

TL;DR

This study uses ALMA observations to investigate the origin of SiC$_2$ in the circumstellar envelopes of carbon-rich AGB stars, suggesting it is a daughter species formed by photochemistry rather than a parent species.

Contribution

It provides observational evidence supporting the photochemical formation of SiC$_2$ in AGB star envelopes, challenging previous assumptions about its origin.

Findings

SiC$_2$ shows an annular, shell-like distribution in observed stars.

Results support SiC$_2$ being a daughter species formed by photochemistry.

Findings offer new insights for chemical modeling of AGB star envelopes.

Abstract

Whether SiC$_2$ is a parent species, that is formed in the photosphere or as a by-product of high-temperature dust formation, or a daughter species, formed in a chemistry driven by the photodestruction of parent species in the outer envelope, has been debated for a long time. Here, we analyze the ALMA observations of four SiC$_2$ transitions in the CSEs of three C-rich AGB stars (AI Vol, II Lup, and RAFGL 4211), and found 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 can provide important references for future chemical models.