SiC2 molecule found to form in outer envelopes of carbon-rich AGB stars

Circumstellar envelopes (CSEs) surrounding asymptotic giant branch (AGB) stars are rich in molecules, comprising about one-third of all molecules detected in interstellar space.

These envelopes contain both gas and dust, with SiC2 being a significant component of dust grains in carbon-rich AGB stars. The debate persists: Is SiC2 a “parent” molecule originating in the photosphere or formed during high-temperature dust creation, showing a “solid” distribution? Or is it a “daughter” molecule, resulting from the photodissociation of “parent” molecules in outer envelopes, displaying an annular pattern?

Researchers, led by Ph.D. candidate Feng Yanan and Prof. Li Xiaohu from the Xinjiang Astronomical Observatory (XAO) of the Chinese Academy of Sciences, employed the Atacama Large Millimeter/Submillimeter Array (ALMA) to observe SiC2 in circumstellar envelopes of three carbon-rich AGB stars (AI Vol, II Lup, and RAFGL 4211). Their findings, published in Frontiers in Astronomy and Space Sciences on Aug. 14, reveal that the spatial distribution of SiC2 molecules’ rotational transition spectral lines around these sources forms an annular pattern, indicating its “daughter” nature.

Comparatively, ALMA results for SiC2 and SiO molecules in AI Vol show that SiO exhibits a “solid” distribution, consistent with being a “parent” molecule, as established in prior research.

Prof. Li suggests that future studies should revisit SiC2’s formation mechanism in the CSEs of evolved stars.

Source: Chinese Academy of Sciences

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