The circumstellar envelopes (CSE) of asymptotic giant branch (AGB) stars contain a large number of molecules, which account for about one-third of all molecules discovered in interstellar space.

Gas and dust are essential components of CSEs, and SiC₂ is one of the significant constituents of dust grains in carbon-rich AGB stars. Whether SiC₂ is "parent" molecules formed in the photosphere or during the high-temperature dust formation process (exhibiting a "solid" spatial distribution), or a "daughter" molecule formed through photodissociation of "parent" molecules in the outer envelopes (exhibiting an annular distribution), is a debate existing for a long time.

Researchers led by Ph.D. candidate FENG Yanan and Prof. LI Xiaohu from the Xinjiang Astronomical Observatory (XAO) of the Chinese Academy of Sciences have conducted observational work on the SiC₂ molecule in circumstellar envelopes of three carbon-rich AGB stars (AI Vol, II Lup, and RAFGL 4211) using the Atacama Large Millimeter/Submillimeter Array (ALMA).

The study was published in Frontiers in Astronomy and Space Sciences on Aug. 14.

They found that the spatial distribution of the four rotational transition spectral lines of SiC₂ molecules around these three sources exhibited an annular distribution, indicating the feature of a typical "daughter" species.

Then they compared the ALMA results of SiC₂ and SiO molecules in AI Vol. The SiO molecule exhibited a "solid" distribution feature, indicating that it is a "parent" molecule, which is consistent with previous studies.

"In future studies, we need to rethink the formation mechanism of SiC₂ in the CSEs of evolved stars," said Prof. LI.