A Molecule-rich Torus-like Structure In The 21 μm Source IRAS 23304+6147
A long-standing enigma in observational astronomy is the identification of the so-called 21 μm feature in a subset of envelopes of post-asymptotic giant branch (post-AGB) stars.
Identifying this transient feature is important for understanding the chemical processes during the brief post-AGB phase and the enrichment of the interstellar medium. Understanding the structures and chemical environments of these objects is a prerequisite for such an endeavor.
We investigate the structure of the circumstellar envelope and the spatial distribution of gas-phase molecules in the 21 μm source IRAS 23304+6147, aiming to explore the potential physicochemical conditions required for the emergence of the 21 μm feature. Molecular line observations toward IRAS 23304+6147 at the 1.3 mm band were performed using the Northern Extended Millimeter Array. A morpho-kinematic model was built to reproduce the observed 13CO images and to decipher the structures of the nebula.
The imaging results reveal an elliptically elongated shell with an equatorial density enhancement (or a torus-like structure),and in detail how the various molecules distribute in the envelope. The nebular morphology points to a binary system in which the ultraviolet radiation from the companion may trigger photochemistry in the inner regions.
The torus-like structure exhibits an enrichment of linear carbon-chain molecules and a depletion of silicon-bearing molecules. The chemically stratified structure of 13CN, HC3N, and C4H represents an observational evidence of the internal radiation that initiates photochemistry.
The carbon-rich torus-like structure probably offers a conducive environment for the formation of dust and complex molecules implicated in the rare 21 μm emission.
Hao-Min Sun, Yong Zhang, Xu-Jia Ouyang, Sheng-Li Qin, Junichi Nakashima, Jian-Jie Qiu, Xiao-Hu Li
Comments: 19 pages,15figures
Subjects: Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2503.12054 [astro-ph.GA] (or arXiv:2503.12054v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2503.12054
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Submission history
From: Haomin Sun
[v1] Sat, 15 Mar 2025 09:00:57 UTC (16,940 KB)
https://arxiv.org/abs/2503.12054
astrobiology
Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 
