A molecule-rich torus-like structure in the 21 µm source IRAS 23304+6147

¹ School of Physics and Astronomy, Sun Yat-sen University, 2 Daxue Road, Tangjia, Zhuhai 519082, Guangdong Province, China
² Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 150 Science 1-Street, Urumqi 830011, China
³ CSST Science Center for the Guangdong-Hongkong-Macau Greater Bay Area, Sun Yat-Sen University, Guangdong Province, China
⁴ School of physics and astronomy, Yunnan University, Kunming 650091, China
⁵ School of Mathematics and Physics, Jinggangshan University, 28 Xueyuan Road, Qingyuan District, Ji’an 343009, Jiangxi Province, China
⁶ Xinjiang Key Laboratory of Radio Astrophysics, 150 Science 1-Street, Urumqi, Xinjiang 830011, China
⁷ Key Laboratory of Radio Astronomy and Technology, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District, Beijing 100101, China

^★ Corresponding author; zhangyong5@mail.sysu.edu.cn

Received: 24 September 2024
Accepted: 14 March 2025

Abstract

Context. 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 this endeavor.

Aims. We investigate the structure of the circumstellar envelope and the spatial distribution of gas-phase molecules in the 21 µm source IRAS 23304+6147 with the aim to explore the potential physicochemical conditions required for the emergence of the 21 µm feature.

Methods. 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.

Results. The imaging results reveal an elliptically elongated shell with an equatorial density enhancement (or a torus-like structure), which shows in detail how the various molecules are distributed in the envelope. The nebular morphology indicates a binary system in which the ultraviolet radiation from the companion might 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. This nebula has a lower mass limit of 1.3 × 10⁻² M_⊙ and might exhibit a low ¹²CO/¹³CO abundance ratio.

Conclusions. The chemically stratified structure of ¹³CN, HC₃N, and C₄H represents 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 that are part of the rare 21 µm emission. We hypothesize that the 21 µm sources probably descend from J-type carbon stars via a binary evolutionary channel.