2025A&A...696A.102S

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.