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

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.