2024MNRAS.532..734V

The nearby oxygen-rich AGB star L$_2$ Pup hosts a well-studied nearly edge-on disc. To date, discs around AGB stars have not been chemically studied in detail. By combining a parametrization commonly used for protoplanetary discs and archival ALMA observations, we retrieved an updated density and temperature structure of this disc. This physical model was then used as input to the first chemical model of an AGB disc. The model shows that the physical structure of the disc has a large impact on its chemistry, with certain species showing large changes in column density relative to a radial outflow, indicating that chemistry could be used as a tracer of discs that cannot be directly imaged. Despite its oxygen-rich nature, the daughter species formed within the disc are surprisingly carbon-rich. Two chemical regimes can be distinguished: cosmic-ray induced chemistry in the midplane and photochemistry induced by the interstellar radiation field in the outer regions. Certain complex organic molecules are formed in the midplane. This occurs via gas-phase chemistry only, as the disc is too warm for dust-gas chemistry. The photochemistry in the outer regions leads to the efficient formation of (long) carbon-chains. The predictions of the model allow us to tentatively put the disc's age $\lesssim 10^5$ yr. Additional observations are necessary to better constrain the physical structure of L$_2$ Pup's disc and are essential to test the predictions made by the chemical model. Our exploratory work paves the way for a more general study of the chemistry of AGB discs.