2025MNRAS.537..598K

Most stars form in dense clusters within high-mass star-forming regions, where protoplanetary discs may be exposed to intense UV radiation from nearby massive stars. While previous studies have typically focussed on isolated sources in low-mass regions, recent observational campaigns have started to probe the chemistry of irradiated discs in unprecedented detail. Interpreting this data requires complex chemical models, yet few studies have examined these discs' chemistry, and none have incorporated the photoevaporative wind launched by external UV fields into their physical structure. In this study, we post-process radiation hydrodynamics simulations of externally irradiated protoplanetary discs using the thermochemical code DALI, comparing models with and without the wind to assess its impact on disc chemistry. Results show that UV radiation is rapidly attenuated by the disc in both cases. However, thermal re-emission from the wind at longer wavelengths enhances disc heating, increasing the gas-phase abundances of some key volatiles. Synthetic line fluxes vary by orders of magnitude between wind and windless models, primarily due to emission from the wind itself rather than abundance variations within the disc. Our findings demonstrate that the photoevaporative wind significantly influences the physical and chemical structure, and observational characteristics, of externally irradiated discs. We conclude that incorporating the wind into chemical models is essential for accurately predicting chemical abundances, interpreting observations, and ultimately understanding planet formation in these common yet complex environments.