2025RNAAS...9..330P

The chemistry of protoplanetary disks is influenced by stellar irradiation and dust evolution, both of which shape the physical conditions governing planet formation. In particular, molecules such as HCO⁺ and CN trace ionization and photochemistry in surface layers exposed to UV radiation. Here, we present Submillimeter Array observations of HCO⁺J = 3-2, 4-3 and CN N = 3-2 in five transition disks (V4046 Sgr, GM Aur, J1604, GG Tau, LkCa 15) with large dust cavities to assess how disk structure influences the observed emission. When combined with literature data, we identify a tentative trend of increasing line fluxes with cavity size, suggesting that reduced dust opacities in larger cavities enhance UV penetration and stimulate ion-radical chemistry. These results support a scenario in which disk structure modulates chemistry and highlights the potential of HCO⁺ and CN as tracers of UV-driven processes.