2025ApJ...980..147B

High-resolution transmission spectroscopy of the warm gas giant WASP-69 b has revealed the presence of H₂O, CO, CH₄, NH₃, and C₂H₂ in its atmosphere. This study investigates the impact of vertical diffusion and photochemistry on its atmospheric composition, with a focus on the detected species plus HCN and CO₂, to constrain the atmospheric C/O ratio. We utilize nonequilibrium gas-phase simulations to conduct a parameter study for vertical diffusion strength, local gas temperature, and C/O ratio. Our results indicate that a carbon-rich atmosphere enhances CH₄ and C₂H₂ concentrations, while NH₃ undergoes chemical conversion into HCN in carbon-rich, high-temperature environments. Consequently, HCN is abundantly produced in such atmospheres, though its strong spectral features remain undetected in WASP-69 b. Photochemical production of HCN and C₂H₂ is highly sensitive to vertical diffusion strength, with weaker diffusion resulting in higher concentrations. Cross-correlation of synthetic spectra with observed data shows that models with C/O = 2 best match observations, but models with C/O = 0.55 and 0.9 lead to statistically equivalent fits, leaving the C/O ratio unconstrained. We highlight the importance of accurately modeling NH₃ quenching at pressures greater than 100 bars. Models for WASP-69 b capped at 100 bars bias cross-correlation fits toward carbon-rich values. We suggest that if the atmosphere of WASP-69 b is indeed carbon-rich with a solar metallicity, future observations should reveal the presence of HCN.