The impact of the atmospheric C/O ratio on hot Jupiter interiors

Atmospheric boundary conditions play a critical role in interior modelling because they determine the pressure and temperature at the radiative-convective boundary, which sets the interior adiabat. These boundary conditions depend on the assumed atmospheric composition, which observations of hot Jupiter atmospheres now constrain in increasing detail. However, interior models typically include only atmospheric metallicity in the boundary condition and assume solar values for all other elemental ratios. In this talk, we show how the C/O ratio affects the atmospheric boundary and, consequently, the inferred interior properties of hot Jupiters. We couple a grid of self-consistent 1D radiative-chemical equilibrium models to a static interior structure model. This framework allows atmospheric composition to be incorporated consistently both as an atmospheric boundary condition and as a constraint on the envelope metallicity. We find that variations in the atmospheric C/O ratio can change the temperature at the atmospheric boundary up to 200 K. These temperature differences propagate throughout the planetary interior and can produce radius differences of up to 40% for a fixed interior structure, depending on the equilibrium temperature and atmospheric metallicity. For equilibrium temperatures above 1500 K and super-solar metallicities the assumption of a solar C/O ratio introduces radius errors larger than current observational uncertainties. Finally, we retrieve the interior structure of WASP-19b for different atmospheric C/O ratios and show that assumptions about atmospheric composition alter the inferred intrinsic temperature. Our results highlight the importance of using chemically informed atmospheric boundary conditions when atmospheric abundance constraints are available.