The atmosphere of HD 149026b: Low metal-enrichment and weak energy transport

Recent JWST eclipse spectra of the high-density hot Saturn HD 149026b between 2. 35 and 5. 08 m has allowed for in-depth study of its atmosphere. To understand its atmospheric properties, we have created a grid of 1D radiative-convective-thermochemical equilibrium atmosphere models and spectra with PICASO 3. 0. In agreement with previous work, we find that the presence of gaseous TiO creates a thermal inversion, which is inconsistent with the data. The presence of gaseous VO, however, which condenses at temperatures 200 K cooler, does not cause such inversions but alters the temperature-pressure profile of the atmosphere. We estimate an atmospheric metallicity of 14^+12-₈ solar without VO and 20^+11-₈ solar with VO, a factor of 10 times smaller than previous work from Bean et al. (2023), who relied on atmosphere retrievals. We attribute this significant difference in metallicity to a larger temperature gradient at low pressures in radiative equilibrium models. Such models with lower metallicities readily fit the strong CO₂ feature at 4. 3 m. Our lower estimated metallicity makes HD 149026b more consistent with the mass-metallicity relationship for other giant planets. We find a C/O ratio of 0. 67^+0. 06-₀. ₂₇ with and without VO. The best-fit heat redistribution factor without VO is 1. 17, a very high value suggesting very little dayside energy transport and no energy transport to the night side. The heat redistribution factor shrinks to a more plausible value of 0. 91^+0. 05-₀. ₀₅, with VO, which we regard as circumstantial evidence for the molecule in the atmosphere of HD 149026b.