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 mu m have 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 14-8+12x solar without VO and 20-8+11x solar with VO, a factor of similar to 10 times smaller than previous work from Bean et al., 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 mu 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 0.67-0.27+0.06 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 nightside. 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.