The Atmosphere of HD 149026b: Low Metal Enrichment and Weak Energy Transport

Abstract

Recent JWST eclipse spectra of the high-density hot Saturn HD 149026b between 2.35 and 5.08 μ 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}^{+12}\times $ solar without VO and ${20}_{-8}^{+11}\,\times $ solar with VO, a factor of ∼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 CO _2 feature at 4.3 μ 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.

Keywords

• Exoplanet atmospheres
• Atmospheric composition