Revisiting the Atmosphere of HD 149026b: The Role of Stellar Abundances and Choice of Opacities in Exoplanet Atmosphere Modeling

Abstract

Planet formation occurs within the same molecular cloud as the host star, suggesting a link between the elemental abundances of star and the planet. Exoplanet atmosphere studies often assume solar abundances for host stars, however, specific host star abundances might lead to more accurate constraints. In this work, we perform sensitivity studies for a metal rich stellar host HD 149026 and its exoplanet HD 149026b, to understand the effect of solar versus stellar abundance choice on the $P$-$T$ profiles, equilibrium chemical abundances and emission spectra, using self-consistent atmosphere models. We find that the differences are dependent on the model parameters, particularly C/O ratio, and for HD 149026b the difference in the eclipse depth is maximum $\sim$80 ppm, for C/O between 0.75-0.85. Recent JWST NIRCam observations of HD 149026b have yielded widely varying metallicity ranges, highly super-solar (59-275$\times$) using chemical equilibrium retrievals and 12-31$\times$ solar using self-consistent models, both using solar abundances. In this work, we constrain the metallicity of HD 149026b to be 53-113$\times$ solar, with solar abundances and 39-78$\times$ stellar, with stellar abundances. We constrain the self-consistent $P$-$T$ profile of HD 149026b to be substantially cooler (upto 500 K) than the self-consistent best-fit model in the previous work, in the emission spectra probed region, thus requiring higher CO$_2$ abundance to explain the observations, leading to comparatively higher metallicity constraint. We find that the inclusion of Fe opacity in computing self-consistent $P$-$T$ profiles for HD 149026b in our models is the major reason for these differences. We constrain the C/O ratio to 0.47-0.68 and the heat redistribution factor to 0.70-0.76, indicating higher heat redistribution than previously estimated.