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

Priyanka Baghel, Jayesh Goyal, Gaurav Shukla

TL;DR

This study revisits the atmosphere of HD 149026 b by contrasting solar versus host-star elemental abundances within self-consistent radiative–convective-thermochemical equilibrium models using ATMO. It constructs five model grids featuring varying $C/O$, metallicity, and opacities (with and without VO) and applies both chi-squared minimization and Bayesian grid retrieval to JWST NIRCam emission spectra. The results show that stellar abundances generally yield high atmospheric metallicities (up to ~100× solar) with $C/O$ in the range ~0.46–0.68, while solar-abundance grids produce similar $C/O$ constraints but metallicities that can be higher or lower depending on opacity choices; Fe opacity notably lowers deeper $P$-$T$ temperatures, driving higher CO$_2$ and metallicities to fit the data. The work highlights the importance of using stellar abundances when available, clarifies the role of opacities (notably Fe) in shaping retrieved properties, and places the HD 149026 b results in the context of prior Bean (chemical equilibrium) and gagnebin (self-consistent) analyses, with implications for exoplanet formation and atmospheric interpretation.

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.

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

TL;DR

This study revisits the atmosphere of HD 149026 b by contrasting solar versus host-star elemental abundances within self-consistent radiative–convective-thermochemical equilibrium models using ATMO. It constructs five model grids featuring varying , metallicity, and opacities (with and without VO) and applies both chi-squared minimization and Bayesian grid retrieval to JWST NIRCam emission spectra. The results show that stellar abundances generally yield high atmospheric metallicities (up to ~100× solar) with in the range ~0.46–0.68, while solar-abundance grids produce similar constraints but metallicities that can be higher or lower depending on opacity choices; Fe opacity notably lowers deeper - temperatures, driving higher CO and metallicities to fit the data. The work highlights the importance of using stellar abundances when available, clarifies the role of opacities (notably Fe) in shaping retrieved properties, and places the HD 149026 b results in the context of prior Bean (chemical equilibrium) and gagnebin (self-consistent) analyses, with implications for exoplanet formation and atmospheric interpretation.

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 - 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 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) using chemical equilibrium retrievals and 12-31 solar using self-consistent models, both using solar abundances. In this work, we constrain the metallicity of HD 149026b to be 53-113 solar, with solar abundances and 39-78 stellar, with stellar abundances. We constrain the self-consistent - 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 abundance to explain the observations, leading to comparatively higher metallicity constraint. We find that the inclusion of Fe opacity in computing self-consistent - 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.

Paper Structure

This paper contains 23 sections, 1 equation, 17 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Methodology
  3. Atmospheric model
  4. Host star abundances
  5. HD-149026 b Grid of Model Simulations
  6. Grid with solar abundances varying O/H
  7. Grid with solar abundances varying C/H
  8. Grid with stellar abundance
  9. Grid similar to gagnebin2024 without VO grid
  10. Grid similar to gagnebin2024 with VO grid
  11. Grid Fitting Techniques
  12. Fitting with $\chi^2$ minimization
  13. Fitting with Bayesian Grid Retrieval
  14. Sensitivity Tests
  15. Case 1: Vary $C/O$ Value
  16. ...and 8 more sections

Figures (17)

  • Figure 1: These plots show the comparison between PT profiles, emission spectra and molecular abundances when solar and HD-149026 abundances are considered for three different $C/O$ values (0.1, 0.8, 1.6) while recirculation factor and logZ are kept constant with values 0.7 and 0.0 respectively. The overall effect of solar and host star abundances on PT profile, emission spectra and atmospheric abundances is different for different $C/O$ values. It's negligible for $C/O$ value = 0.1 and 1.6, but the difference is significant for $C/O$ value = 0.8.
  • Figure 2: These plots show the comparison between PT profiles, emission spectra and molecular abundances when solar and HD-149026 abundances are considered for three different logZ values (-0.1, 1.0, 2.0) while recirculation factor and $C/O$ are kept constant with values 0.7 and 0.8 respectively. The overall effect of solar and host star abundances on PT profile, emission spectra and atmospheric abundances differs for different logZ values.
  • Figure 3: Best-fit emission spectra derived from a grid of ATMO models using solar elemental abundances (blue) and stellar elemental abundances (orange), fitted to JWST NIRCam observations (black). The minimum $\chi^2$ values of 119.32 and 118.74 correspond to the best-fit solar and stellar models, respectively, both at a recirculation factor of 0.7. The best-fit solar model has logZ = 1.8, while the best-fit stellar model has logZ = 1.7, both with a $C/O$ ratio of 0.7.
  • Figure 4: This Figure shows a comparison between the pressure-temperature profile for the best-fit spectra obtained from the ATMO model grid constructed using solar abundances and host star HD-149026 abundances (stellar abundances). The blue line represents the PT profile of the solar abundance best-fit model, and the orange line represents the PT profile of the stellar abundance best fit model. The Figure shows that the PT profile with host star abundances is colder than the solar abundance PT profile in the lower atmosphere.
  • Figure 5: This plot compares the atmospheric chemical abundances of HD-149026 b corresponding to the best-fit emission spectra from solar and host star abundance grid. The solid line represents the atmospheric abundances for the solar abundance best fit, while the dotted line represents the stellar abundance best fit.
  • ...and 12 more figures