The asymmetric limbs of HD 209458 b observed with JWST NIRCam F322W2/F444W

We present a reanalysis of JWST NIRCam F322W2/F444W transit observations of the hot Jupiter HD 209458 b to extract morning and evening transmission spectra. To determine whether asymmetries in the transit light curves arise from atmospheric structure, we refine the HD 209458 system parameters through a reanalysis of archival transit and radial velocity data. We then extract limb-resolved spectra using two independent light curve modelling frameworks, Harmonica and catwoman,which yield consistent results. We interpret the transmission spectra using both 1D and 1.5D (i.e.,retrievals in which each limb of the planet is modelled independently) atmospheric retrievals. From our 1.5D free-chemistry retrievals, we find a strong preference (Delta ln Z = 2.81 +/- 0.49) for a model in which the morning and evening limbs are retrieved independently over one in which they share atmospheric parameters, despite the additional free parameters this requires. This asymmetry is most pronounced in the retrieved cloud properties, consistent with 3D atmospheric circulation models that predict longitudinally varying cloud distributions, and to a lesser extent in the CH4 and CO abundances. In contrast, the H2O and CO2 abundances and the isothermal temperature structure remain largely consistent between limbs. Similarly, our 1.5D equilibrium chemistry retrieval reveals a uniform composition between the limbs, alongside a marginally hotter evening limb relative to the morning limb. While the retrieved temperatures, chemical abundances, metallicities and carbon-to-oxygen ratios (C/O) are uniform across both limbs, they exhibit significant differences from our 1D retrieval results, highlighting the importance of multidimensional modelling in mitigating 1D retrieval biases.