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Arxiv: Retrievals on NIRCam transmission and emission spectra of HD 189733b with PLATON 6, a GPU code for the JWST era Published: 10/29/2024 8:00:00 PM Updated: 10/29/2024 8:00:00 PM

Paper abstract: We present the 2.4--5.0 \um JWST/NIRCam emission spectrum of HD 189733b,along with an independent re-reduction of the previously published transmissionspectrum at the same wavelengths. We use an upgraded version of PLanetaryAtmospheric Tool for Observer Noobs (PLATON) to retrieve atmospheric parametersfrom both geometries. In transit, we obtain [M/H]=0.53_{-0.12}^{+0.13} andC/O=0.41_{-0.12}^{+0.13}, assuming a power-law haze and equilibrium chemistrywith methane depletion. In eclipse, we obtain [M/H]=0.68_{-0.11}^{+0.15} andC/O=0.43_{-0.05}^{+0.06}, assuming a clear atmosphere and equilibriumchemistry without methane depletion. These results are consistent with eachother, and with a re-run of our previously published joint retrieval of HST andSpitzer transmission and emission spectra. Accounting for methane depletiondecreases the C/O ratio by 0.14/0.04 (transmission/emission), but changing thelimb cloud parameterization does not affect the C/O ratio by more than 0.06. Wedetect H_2O, CO_2, CO, and H_2S in both the NIRCam transmission andemission spectra, find that methane is depleted on the terminator, and confirmwith VULCAN that photochemistry is a potential cause of this depletion. We alsofind tentative (1.8\sigma) evidence of a dayside thermal inversion atmillibar pressures. Finally, we take this opportunity to introduce a newversion of PLATON. PLATON 6 supports GPU computation, speeding up the code upto 10x. It also supports free retrievals using both volume mixing ratio andcentered-log ratio priors; emission from planetary surfaces of differentcompositions; updated opacities at improved resolution; and Pareto smoothedimportance sampling leave-one-out cross validation (PSIS-LOO).