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Arxiv: The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems V: Do Self-Consistent Atmospheric Models Represent JWST Spectra? A Showcase With VHS 1256 b Published: 12/6/2023 9:07:10 PM Updated: 1/31/2024 3:59:59 PM

Paper abstract: The unprecedented medium-resolution (R~1500-3500) near- and mid-infrared(1-18um) spectrum provided by JWST for the young (140+/-20Myr) low-mass(12-20MJup) L-T transition (L7) companion VHS1256b gives access to a catalogueof molecular absorptions. In this study, we present a comprehensive analysis ofthis dataset utilizing a forward modelling approach, applying our Bayesianframework, ForMoSA. We explore five distinct atmospheric models to assess theirperformance in estimating key atmospheric parameters: Teff, log(g), [M/H], C/O,gamma, fsed, and R. Our findings reveal that each parameter's estimate issignificantly influenced by factors such as the wavelength range considered andthe model chosen for the fit. This is attributed to systematic errors in themodels and their challenges in accurately replicating the complex atmosphericstructure of VHS1256b, notably the complexity of its clouds and dustdistribution. To propagate the impact of these systematic uncertainties on ouratmospheric property estimates, we introduce innovative fitting methodologiesbased on independent fits performed on different spectral windows. We finallyderived a Teff consistent with the spectral type of the target, considering itsyoung age, which is confirmed by our estimate of log(g). Despite theexceptional data quality, attaining robust estimates for chemical abundances[M/H] and C/O, often employed as indicators of formation history, remainschallenging. Nevertheless, the pioneering case of JWST's data for VHS1256b haspaved the way for future acquisitions of substellar spectra that will besystematically analyzed to directly compare the properties of these objects andcorrect the systematics in the models.