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The unprecedented medium-resolution (R=1500-3500) near- and mid-infrared(1-18um) spectrum provided by JWST for the young (120-160Myr) low-mass(12-20MJup) L-T transition (L7) companion VHS1256b has opened new avenues forthe in-depth characterization of substellar object atmospheres. In this study,we present a comprehensive analysis of this dataset utilizing a forwardmodelling approach. We explore five distinct atmospheric models, each aiming toencompass diverse physical and chemical phenomena proposed to happen in coolatmospheres in a self-consistent way. Our aim is to assess their performance inestimating key atmospheric parameters: Teff, log(g), [M/H], C/O, gamma, fsed,and R. To achieve this, we apply our Bayesian framework, ForMoSA, employing anoptimized nested sampling algorithm for model-data comparison. Our findingsreveal that each parameter's estimate is significantly influenced by factorssuch as the wavelength range considered, the spectral features used, thesignal-to-noise ratio, and the model chosen for the fit. The observed parameterdispersion may be attributed to systematic error in the models, resulting fromtheir difficulties in accurately replicating the complex atmospheric structureof VHS1256b, notably the complexity of its clouds and dust distribution. Topropagate the impact of these systematic uncertainties on our atmosphericproperty estimates, we introduce innovative fitting methodologies based onindependent fits performed on different spectral windows. We finally derived aTeff consistent with the spectral type of the target, considering its youngage, which is confirmed by our estimate of log(g). Despite the exceptional dataquality, attaining robust estimates for chemical abundances [M/H] and C/O,often employed as indicators of formation history, remains challenging withestimates dispersed on the entire range explored by the model grids.

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