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 1256–1257 b
Authors: Petrus Simon , Whiteford Niall , Patapis Polychronis , Biller Beth A. , Skemer Andrew , Hinkley Sasha , Suárez Genaro , Palma-Bifani Paulina , Morley Caroline V. , Tremblin Pascal , Charnay Benjamin , Vos Johanna M. , Wang Jason J. , Stone Jordan M. , Bonnefoy Mickaël , Chauvin Gaël , Miles Brittany E. , Carter Aarynn L. , Lueber Anna , Helling Christiane , Sutlieff Ben J. , Janson Markus , Gonzales Eileen C. , Hoch Kielan K. W. , Absil Olivier , Balmer William O. , Boccaletti Anthony , Bonavita Mariangela , Booth Mark , Bowler Brendan P. , Briesemeister Zackery W. , Bryan Marta L. , Calissendorff Per , Cantalloube Faustine , Chen Christine H. , Choquet Elodie , Christiaens Valentin , Cugno Gabriele , Currie Thayne , Danielski Camilla , De Furio Matthew , Dupuy Trent J. , Factor Samuel M. , Faherty Jacqueline K. , Fitzgerald Michael P. , Fortney Jonathan J. , Franson Kyle , Girard Julien H. , Grady Carol A. , Henning Thomas , Hines Dean C. , Hood Callie E. , Howe Alex R. , Kalas Paul , Kammerer Jens , Kennedy Grant M. , Kenworthy Matthew A. , Kervella Pierre , Kim Minjae , Kitzmann Daniel , Kraus Adam L. , Kuzuhara Masayuki , Lagage Pierre-Olivier , Lagrange Anne-Marie , Lawson Kellen , Lazzoni Cecilia , Leisenring Jarron M. , Lew Ben W. P. , Liu Michael C. , Liu Pengyu , Llop-Sayson Jorge , Lloyd James P. , Macintosh Bruce , Mâlin Mathilde , Manjavacas Elena , Marino Sebastián , Marley Mark S. , Marois Christian , Martinez Raquel A. , Matthews Elisabeth C. , Matthews Brenda C. , Mawet Dimitri , Mazoyer Johan , McElwain Michael W. , Metchev Stanimir , Meyer Michael R. , Millar-Blanchaer Maxwell A. , Mollière Paul , Moran Sarah E. , Mukherjee Sagnick , Pantin Eric , Perrin Marshall D. , Pueyo Laurent , Quanz Sascha P. , Quirrenbach Andreas , Ray Shrishmoy , Rebollido Isabel , Adams Redai Jea , Ren Bin B. , Rickman Emily , Sallum Steph , Samland Matthias , Sargent Benjamin , Schlieder Joshua E. , Stapelfeldt Karl R. , Tamura Motohide , Tan Xianyu , Theissen Christopher A. , Uyama Taichi , Vasist Malavika , Vigan Arthur , Wagner Kevin , Ward-Duong Kimberly , Wolff Schuyler G. , Worthen Kadin , Wyatt Mark C. , Ygouf Marie , Zurlo Alice , Zhang Xi , Zhang Keming , Zhang Zhoujian , Zhou Yifan 
Published: April 2024

Abstract
The unprecedented medium-resolution (R

λ
∼ 1500–3500) near- and mid-infrared (1–18 μm) spectrum provided by JWST for the young (140 ± 20 Myr) low-mass (12–20 M
Jup) L–T transition (L7) companion VHS 1256 b gives access to a catalog of molecular absorptions. In this study, we present a comprehensive analysis of this data set utilizing a forward-modeling approach applying our Bayesian framework, ForMoSA. We explore five distinct atmospheric models to assess their performance in estimating key atmospheric parameters: T
eff, log(g), [M/H], C/O, γ, f
sed, and R. Our findings reveal that each parameter’s estimate is significantly influenced by factors such as the wavelength range considered and the model chosen for the fit. This is attributed to systematic errors in the models and their challenges in accurately replicating the complex atmospheric structure of VHS 1256 b, notably the complexity of its clouds and dust distribution. To propagate the impact of these systematic uncertainties on our atmospheric property estimates, we introduce innovative fitting methodologies based on independent fits performed on different spectral windows. We finally derived a T
eff consistent with the spectral type of the target, considering its young age, which is confirmed by our estimate of log(g). Despite the exceptional data quality, attaining robust estimates for chemical abundances [M/H] and C/O, often employed as indicators of formation history, remains challenging. Nevertheless, the pioneering case of JWST’s data for VHS 1256 b has paved the way for future acquisitions of substellar spectra that will be systematically analyzed to directly compare the properties of these objects and correct the systematics in the models.