Atmospheric properties of AF Lep b with forward modeling

Context. About a year ago, a super-Jovian planet was directly imaged around the nearby young solar-type star AF Lep. The 2.8 MJup planet orbiting at a semimajor axis of 8.2 au matches the predicted location based on the HIPPARCOS-Gaia astrometric acceleration.Aims. Our aim is to expand the atmospheric exploration of AF Lep b by modeling all available observations obtained with SPHERE at VLT (in the range 0.95–1.65, at 2.105, and at 2.253 µm), and NIRC2 at Keck (at 3.8 µm) with self-consistent atmospheric models.Methods. To understand the physical properties of this exoplanet, we used ForMoSA. This forward-modeling code compares observations with grids of pre-computed synthetic atmospheric spectra using Bayesian inference methods. We used Exo-REM, an atmospheric radiative-convective equilibrium model, including the effects of nonequilibrium processes and clouds.Results. From the atmospheric modeling we derived solutions at the low Teff of ~750 K. Our analysis also favors a metal-rich atmosphere (>0.4) and solar to super-solar carbon-to-oxygen ratio (~0.6). We tested the robustness of the estimated values for each parameter by cross-validating our models using the leave-one-out strategy, where all points are used iteratively as validation points. Our results indicate that the photometry point at 3.8 µm strongly drives the metal-rich and super-solar carbon-to-oxygen solutions.Conclusions. Our atmospheric forward-modeling analysis strongly supports the planetary nature of AF Lep b. Its spectral energy distribution is consistent with that of a young, cold, early-T super-Jovian planet. We recover physically consistent solutions for the surface gravity and radius, which allows us to reconcile atmospheric forward modeling with evolutionary models, in agreement with the previously published complementary analysis done by retrievals. Finally, we identified that future data at longer wavelengths are necessary before we can conclude about the metal-rich nature of AF Lep b.