2026/05/27 by Alice Radcliffe, Benjamin Charnay, Anne-Marie Lagrange +9 · 1 citation
Physics and Astronomy · #Astrophysics and Star Formation Studies #Stellar, planetary, and galactic studies #Astronomy and Astrophysical Research
paper · pdf · doi:10.1051/0004-6361/202659006
Condensate clouds are a defining feature of brown dwarf and exoplanet atmospheres, producing a broad range of colours on the colour–magnitude diagram (CMD) and giving rise to spectral features such as the distinct ∼10 μm spectral imprint, a prominent diagnostic for silicate clouds. Cloud cover is likely to be heterogeneous in many objects, with observed rotational variability providing key evidence of the presence of thick and thin cloud regions rotating in and out of view. Yet current one-dimensional atmosphere models, which often lack a parameter to tune cloud optical thickness, typically fail to reproduce the spectra of highly cloudy substellar objects, especially those with complex cloud structures. Our goal is to address this limitation by upgrading the Exo-REM atmosphere model and by devising a more nuanced approach to better describe heterogeneous cloud cover with pre-computed 1D grids. Here, we present new self-consistent low- ( R = 500) and medium-resolution ( R = 10 000) Exo-REM grids, hereafter Exo-REM k26 , featuring critical updates: (1) the incorporation of a cloud sedimentation parameter, f sed , to govern cloud opacity, thereby enabling even the reddest of objects to be accessed on a CMD and revealing a trend of decreasing f sed along the L–T transition; (2) a substantial update of the molecular opacities and abundances used, including new experimentally validated alkali line lists; and (3) the implementation of strict convergence criteria that entirely avoid unstable model solutions. Correcting an erroneous CH 3 D abundance leads to marked spectral changes for low- T eff (methane-rich) objects. As a consequence, applying Exo-REM k26 to the cool GJ 504 b results in a revision of its parameters ( T eff = 473 −12 +14 K, log g = 4.0 ± 0.1 dex). For the notoriously variable VHS 1256 b, a two-column Exo-REM k26 framework that emulates cloud heterogeneities achieves a significantly improved global fit over a single 1D model. Here, a ∼ 60–40% split of thick and thin clouds best describes its atmosphere, further confirming the presence of patchy clouds. In particular, this reproduces the strong 10 μm silicate absorption in the MIRI/MRS (JWST) data of VHS 1256 b, where 1D grids had previously failed, due to the formerly unexplored low- f sed regime in the new model. The addition of the f sed dimension as well as the two-column approach for heterogeneous cloud distributions prove vital for accurately characterising cloudy sub-stellar objects.