The MSG model for cloudy sub-stellar atmospheres

A grid of self-consistent sub-stellar atmosphere models with microphysical cloud formation

¹ Centre for ExoLife Sciences, Niels Bohr Institute, Øster Voldgade 5, 1350 Copenhagen, Denmark
² Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
³ TU Graz, Fakultät für Mathematik, Physik und Geodäsie, Petersgasse 16, 8010 Graz, Austria
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Centre for Science at Extreme Conditions, School of Physics and Astronomy, The University of Edinburgh, Edinburgh, UK
⁶ Scottish Universities Physics Alliance, School of Physics and Astronomy, The University of Edinburgh, Edinburgh, UK
⁷ School of GeoSciences, The University of Edinburgh, Edinburgh, UK
⁸ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
⁹ Kapteyn Astronomical Institute, University of Groningen, Groningen, The Netherlands
¹⁰ Netherlands Space Research Institute (SRON), Leiden, The Netherlands
¹¹ Centre for Exoplanet Science, The University of Edinburgh, Edinburgh, UK
¹² Department of Astrophysics, American Museum of Natural History, NY, USA

^★ Corresponding author; becampos@mpia.de

Received: 18 July 2024
Accepted: 8 January 2025

Abstract

Context. State-of-the-art JWST observations are unveiling unprecedented views into the atmospheres of sub-stellar objects in the infrared, further highlighting the importance of clouds. Current forward models struggle to fit the silicate clouds absorption feature at ∼10 µm observed in sub-stellar atmospheres.

Aims. In the MSG model, we aim to couple the MARCS 1D radiative-convective equilibrium atmosphere model with the 1D kinetic, stationary, non-equilibrium cloud formation model DRIFT, also known as StaticWeather, to create a new grid of self-consistent cloudy sub-stellar atmosphere models with microphysical cloud formation. We aim to test if this new grid is able to reproduce the silicate cloud absorption feature at ∼10 µm.

Methods. We modelled sub-stellar atmospheres with effective temperatures in the range T_eff = 1200–2500 K and with log(ɡ) = 4.0. We computed atmospheric structures that self-consistently account for condensate cloud opacities based on microphysical properties. We present an algorithm based on control theory to help converge such self-consistent models. Synthetic atmosphere spectra were computed for each model to explore the observable impact of the cloud microphysics. We additionally explored the impact of choosing different nucleation species (TiO₂ or SiO) and the effect of less efficient atmospheric mixing on these spectra.

Results. The new MSG cloudy grid using TiO₂ nucleation shows spectra that are redder in the near-infrared compared to the currently known population of sub-stellar atmospheres. We find that the models with SiO nucleation and models with reduced mixing efficiency are less red in the near-infrared.

Conclusions. We present a new grid of MSG models for cloudy sub-stellar atmospheres that include cloud radiative feedback from microphysical clouds. The grid is unable to reproduce silicate features similar to the ones found in recent JWST observations and Spitzer archival data. We thoroughly discuss further work that may better approximate the impact of convection in cloud-forming regions and steps that may help resolve the silicate cloud feature.