Dynamics of Y dwarf atmospheres

¹ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
² Faculty of Physics, Ludwig Maximilian University, Scheinerstraße 1, 81679 München, Deutschland
³ ARTORG Center for Biomedical Engineering Research, University of Bern, Murtenstrasse 50, 3008 Bern, Switzerland
⁴ University College London, Department of Physics & Astronomy, Gower St, London WC1E 6BT, UK
⁵ Astronomy & Astrophysics Group, Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁶ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 25 August 2025
Accepted: 2 February 2026

Abstract

Context. The global circulation regime of the coolest class of brown dwarfs, known as the Y dwarfs, remains largely unexplored.

Aims. We aim to investigate the interplay between convection, rotation, and cloud thermal feedback through a selected sample of Y dwarf atmospheric models. We explore a range of effective temperatures 400 K ≤ T_eff ≤ 600 K and rotation rates P_rot = 2.5–20 h. In this temperature range, salt and sulfide condensates are expected to form. We include KCl, Na₂S, and MnS clouds in our simulations to study their effect on the atmosphere. Our goal is to identify circulation regimes and emergent trends across this space, providing insights into the dynamical processes governing Y dwarf atmospheres.

Methods. We ran a suite of twelve general circulation models (GCMs) across the outlined parameter grid. For this purpose, we developed additional physics modules for the THOR GCM to model brown dwarf atmospheres. The THOR dynamical core was coupled to modules for interior thermal perturbations near the radiative–convective boundary, a mixing-length convection scheme, a gray two-stream radiative transfer module using Rosseland-mean opacities, and simple cloud tracers with thermal feedback and scattering.

Results. Across all simulations, the circulation resides in a radiative-forcing-dominated regime with weak winds, minimal horizontal temperature contrasts, and no persistent jets. Convection controls vertical mixing and sets the extent of the salt and sulfide cloud layers that form below the photosphere. Thermal structures equilibrate quickly and cloud radiative feedback remains insignificant, with limited variability.

Conclusions. Y dwarf atmospheres in this parameter range, within the gray radiative transfer framework adopted in this work, remain controlled by thermal radiation from the interior, with small variability primarily set by rotation and clouds playing a secondary role. As our single-band approach does not capture spectral windows that could probe deeper into the cloud-rich layers, our constraints on cloud radiative feedback are likely conservative, and we outline possible causes and pathways toward more active regimes.