Dynamics of Y Dwarf Atmospheres

Abstract

The global circulation regime of the coolest brown dwarfs, the Y dwarfs, remains largely unexplored. We investigate the interplay between convection, rotation, and cloud thermal feedback using a selected sample of Y dwarf atmospheric models. We explore effective temperatures $400~\mathrm{K} \leq T_{\mathrm{eff}} \leq 600~\mathrm{K}$ and rotation periods $P_{\mathrm{rot}} = 2.5 \text{--} 20\ \mathrm{h}$, where salt and sulfide condensates are expected. We include $\mathrm{KCl,~Na_{2}S}$, and $\mathrm{MnS}$ clouds to assess their atmospheric impact and identify circulation regimes across parameter space. We run twelve general circulation models (GCMs) spanning this grid and develop additional physics modules for the THOR GCM to model brown dwarf atmospheres. The dynamical core is coupled to interior thermal perturbations near the radiative-convective boundary, a mixing-length convection scheme, gray two-stream radiative transfer with Rosseland-mean opacities, and simple cloud tracers including thermal feedback and scattering. All simulations exhibit 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 salt and sulfide cloud layers below the photosphere. Thermal structures equilibrate quickly and cloud radiative feedback remains insignificant, with limited variability. Within the gray radiative transfer framework adopted here, Y dwarf atmospheres in this parameter space are controlled by interior thermal radiation. Rotation sets modest variability, while clouds play a secondary role. Because our single-band approach does not capture spectral windows that could probe deeper cloud layers, our constraints on cloud radiative feedback are likely conservative, and we outline pathways toward more active regimes.