Dynamics of Y Dwarf Atmospheres

TL;DR

This study investigates the global dynamics of Y-dwarf atmospheres by performing a grid of twelve THOR GCM runs that couple interior thermal forcing, mixing-length convection, gray radiative transfer, and simple salt-and-sulfide cloud tracers. Across $T_{\mathrm{eff}}$ = 400–600 K and $P_{\mathrm{rot}}$ = 2.5–20 h, the atmospheres reside in a radiative-forcing-dominated regime with weak winds, near-adiabatic interiors, and negligible jet formation; convection sets cloud vertical extents while cloud radiative feedback remains subdominant, yielding small horizontal temperature variations and limited variability. The results suggest that, within the gray single-band framework, Y-dwarf atmospheres are primarily controlled by interior thermal radiation and rotation, with clouds playing a secondary role; deeper or non-gray radiative transfer and more sophisticated cloud microphysics could reveal more dynamical activity in other regions of parameter space. This work provides a baseline for interpreting JWST variability observations and outlines concrete avenues—non-gray radiative transfer, richer cloud physics, and disequilibrium chemistry—to explore regimes where cloud-radiation feedback might drive more active dynamics.

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.

Figures (10)

• Figure 1: A summary of our calculations, presented in Section \ref{['sec:characteristic_flow_quantities']}. Highlighted are the cases with gravity $\log g = 4.5$, a wave-driving efficiency of $\eta=10^{-3}$ and a vertical scale of $\Delta \tilde{z} = 2$ at the approximate photosphere level $p_{\mathrm{photosphere}} = 0.1 \rm~bar$. Other possible combinations are overplotted in fainter colors to illustrate the complete space spanned by the range of values considered in this study. Dotted lines mark the edges of minimum and maximum values. The red star corresponds to ($T_{\mathrm{eff}} = 400~\mathrm{K}~;~ P_{\mathrm{rot}} = 10~ \rm h$; $\log g = 4.46$) and is from a recent GCM study, included as a reference point.
• Figure 2: Various $T\text{--}p$ profiles overplotted to illustrate the initial conditions from which the simulations were spun up. The solid lines correspond to the $\kappa_{\mathrm{lw}} = 2.5 \times 10^{-3} ~\rm m^{2}~kg^{-1}$ curves with the lighter shades of each color illustration curves corresponding to the same internal temperature $T_{\mathrm{int}}$ with varying long-wave opacities of $\kappa_{\mathrm{lw}} = [2.5 \times 10^{-5}, 2.5 \times 10^{-4}, 2.5 \times 10^{-2}] \rm ~m^{2}~kg^{-1}$. Also overplotted are condensation curves for the cloud condensate species $\rm KCl,~MnS,~Na_2S$. The shaded horizontal region ($p = 0.1~\text{--}~1~\rm bar$) corresponds to the approximate photosphere.
• Figure 3: Effective radiating temperature maps of individual runs at $t_{\mathrm{run}} = 1000~d$. The shown results are time-averaged over the last 10 days of the given run time. Run cases vary in effective temperature from $T_{\mathrm{eff}} = 400~K$ to $T_{\mathrm{eff}} = 600~\rm K$ from top to bottom and correspond to a rotation period of $P_{\mathrm{rot}} = 2.5 ~\text{--}~ 20~ \rm h$ from left to right.
• Figure 4: Vertical mixing profiles of all simulations. The $T\text{--}p$ profile is overplotted with the convective mixing $K_{zz}$ and the advective mixing contribution $H \left(z \right) \sqrt{\langle w^{2} \left( z \right ) \rangle}_{h}$. The pressure extents of convective regions are shown as gray horizontal bands in each panel.
• Figure 5: Globally averaged vertical profiles of cloud condensate (blue) and vapour (red) VMRs, together with the saturation vapour VMR (green), for three cloud species (columns, left to right: $\mathrm{KCl}$, $\mathrm{Na_{2}S}$, $\mathrm{MnS}$) and three effective temperatures (rows, top to bottom: $400 \text{--}600$ K). Each panel shows results at a simulation time of $t_{\mathrm{run}} = 1000\,\text{d}$, averaged over the final $10$ days, for a fixed rotation period of $P_{\mathrm{rot}} = 20\,\text{h}$. The horizontal gray shaded area marks the photosphere for the single-band thermal emission.