Atmospheric Circulation of Brown Dwarfs and Directly Imaged Extrasolar Giant Planets

Abstract:

Growing observations of brown dwarfs (BDs) and directly imaged extrasolar giant planets (EGPs), such as brightness variability and surface maps have provided evidence for strong atmospheric circulation on these worlds. Previous studies that serve to understand the atmospheric circulation of BDs include modeling of convection from the interior and its interactions with stably stratified atmospheres. These models show that such interactions can drive an atmospheric circulation, forming zonal jets and/or vortices. However, these models are dry, not including condensation of various chemical species. Condensation of water has previously been shown to play an important role on driving the zonal jets on four giant planets in our solar system. As such, condensation cycles of various chemical species are believed to be an important source in driving the atmospheric circulation of BDs and directly imaged EGPs. Here we present results from three-dimensional simulations for the stably stratified atmospheres of BDs and EGPs based on a general circulation model that includes the effect of a condensate cycle. Large-scale latent heating and molecular weight effect due to condensation of a single species are treated explicitly. We examine the circulation patterns caused by large-scale latent heating which results from condensation of silicate vapor in hot dwarfs and water vapor in the cold dwarfs. By varying the abundance of condensable vapor and the radiative timescale, we conclude that under normal conditions of BDs (near 1x solar abundance and relatively short radiative timescale), latent heating alone by silicate vapors is unable to drive a global circulation, leaving a quiescent atmosphere, because of the suppression to moist instability by downward transport of dry air. Models with high abundance of condensates (~5x solar) and relatively long radiative timescale, which may be the case of directly imaged EGPs, tend to maintain an active hydrological cycle and develop zonal jets. Once condensation happens, storms driven by moist instability can extend vertically over several scale heights, reaching the photosphere and being able to induce flux variability — helping to explain patchy clouds inferred from near-IR light curves and the inferred surface map of BDs.