WEATHER IN BROWN DWARFS AND DIRECTLY IMAGED EXTRASOLAR GIANT PLANETS

Abstract

Observational evidence has suggested that there is active meteorology in the atmospheres of brown dwarfs. In particular, a number of surveys of brown dwarfs have shown that near-IR brightness variability is common for L and T dwarfs. Similar near-IR variability has also been found for directly imaged extrasolar giant planets (EGPs), which can be viewed as low-gravity versions of brown dwarfs. A number of possibilities have been proposed as the driving sources for the variability in brown dwarf atmospheres, and yet they need further detailed demonstration. Clouds are believed to play an important role in shaping the thermal structure, spectral properties and perhaps dynamics of these atmospheres, and we expect the same for inducing short-term atmospheric variability. In this work we propose a robust mechanism for the variability in which condensational cloud cycles controlled by particle gravitational settling and convective mixing interact with thermal radiation via cloud opacity loading. This mechanism naturally drive oscillations in both temperature and cloud structure. We demonstrate the variability using a simple and self-consistent one-dimensional model. We show that the detailed evolution of variability is sensitive to the assumed cloud microphysics, but the existence of variability is robust for a wide range of model assumptions. The mechanism lies in the transition between growth of cloud layer thickness and the inevitable decay of clouds. Our proposed mechanism has important implications for the observed flux variability, especially brown dwarfs that evolve in a short timescale and with complex vertical structure. It is also a promising mechanism for cloud breaking, which has been proposed to explain the L/T transition of brown dwarfs. This work motivates the pressing need for global 3D circulation modeling for brown dwarfs and directly imaged EGPs including radiative cloud feedback to fully understand cloudy brown dwarf atmospheres.