Meteorologies of Brown Dwarfs and Extrasolar Giant Planets

Abstract

This dissertation explores the consequences of atmospheric dynamics for observations of substellar mass objects (SMOs). Discussed first is the growth of cloud particles of various compositions in brown dwarfs of different surface gravities and effective temperatures. The structure of these objects is calculated with a one-dimensional radiative transfer model. To determine particle sizes, the timescales for microphysical growth processes, including nucleation, coagulation, and coalescence, are compared to the timescale for gravitational sedimentation. The model also allows for sustained uplifting of condensable vapor in convective regions. The results show that particle sizes vary greatly over the range of objects studied. In most cases, clouds on brown dwarfs do not dominate the opacity. Rather, they smooth the emergent spectrum and partially redistribute the radiative energy. The focus then shifts to extrasolar giant planets (EGPs). Results are presented from a three-dimensional model of atmospheric dynamics on the transiting Jupiter-like planet HD 209458b. As a close-in orbiter (known as a “roaster”), HD 209458b is super-heated on its dayside. Due to tidal locking of the interior, the dayside hemisphere faces the star in perpetuity, which leads to very different dynamics than is seen on Jupiter. The flow is characterized by an eastward supersonic jet (u ∼ 4 km s⁻¹) extending from the equator to the mid-latitudes. Temperature contrasts are ∼ 500 K at the photosphere. At 220 mbar, winds blow the hottest regions downstream from the substellar point by ∼ 60°, with direct implications for the infrared light curve. These simulations are extended to the study of carbon chemistry in HD 209458b’s atmosphere by coupling the CO/CH₄ reaction kinetics to the dynamics. Disequilibrium results from slow reaction rates at low temperatures and pressures. Effective vertical quenching near the ∼ 3 bar level leads to uniformly high concentrations of CO at the photosphere, even in cool regions where CH₄ is strongly favored thermodynamically. Observations are underway to detect these signatures of meteorology on HD 209458b and similar planets.