Chemical Equilibrium Abundances in Brown Dwarf and Extrasolar Giant Planet Atmospheres

Abstract

We explore detailed chemical equilibrium abundance profiles for a variety of brown dwarf and extrasolar giant planet atmosphere models, focusing in particular on Gl 229B, and derive the systematics of the changes in the dominant reservoirs of the major elements with altitude and temperature. We assume an Anders & Grevesse solar composition of 27 chemical elements and track 330 gas-phase species, including the monatomic forms of the elements, as well as about 120 condensates. In our fiducial, limiting reference model, we do not assume that condensation sequesters elements at depth and present true chemical equilibrium calculations for the solar element mix. However, there is evidence in the atmosphere of Gliese 229B that it is depleted in heavy elements. To explore depletion and rainout, which are fundamentally nonequilibrium processes, we perform a series of rainout calculations in which refractory elements are withdrawn from the equilibrium in stoichiometric ratios via an ad hoc algorithm. Then we compare the true equilibrium results with the rainout results, with an eye to understanding the near-infrared spectrum of Gliese 229B, as well as the new "L dwarfs" with T eff 's above ~1500 K. We address the issue of the formation and composition of clouds in the cool atmospheres of substellar objects. We conclude that the opacity of clouds of low-temperature (≤900 K), small-radius condensables (specific chlorides and sulfides, not silicates), may in part be responsible for the steep spectrum of Gl 229B observed in the near-IR below 1 μm. We assemble a temperature sequence of chemical transitions in substellar atmospheres that may be used to anchor and define a sequence of spectral types for substellar objects with T eff 's from ~2200 to ~100 K.