Freiburg 2008 – wissenschaftliches Programm
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EP: Fachverband Extraterrestrische Physik
EP 9: Planets and Small Bodies I
EP 9.6: Vortrag
Mittwoch, 5. März 2008, 10:15–10:30, KGI-Aula
Extrasolar climate modelling – condensation and chemical processes in the atmospheres of gas giant planets and substellar objects — •Derek Homeier1 and France Allard2 — 1Institut für Astrophysik, Georg-August-Universität Göttingen, Germany — 2Centre de Recherche Astrophysique de Lyon, ENS Lyon, France
Condensate cloud formation and greenhouse heating by molecular absorption are primary factors in forming the thermal structure and spectral appearance of cool compact objects from the stellar/substellar boundary into the regime of gas giant planets. Condensation and chemical reactions in these cool atmospheres are often far removed from a thermodynamical equilibrium state. We present a model describing the vertical distribution of dust particles as an equilibrium between the timescales involved in the condensation, growth and sedimentation of grains, and the replenishment of gaseous material by convective mixing and overshoot. The velocity fields are described by an extension of classical mixing length theory based on the results of radiative hydrodynamic simulations. We find a strong dependence of mixing efficiency, and thus dust cloud formation, on surface gravity, allowing us to reproduce observed patterns of the spectral energy distribution and use these as a tracer of mass. Departures from equilibrium chemistry caused by finite reaction timescales, such as an enhancement of CO or N2 observed in the coolest known brown dwarfs of spectral type T, can be modelled by the same approach.