Wednesday 21 Nov 2012Stellar and substellar clouds - modelling the dynamical atmospheres of low-mass dwarfs

Dr Derek Homeier - ENS Lyon, France

Physics, 4th floor 14:00-15:00

The coolest dwarf stars, brown dwarfs and gas giant planets are all characterised by complex molecular chemistry, not always in chemical equilibrium, and formation of condensate clouds that affects and often dominates the spectral appearance of these objects. These processes are inherently dynamical and related to temporal and spatial variability in the atmospheres and thus hydrodynamical simulations are key to their understanding. Nonetheless 1D models taking the classical stellar atmosphere approach still play an important role when it comes to capturing the full complexity of chemistry and microphysics, and to doing detailed radiative transfer to synthesise spectra at high resolution.
I will show how the PHOENIX Settl models, by connecting to 2D and 3D radiative hydrodynamic simulations describe photospheric mixing, provide a consistent model of the cloud formation in late M dwarfs to cool brown dwarfs.
They form the first model atmosphere grid to reproduce the transition from L to T dwarfs in the brown dwarf colour-magnitude diagram without adjusting any free parameters, and also predict increased cloud opacity and clouds persistent to lower effective temperatures for low gravity objects. Their application to young planets thus provide an explanation for the, at first glance seemingly unusually dusty atmospheres seen in directly imaged planets.
I will further discuss the impact of additional photospheric mixing due to large scale motions and the need to connect to global circulation models to better characterise cloud physics in fast rotating bodies, irradiated planets etc.
Finally I will present an outlook on future improvements in modelling the coolest T dwarfs, Y dwarfs and exoplanets of corresponding temperature range, with upcoming extensions to the molecular and dust opacity data used in these simulations.

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