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Since the discovery of brown dwarfs in 1994, and the discovery of dust cloudformation in the latest Very Low Mass Stars (VLMs) and Brown Dwarfs (BDs) in1996, the most important challenge in modeling their atmospheres as become theunderstanding of cloud formation and advective mixing. For this purpose, wehave developed radiation hydrodynamic 2D model atmosphere simulations to studythe formation of forsterite dust in presence of advection, condensation, andsedimentation across the M-L-T VLMs to BDs sequence (Teff = 2800 K to 900 K,Freytag et al. 2010). We discovered the formation of gravity waves as a drivingmechanism for the formation of clouds in these atmospheres, and derived a rulefor the velocity field versus atmospheric depth and Teff , which is relativelyinsensitive to gravity. This rule has been used in the construction of the newmodel atmosphere grid, BT-Settl, to determine the microturbulence velocity, thediffusion coefficient, and the advective mixing of molecules as a function ofdepth. This new model grid of atmospheres and synthetic spectra hasbeencomputedfor100,000K>Teff >400K,5.5>logg>-0.5,and[M/H]=+0.5to -1.5, and thereference solar abundances of Asplund et al. (2009). We found that the newsolar abundances allow an improved (close to perfect) reproduction of thephoto- metric and spectroscopic VLMs properties, and, for the first time, asmooth transition between stellar and substellar regimes -- unlike thetransition between the NextGen models from Hauschildt et al. 1999a,b, and theAMES-Dusty models from Allard et al. 2001). In the BDs regime, the BT-Settlmodels propose an improved explanation for the M-L-T spectral transition. Inthis paper, we therefore present the new BT-Settl model atmosphere grid, whichexplains the entire transition from the stellar to planetary mass regimes.

Model Atmospheres From Very Low Mass Stars to Brown Dwarfs