Since the discovery of brown dwarfs in 1994, and the discovery of dust cloud formation in the latest Very Low Mass Stars (VLMs) and Brown Dwarfs (BDs) in 1996, the most important challenge in modeling their atmospheres as become the understanding of cloud formation and advective mixing.
Many cloud models have been constructed to address this problem in brown dwarfs over the past decade, but none treated the mixing properties of the atmosphere, and the resulting diffusion mechanism realistically enough to reproduce the properties of the spectral transition from M through L and T spectral types without changing cloud parameters. One of the most important challenge in modeling the atmospheres and spectral properties of VLMs and brown dwarfs is the formation of dust clouds and its associated greenhouse effects making the infrared colors of late M and early L dwarfs extremely red compared to colors of low mass stars. The cloud composition, according to equilibrium chemistry, is going from zirconium oxide (ZrO2 ), to refractory ceramics (perovskite and corundum; CaTiO3 , Al2O3 ), to silicates (forsterite; Mg2SiO4 ), to salts (CsCl, RbCl, NaCl), and finally to ices (H2O, NH3 , NH4 SH).
An international team leaded by France Allard, co-authored by Homeier and Freytag has now proposed a 2D a new model atmosphere grid, named BT-Settl, computed using an updated version of the atmosphere code Phoenix and compared it to the AMES-Cond/Dusty models, on several parameters and found that the model is quite god on covering the whole range of VLMs and brown dwarfs and beyond: 1000,000 K < T eff < 400 K; -0.5 < logg <5.5; and +0.5 < [M/H] < -4.0, including various values of the alpha element enhancement. (read full article)