Grain Formation in Atmospheres of Cool Dwarfs

We have constructed a grid of model atmospheres for cool dwarf stars and brown dwarfs with T_eff ≤ 3000 K that includes (i) an equation of state which accounts for over 600 gas phase species and 1000 liquids and solids, and (ii) the opacities of corundum (Al₂O₃), iron, enstatite (MgSiO₃) and forsterite (Mg₂SiO₄), as well as amorphous carbon and SiC. We confirm earlier findings of Tsuji, Ohnaka & Aoki (1996a) that grains are abundant in the outer photospheric layers of red and brown dwarfs with spectral type later than M8. We identify high temperature condensates including perovskite (CaTiO₃) that depletes the photospheres of important absorbers including TiO, and we confirm the disappearance of TiO bands in the observed spectra of cool dwarfs. This revised version was published online in September 2006 with corrections to the Cover Date.