Grain Formation in Atmospheres of Cool Dwarfs

Abstract

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.