Grain formation in atmospheres of cool dwarfs

Abstract

We have constructed a grid of model atmospheres for cool dwarf stars and brown dwarfs with Teff ≤ 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.