Dust in Very Cool Dwarfs

Summary

Recent observations and preliminary model atmospheres of very low mass objects (VLMOs) including brown dwarfs and late M dwarfs show that these objects are mostly dusty. We first try to understand the physical reasons why dust can so easily be formed and further be sustained in the photospheres of VLMOs. By considering the thermodynamics of the grain particle formation in the photospheric environment, we found that there is a rather wide regime where dust formation can be treated within the framework of the local thermodynamical equilibrium (LTE). Actually, we consider three cases A, B, and C, which correspond to r_gr = 0, r_gr < r_cr, and r_gr > r_cr, respectively, where r_gr is the grain radius and r_cr the critical radius below which dust is unstable and can be treated by LTE. We discuss a series of model atmospheres for the cases A, B, and C, and also the cases of lower gravities representing the contracting low mass stars in young clusters and associations. Models well above the ZAMS are again dusty for T_eff < 2, 800K. Based on these models, we discuss observable properties such as spectra and colors of VLMOs, including the gravity effect. We re-discuss the two important touchstones of the model atmospheres of VLMOs, namely GD 165B and Gl 229B.