Photospheric Dust in Cool Dwarfs

We investigate the optical and infrared spectra for a range of cool dwarfs. For M dwarfs we find that the match between observation and theory is much improved when comparisons are made with models allowing for the formation of dust. We present two compelling pieces of evidence for dust (1)a reverse in TiO band strengths in the spectra of late-type M dwarfs at wavelengths shorter than 0.75 microns and (2) an excellent fit to the infrared spectrum of the brown dwarf candidate GD165B. We believe that the lack of accounting for dust significantly hampers our ability to make reliable predictions for properties of objects around the M dwarf - brown dwarf transition. At lower temperatures a dust-free model well explains the SED of the ~1000 K brown dwarf GI229B. Although chemical equilibrium means that dust should form in GI229B, the dust may be in clouds below the photosphere. Such a meteorological model is currently beyond the scope of the models presented here. We suggest a number of observational and theoretical tests to empirically constrain the properties of photospheric dust.