Clouds and chemistry: Ultracool dwarf atmospheric properties from optical and infrared colors

The optical and infrared colors of L and T dwarfs are sensitive to cloud sedimentation and chemical equilibrium processes in their atmospheres. The i'-z' versus J-K color-color diagram provides a window into diverse atmospheric processes mainly because different chemical processes govern each color, and cloud opacity largely affects J-K but not i'-z'. Using theoretical atmosphere models that include for the first time a self-consistent treatment of cloud formation, we present an interpretation of the i'-z' versus J-K color trends of known L and T dwarfs. We find that the i'-z' color is extremely sensitive to chemical equilibrium assumptions; chemical equilibrium models accounting for cloud sedimentation predict redder i'-z' colors by up to 2 mag-than models that neglect sedimentation. We explore the previously known J-K color trends in which objects first become redder, then bluer with decreasing effective temperature. Only models that include sedimentation of condensates are able to reproduce these trends. We find that the exact track of a cooling brown dwarf in J-K (and i'-z') is very sensitive to the details of clouds, in particular to the efficiency of sedimentation of condensates in its atmosphere. We also find that clouds still affect the strength of the J-, H-, and K-band fluxes of even the coolest T dwarfs. In addition, we predict the locus in the i'-z' versus J-K color-color diagram of brown dwarfs cooler than yet discovered.