L and T Dwarf Models and the L to T Transition

Using a model for refractory clouds, a novel algorithm for handling them, andthe latest gas-phase molecular opacities, we have produced a new series of Land T dwarf spectral and atmosphere models as a function of gravity andmetallicity, spanning the \teff range from 2200 K to 700 K. The correspondencewith observed spectra and infrared colors for early- and mid-L dwarfs and formid- to late-T dwarfs is good. We find that the width in infraredcolor-magnitude diagrams of both the T and L dwarf branches is naturallyexplained by reasonable variations in gravity and, therefore, that gravity isthe "second parameter" of the L/T dwarf sequence. We investigate the dependenceof theoretical dwarf spectra and color-magnitude diagrams upon various cloudproperties, such as particle size and cloud spatial distribution. In the regionof the L$\to$T transition, we find that no one cloud-particle-size and gravitycombination can be made to fit all the observed data. Furthermore, we note thatthe new, lower solar oxygen abundances of Allende-Prieto, Lambert, & Asplund(2002) produce better fits to brown dwarf data than do the older values.Finally, we discuss various issues in cloud physics and modeling and speculateon how a better correspondence between theory and observation in theproblematic L$\to$T transition region might be achieved.