L and T dwarf models and the L to T transition

Using a model for refractory clouds, a novel algorithm for handling them, and the latest gas-phase molecular opacities, we have produced a new series of L and T dwarf spectral and atmosphere models as a function of gravity and metallicity, spanning the T-eff range from 2200 to 700 K. The correspondence with observed spectra and infrared colors for early and mid-L dwarfs and for mid-to late T dwarfs is good. We find that the width in infrared color-magnitude diagrams of both the T and L dwarf branches is naturally explained by reasonable variations in gravity and therefore that gravity is the second parameter'' of the L-T dwarf sequence. We investigate the dependence of theoretical dwarf spectra and color-magnitude diagrams on various cloud properties, such as particle size and cloud spatial distribution. In the region of the L -> T transition, we find that no single combination of cloud particle size and gravity can be made to fit all the observed data. Our results suggest that current ignorance of detailed cloud meteorology renders ambiguous the extraction of various physical quantities such as Teff and gravity for mid-L to early T dwarfs. Nevertheless, for decreasing Teff, we capture with some accuracy the major spectral features and signatures observed. We speculate that the subdwarf branch of the L dwarfs would be narrower in effective temperature and that for low enough metallicity the L dwarfs would disappear altogether as a spectroscopic class. Furthermore, we note that the new, lower solar oxygen abundances of Allende-Prieto and coworkers produce better fits to brown dwarf data than do the older values. Finally, we discuss various issues in cloud physics and modeling and speculate on how a better correspondence between theory and observation in the problematic L -> T transition region could be achieved.