2004ApJ...607..511T

We report an attempt to interpret the spectra of L and T dwarfs with the use of the unified cloudy model (UCM). For this purpose, we extend the grid of the UCMs to cases of logg=4.5 and 5.5. The dust column density relative to the gas column density in the observable photosphere is larger at higher gravities, and molecular line intensity is generally smaller at higher gravities. The overall spectral energy distributions (SEDs) are f_J<f_H<f_Kin middle and late L dwarfs, f_J<f_H>f_Kin early T dwarfs (L/T transition objects), and finally f_J>f_H>f_Kin middle and late T dwarfs, where f_J,f_H, and f_Kare the peak fluxes at J, H, and K bands, respectively, in f_νunits. This tendency is the opposite of what is expected for the temperature effect, but it can be accounted for as the effect of thin dust clouds formed deep in the photosphere together with the effect of the gaseous opacities, including H₂(collision-induced absorption), H₂O, CH₄, and K I. Although the UCMs are semiempirical models based on a simple assumption that thin dust clouds form in the region of T_cr≲T≲T_cond(T_cr~1800 K is only an empirical parameter, while T_cond~2000 K is fixed by the thermodynamical data), the major observations, including the overall SEDs and the strengths of the major spectral features, are consistently accounted for throughout L and T dwarfs. In view of the formidable complexities of the cloud formation, we hope that our UCM can be of some use as a guide for future modeling of ultracool dwarfs and for interpretation of observed data of L and T dwarfs.