Dust in the photospheric environment. II. Effect on the near-infrared spectra of L and T dwarfs

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 log g 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(K) in middle and late L dwarfs, f(J) < f(H) > f(K) in early T dwarfs (L/T transition objects), and finally f(J) > f(H) > f(K) in middle and late T dwarfs, where f(J); f(H), and f(K) are the peak fluxes at J, H, and K bands, respectively, in f(v) units. 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 H-2 (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 less than or similar to T less than or similar to T-cond (T-cr approximate to 1800 K is only an empirical parameter, while T-cond approximate to 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.