Abstract

Dust should be formed in the photosphere of cool brown dwarfs, but it is not clear in what form the dust exists. We propose a model in which a rather warm dust layer exists deep in the photosphere. At relatively high temperature there, dust grains are in detailed balance with the gas and the homogeneous dust-gas mixture works as an efficient source of opacity. The warm dust layer, which can be optically thick in the optical region, effectively blocks the optical radiation and emits like a blackbody of rather high temperature. The dust layer, however, can be optically thin in the infrared and has little effect on the infrared radiation. In the cool upper region, dust grows too large to be sustained with the gas and segregates from the gaseous atmosphere. Dust no longer works as an opacity source, and volatile molecules such as CH4 and H2O take the role. Also, nonrefractory elements such as alkali metals remain in gaseous form, and the neutral atoms contribute significantly to block the optical radiation (e.g., K I doublet). We show that such a hybrid model can be constructed without any ad hoc assumption except that the transition from the dust-gas detailed balance regime to the dust-gas segregation phase takes place at a certain transition temperature T-tr. By a sudden increase of opacity due to dust at T-tr, an outer convective zone appears, followed by intermediate radiative and inner deep convective zones. The emergent spectrum is also a hybrid of the dust- and gas-dominated cases and offers a natural explanation of the observed spectrum of Gliese 229B through the optical to the infrared.