Warm dust in the cool brown dwarf Gliese 229B and spectroscopic diagnosis of dusty photospheres

Dust should be formed in the photosphere of cool brown dwarfs, but it is no t 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 temperat ure there, dust grains are in detailed balance with the gas and the homogen eous dust-gas mixture works as an efficient source of opacity. The warm dus t layer, which can be optically thick in the optical region, effectively bl ocks the optical radiation and emits like a blackbody of rather high temper ature. The dust layer, however, can be optically thin in the infrared and h as 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 gaseou s atmosphere. Dust no longer works as an opacity source, and volatile molec ules such as CH4 and H2O take the role. Also, nonrefractory elements such a s alkali metals remain in gaseous form, and the neutral atoms contribute si gnificantly to block the optical radiation (e.g., K I doublet). We show tha t such a hybrid model can be constructed without any ad hoc assumption exce pt that the transition from the dust-gas detailed balance regime to the dus t-gas segregation phase takes place at a certain transition temperature T-t r. 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-dominat ed cases and offers a natural explanation of the observed spectrum of Glies e 229B through the optical to the infrared.