2010ApJ...722..682Y

Near-infrared medium-resolution spectra of seven bright brown dwarfs are presented. The spectra were obtained with the Infrared Camera on board the infrared astronomical satellite AKARI, covering 2.5-5.0 µm with a spectral resolution of approximately 120. The spectral types of the objects range from L5 to T8 and enable us to study the spectral evolution of brown dwarfs. The observed spectra are in general consistent with predictions from previous observations and photospheric models; spectra of L-type dwarfs are characterized by continuum opacity from dust clouds in the photosphere, while very strong molecular absorption bands dominate the spectra in T-type dwarfs. We find that the CO fundamental band around 4.6 µm is clearly seen even in the T8 dwarf 2MASS J041519-0935, confirming the presence of a non-equilibrium chemical state in the atmosphere. We also identify the CO₂fundamental stretching-mode band at 4.2 µm for the first time in the spectra of late-L- and T-type brown dwarfs. As a preliminary step towards interpretation of the data obtained by AKARI, we analyze the observed spectra by comparing with those predicted by the unified cloudy model (UCM). Although overall spectral energy distributions can be reasonably fitted with the UCM, observed CO and CO₂ bands in late-L and T dwarfs are unexpectedly stronger than the model predictions assuming local thermodynamical equilibrium. We examine the vertical mixing model and find that this model explains the CO band at least partly in the T dwarfs 2MASS J041519-0935 and 2MASS J055919-1404. The CO fundamental band also shows excess absorption against the predicted one in the L9 dwarf SDSS J083008+4828. Since CO is already highly abundant in the upper photospheres of late-L dwarfs, the extra CO due to vertical mixing has little effect on the CO band strengths, and the vertical mixing model cannot be applied to this L dwarf. A more serious problem is that the significant enhancement of the CO₂4.2 µm band in both the late-L and T dwarfs cannot be explained at all by the vertical mixing model. The cause of this enhancement of the CO₂ band remains to be explained.