We present the latest results from the Mission Program NIRLT (PI: I.Yamamura), the near-infrared spectroscopy of brown dwarfs using the AKARI/IRC grism mode with the spectral resolution of ~ 120. The near-infrared spectra in the wavelength range between 2.5 and 5.0 μm are especially important to study the brown dwarf atmospheres because of the presence of major molecular bands, including CH4 at 3.3 μm, CO2 at 4.2 μm, CO at 4.6 μm, and H2O around 2.7 μm. We observed 27 sources, and obtained 16 good spectra. Our model fitting reveals deviations between theoretical model and observed spectra in this wavelength range, which may be attributed to the physical condition of the upper atmosphere. The deviations indicate additional heating, which we hypothesize to be due to chromospheric activity. We test this effect by modifying the brown dwarf atmosphere model to artificially increase the temperature of the upper atmosphere, and compare the revised model with observed spectra of early- to mid-L type objects with Hα emission. We find that the chemical structure of the atmosphere changes dramatically, and the heating model spectra of early-type brown dwarfs can be considerably improved to match the observed spectra. Our result suggests that chromospheric activity is essential to understand early-type brown dwarf atmospheres.

We present the latest results from the Mission Program NIRLT, the NIR spectroscopic observations of brown dwarfs using the IRC on board AKARI. The near-infrared spectra in the wavelength range between 2.5 and 5.0 μm is especially important to study the brown dwarf atmospheres because of the presence of non-blended bands of major molecules, including CH4 at 3.3 μm , CO2 at 4.2 μm , CO at 4.6 μm and H2O around 2.7 μm . Our observations were carried out in the grism-mode resulting in a spectral resolution of ~ 120. In total, 27 sources were observed and 18 good spectra were obtained. We investigate the behavior of three molecular absorption bands, CO, CH4 and CO2 , in brown dwarf spectra relative to their spectral types. We find that the CH4 band appears in the spectra of dwarfs later than L5 and CO band is seen in the spectra of all spectral types. CO2 is detected in the spectra of late-L and T type dwarfs.