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.

ABSTRACT
1. INTRODUCTION
2. NEW ATMOSPHERIC MODEL, "HEATING MODEL"
3. RESULTS AND DISCUSSIONS
REFERENCES

• Issei Yamamura(Department of Space Astronomy and Astrophysics, Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA))
• Takeru K. Suzuki(Department of Physics, Nagoya University)
• Satoko Sorahana(Department of Astronomy, The University of Tokyo/Department of Physics, Nagoya University)