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We investigate variations in the strengths of three molecular bands, CH4 at3.3 um, CO at 4.6 um and CO2 at 4.2 um, in 16 brown dwarf spectra obtained byAKARI. Spectral features are examined along the sequence of source classes fromL1 to T8. We find that the CH4 3.3 um band is present in the spectra of browndwarfs later than L5, and the CO 4.6 um band appears in all spectral types. TheCO2 absorption band at 4.2 um is detected in late-L and T type dwarfs. Tobetter understand brown dwarf atmospheres, we analyze the observed spectrausing the Unified Cloudy Model (UCM). The physical parameters of the AKARIsample, i.e., atmospheric effective temperature Teff, surface gravity logg andcritical temperature Tcr are derived. We also model IRTF/SpeX and UKIRT/CGS4spectra in addition to the AKARI data in order to derive the most probablephysical parameters. Correlations between the spectral type and the modeledparameters are examined. We confirm that the spectral type sequence of late-Ldwarfs is not related to Teff, but instead originates as a result of the effectof dust.

AKARI Observations of Brown Dwarfs. III. CO, CO2 and CH4 Fundamental Bands and Physical Parameters