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

We investigate variations in the strengths of three molecular bands, CH4 at 3.3 um, CO at 4.6 um and CO2 at 4.2 um, in 16 brown dwarf spectra obtained by AKARI. Spectral features are examined along the sequence of source classes from L1 to T8. We find that the CH4 3.3 um band is present in the spectra of brown dwarfs later than L5, and the CO 4.6 um band appears in all spectral types. The CO2 absorption band at 4.2 um is detected in late-L and T type dwarfs. To better understand brown dwarf atmospheres, we analyze the observed spectra using the Unified Cloudy Model (UCM). The physical parameters of the AKARI sample, i.e., atmospheric effective temperature Teff, surface gravity logg and critical temperature Tcr are derived. We also model IRTF/SpeX and UKIRT/CGS4 spectra in addition to the AKARI data in order to derive the most probable physical parameters. Correlations between the spectral type and the modeled parameters are examined. We confirm that the spectral type sequence of late-L dwarfs is not related to Teff, but instead originates as a result of the effect of dust.