2011ApJ...734...73T

Recent observations with the infrared astronomical satellite AKARI have shown that the CO₂ bands at 4.2 µm in three brown dwarfs are much stronger than expected from the unified cloudy model (UCM) based on recent solar C & O abundances. This result has been a puzzle, but we now find it is simply due to the effect of C & O abundances. We show that these strong CO₂bands can be explained with the UCMs based on the classical C & O abundances (log A_C and log A_O), which are about 0.2 dex larger compared to the recent values. Since three other brown dwarfs could be interpreted fairly well with the recent solar C & O abundances, we require at least two model sequences based on the different chemical compositions to interpret all the AKARI spectra. The reason is that the CO₂ band is especially sensitive to C & O abundances, since the CO₂ abundance depends approximately on A_CA ²_O–the cube of C & O abundances. For this reason, even low-resolution spectra of very cool dwarfs, especially of CO₂, cannot be understood unless a model with proper abundances is applied. For the same reason, CO₂ is an excellent indicator of C & O abundances, and we can now estimate C & O abundances of brown dwarfs as follows: Three of the six brown dwarfs observed with AKARI should have high C & O abundances similar to the classical solar values (e.g., log A_C= 8.60 and log A_O= 8.92), but the other three may have low C & O abundances similar to the recent solar values (e.g., log A_C= 8.39 and log A_O= 8.69). This result implies that three of the six brown dwarfs are highly metal-rich relative to the Sun, if the recent solar C & O abundances are correct.