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Akari Observations of Brown Dwarfs. II CO2 as Probe of Carbon and Oxygen Abundances in Brown Dwarfs

Recent observations with the infrared astronomical satellite AKARI have shown that the CO2 bands at 4.2 micron 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 that this is simply an abundance effect: We show that these strong CO2 bands can be explained with the UCMs based on the classical C & O abundances (log Ac and log Ao), which are about 0.2 dex larger compared to the recent values. Since three other brown dwarfs could be well interpreted 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 for this is that the CO2 band is especially sensitive to C & O abundances, since the CO2 abundance depends approximately on AcAo^2 --- 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: Three out of six brown dwarfs observed with AKARI should have high C & O abundances similar to the classical solar values (e.g. logAc = 8.60 and logAo = 8.92), but the other three may have low C & O abundances similar to the recent solar values (e.g. logAc = 8.39 and logAo = 8.69). This result implies that three out of six brown dwarfs are highly metal rich relative to the Sun if the recent solar C & O abundances are correct.