AKARI OBSERVATIONS OF BROWN DWARFS. IV. EFFECT OF ELEMENTAL ABUNDANCES ON NEAR-INFRARED SPECTRA BETWEEN 1.0 AND 5.0 μm

摘要

The detection of the CO2 absorption band at 4.2 μm in brown dwarf spectra by AKARI has made it possible to discuss CO2 molecular abundance in brown dwarf atmospheres. In our previous studies, we found an excess in the 4.2 μm CO2 absorption band of three brown dwarf spectra, and suggested that these deviations were caused by high C and O elemental abundances in their atmospheres. To validate this hypothesis, we have constructed a set of models of brown dwarf atmospheres with various elemental abundance patterns, and we investigate the variations of the molecular composition and the thermal structure, and how they affect the near-infrared spectra between 1.0 and 5.0 μm. The 4.2 μm CO2 absorption band in some late-L and T dwarfs taken by AKARI is stronger or weaker than predicted by corresponding models with solar abundance. By comparing the CO2 band in the model spectra to the observed near-infrared spectra, we confirm possible elemental abundance variations among brown dwarfs. We find that the band strength is especially sensitive to O abundance, but C is also needed to reproduce the entire near-infrared spectra. This result indicates that both the C and O abundances should increase and decrease simultaneously for brown dwarfs. We find that a weaker CO2 absorption band in a spectrum can also be explained by a model with lower "C and O" abundances.