Protostars and Planets VI, Heidelberg, July 15-20, 2013
ELEMENTAL ABUNDANCE EFFECTS ON THE CO2 ABSORPTION BAND AT 4.2 µM IN SPECTRA OF BROWN DWARFS
Sorahana, Satoko (Nagoya University)
Yamamura, Issei (ISAS/JAXA)
We present possible elemental abundance variations among brown dwarfs based on the CO2 absorption band at 4.2 µm. We obtained a continuous brown dwarf spectral data set for a new wavelength range of 2.5 to 5.0 µm with AKARI. Such spectra are the most powerful tools for obtaining physical and chemical information on brown dwarf atmospheres because they sample various molecular bands: H2O at 2.7 µm, CH4 at 3.3 µm, CO2 at 4.2 µm, and CO at 4.6 µm. These are fundamental and non-blended bands so they are suitable for analysis with relatively low-resolution spectra. We observed the CO2 absorption band at 4.2 µm for the first time. This detection of this band has made it possible to discuss CO2 molecular abundances in brown dwarf atmospheres. In our study, the observed CO2 absorption band in some late-L and T dwarfs is stronger or weaker than predicted by atmosphere models with solar metallicity under LTE. These unusual CO2 abundances can not be explained by vertical mixing, which was suggested by previous studies. As our first trial for improving the Unified Cloudy Model (UCM) for brown dwarf atmosphere, we focus on the elemental abundances of brown dwarfs to account for the observed CO2 band strengths. We construct a set of models of brown dwarf atmospheres with various elemental abundances, and investigate the variations of the molecular composition and thermal structure and their effects on near-infrared spectra between 1.0 and 5.0 µm. We find that the CO2 band is better reproduced by the model with revised C & O abundances than the solar elemental abundance model, except for very late-T dwarfs. This indicates that the CO2 band strength is especially sensitive to the combination of C and O abundances. On the other hand, changing only the C or O abundance does not fit the observed spectra. These results indicate that both C and O abundances should increase and decrease simultaneously. Our analysis of elemental abundances can be applied to exoplanet atmospheres. Since metallicity is important to understand planet formation, we will gain insight into the origin of planets.
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