AKARI OBSERVATIONS OF BROWN DWARFS. II. CO2 AS PROBE OF CARBON AND OXYGEN ABUNDANCES IN BROWN DWARFS

作者：Tsuji Takashi^*; Yamamura Issei; Sorahana Satoko

来源：Astrophysical Journal, 2011, 734(2): 73.

DOI：10.1088/0004-637X/734/2/73

• brown dwarfs
• infrared: stars
• stars: abundances
• stars: atmospheres
• stars: individual (2MASS J04151954-0935066, 2MASS J05591914-1404488, 2MASS J15232263+3014562, SDSS J053951.99-005902.0, SDSS J083008.12+482847.4, SDSS J144600.60+002452.0)
• stars: low-mass

Recent observations with the infrared astronomical satellite AKARI have shown that the CO2 bands at 4.2 mu m 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 CO2 bands 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(C)A(O)(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 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.

• 出版日期2011-6-20