AKARI Observations of Brown Dwarfs. IV. Effect of Elemental Abundances to Near-Infrared Spectra between 1.0 and 5.0 {\mu}m

TL;DR

This study investigates how variations in elemental abundances, particularly carbon and oxygen, influence the near-infrared spectra of brown dwarfs, especially the CO_2 absorption band at 4.2 μm, using models and AKARI observations.

Contribution

The paper introduces models with varied elemental abundances to explain spectral deviations, confirming the impact of C and O variations on brown dwarf spectra.

Findings

CO_2 band strength is highly sensitive to O abundance.

Both C and O abundances vary together in brown dwarf atmospheres.

Weaker CO_2 bands can result from lower C and O abundances.

Abstract

The detection of the CO_2 absorption band at 4.2 {\mu}m in brown dwarf spectra by AKARI has made it possible to discuss CO_2 molecular abundance in brown dwarf atmospheres. In our previous studies, we found an excess in the 4.2 {\mu}m CO_2 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 construct a set of models of brown dwarf atmospheres with various elemental abundance patterns, and investigate the variations of the molecular composition, thermal structure and their effects to the near-infrared spectra between 1.0 and 5.0 {\mu}m. The 4.2 {\mu}m CO_2 absorption band in some late-L and T dwarfs taken by AKARI are stronger or weaker than predictions by corresponding models with solar abundance. By comparing CO_2 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 C and O abundances should increase and decrease simultaneously for brown dwarfs. We find that a weaker CO_2 absorption band in a spectrum can also be explained by a model with lower C and O abundances.