A signature of chromospheric activity in brown dwarfs revealed by 2.5-5.0μm AKARI spectra

Abstract

We propose that the 2.7-μm H2O, 3.3-μm CH4 and 4.6-μm CO absorption bands can be good tracers of chromospheric activity in brown dwarfs. In our previous study, we found that there are difficulties in explaining entire spectra between 1.0 and 5.0μm with the unified cloudy model (UCM), a brown dwarf atmosphere model. Based on simple radiative equilibrium, temperature in a model atmosphere usually decreases monotonically with height. However, if a brown dwarf has a chromosphere, as inferred by some observations, the temperature in the upper atmosphere is higher. We construct a simple model that takes into account heating owing to chromospheric activity by setting a temperature floor in the upper atmosphere, and find that the model spectra of three brown dwarfs with moderate Hα emission, an indicator of chromospheric activity, are considerably improved to match the AKARI spectra. Because of the higher temperatures in the upper atmosphere, the number of CH4 molecules is reduced, and the absorption band strengths become weaker. The strengths of the absorption bands of H2O and CO also become weaker. On the other hand, other objects with weak Hα emission cannot be fitted with our treatment. We also briefly discuss magnetic heating processes that possibly operate in the upper atmosphere, by extending our numerical simulations for the Sun and stars with surface convection to brown dwarf atmospheres. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.