Letter to the Editor

Detectability of dirty dust grains in brown dwarf atmospheres

¹ Research and Scientific Support Department, ESTEC/ESA, PO Box 299, 2200 AG Noordwijk, The Netherlands e-mail: chelling@esa.int
² Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
³ Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands

Received: 1 February 2006
Accepted: 11 March 2006

Abstract

Context.Dust clouds influence the atmospheric structure of brown dwarfs, and they affect the heat transfer and change the gas-phase chemistry. However, the physics of their formation and evolution is not well understood. The dust composition can be predicted from thermodynamical equilibrium or time-dependent chemistry that takes into account seed particle formation, grain growth, evaporation, and drift.

Aims.In this Letter, we predict dust signatures and propose a potential observational test of the physics of dust formation in brown dwarf atmospheres based on the spectral features of the different solid components predicted by dust formation theory.

Methods.A momentum method for the formation of dirty dust grains (nucleation, growth, evaporation, and drift) is applied to a static brown dwarf atmosphere structure to compute the dust grain properties, and in particular, the heterogeneous grain composition and the grain size. The effective medium and Mie theories are used to compute the extinction of these spherical grains.

Results.Dust formation results in grains whose composition differs from that of grains formed at equilibrium. Our kinetic model predicts that amorphous SiO₂ (silica) is one of the most abundant solid components, followed by amorphous Mg₂SiO₄ and MgSiO₃[s], while SiO₂ is absent in equilibrium models because it is a metastable solid. Solid amorphous SiO₂ possesses a strong broad absorption feature centered at 8.7 μm, while amorphous Mg₂SiO₄/MgSiO₃[s] absorbs at 9.7 μm in addition to other absorption features at longer wavelengths. Those features at m are detectable in absorption if the grains are small (radius <0.2 μm) in the upper atmosphere, as proposed by our model.

Conclusions.We suggest that the detection of a feature at 8.7 μm in deep infrared spectra could provide evidence for non-equilibrium dust formation that yields grains composed of metastable solids in brown dwarf atmospheres. This feature will shift towards m and broaden if silicates (e.g. fosterite) are much more abundant.