Letter to the Editor

Grain growth and dust settling in a brown dwarf disk^*

Gemini/T-ReCS observations of CFHT-BD-Tau 4

¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany e-mail: apai@as.arizona.edu
² European Southern Observatory, Casilla 19001, Santiago 19, Chile
³ Cerro Tololo Inter-American Observatory, Casilla 603, La Serena, Chile
⁴ Max Planck Institute for Astrophysics, PO Box 1317, 85741 Garching, Germany

Received: 30 June 2004
Accepted: 11 September 2004

Abstract

We present accurate mid-infrared observations of the disk around the young, bona fide brown dwarf CFHT-BD-Tau 4. We report GEMINI/T-ReCS measurements in the 7.9, 10.4 and 12.3 μm filters, from which we infer the presence of a prominent, broad silicate emission feature. The shape of the silicate feature is dominated by emission from 2 μm amorphous olivine grains. Such grains, being an order of magnitude larger than those in the interstellar medium, are a first proof of dust processing and grain growth in disks around brown dwarfs. The object's spectral energy distribution is below the prediction of the classical flared disk model but higher than that of the two-layer flat disk. A good match can be achieved by using an intermediate disk model with strongly reduced but non-zero flaring. Grain growth and dust settling processes provide a natural explanation for this disk geometry and we argue that such intermediate flaring might explain the observations of several other brown dwarf disks as well.