Grain growth and dust settling in a brown dwarf disk: Gemini/T-ReCS observations of CFHT-BD-Tau 4

TL;DR

This study presents mid-infrared observations of a brown dwarf disk, revealing evidence of dust processing and grain growth, and suggests an intermediate disk geometry with reduced flaring.

Contribution

First evidence of large amorphous olivine grains indicating dust processing and grain growth in brown dwarf disks, with a proposed intermediate flaring disk model.

Findings

Detection of broad silicate emission feature

Presence of large 2 micron amorphous olivine grains

Intermediate disk model fits observed spectral energy distribution

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 micron 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 micron 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.