Silicate Evolution in Brown Dwarf Disks

We present a compositional analysis of the 10 micron silicate spectra for brown dwarf disks in the Taurus and Upper Scorpius (UppSco) star-forming regions, using archival Spitzer/IRS observations. A variety in the silicate features is observed, ranging from a narrow profile with a peak at 9.8 micron, to nearly flat, low-contrast features. For most objects, we find nearly equal fractions for the large-grain and crystalline mass fractions, indicating both processes to be active in these disks. The median crystalline mass fraction for the Taurus brown dwarfs is found to be 20%, a factor of ~2 higher than the median reported for the higher mass stars in Taurus. The large-grain mass fractions are found to increase with an increasing strength in the X-ray emission, while the opposite trend is observed for the crystalline mass fractions. A small 5% of the Taurus brown dwarfs are still found to be dominated by pristine ISM-like dust, with an amorphous sub-micron grain mass fraction of ~87%. For 15% of the objects, we find a negligible large-grain mass fraction, but a >60% small amorphous silicate fraction. These may be the cases where substantial grain growth and dust sedimentation has occurred in the disks, resulting in a high fraction of amorphous sub-micron grains in the disk surface. Among the UppSco brown dwarfs, only usd161939 has a S/N high enough to properly model its silicate spectrum. We find a 74% small amorphous grain and a ~26% crystalline mass fraction for this object.