Evolution of Brown Dwarf Disks: A Spitzer Survey in Upper Scorpius

We have carried out a Spitzer survey for brown dwarf disks in the ~5 Myr old Upper Scorpius (UpSco) star-forming region, using IRS spectroscopy from 8 to 12 μm and MIPS photometry at 24 μm. Our sample consists of 35 confirmed very low mass members of UpSco. Thirteen objects in this sample show clear excess flux at 24 μm, explained by dust emission from a circum-substellar disk. The spectral energy distributions (SEDs) of the remaining objects are consistent with pure photospheric emission. Objects without excess emission have either no disks or disks with inner opacity holes of at least ~5 AU radii. Our disk frequency of 37%+/-9% is higher than what has been derived previously for K0-M5 stars in the same region (1.8 σ confidence level), suggesting a mass-dependent disk lifetime in UpSco. The clear distinction between objects with and without disks, as well as the lack of transition objects, shows that disk dissipation inside 5 AU occurs rapidly, probably on timescales of <~10⁵ yr. For the objects with disks, most SEDs are uniformly flat, with flux levels of a few mJy, well modeled as emission from dusty disks affected by dust settling to the midplane. The silicate feature around 10 μm is either absent or weak in our SEDs, arguing for a lack of hot, small dust grains. Compared with younger objects in Taurus, brown dwarf disks in UpSco show less flaring. These results clearly demonstrate that we see disks in an advanced evolutionary state. Dust settling and grain growth are ubiquituous in circum-substellar disks at ages of 5 Myr, arguing for planet-forming processes in brown dwarf disks.