We have carried out a Spitzer survey for brown dwarf disks in the \sim 5 Myr old Upper Scorpius ( UpSco ) star forming region , using IRS spectroscopy from 8 to 12 \mu m and MIPS photometry at 24 \mu m .
Our sample consists of 35 confirmed very low mass members of UpSco .
Thirteen objects in this sample show clear excess flux at 24 \mu m , explained by dust emission from a circum-sub-stellar disk .
The spectral energy distributions ( SEDs ) of the remaining objects are consistent with pure photospheric emission .
Objects without excess emission either have no disks at all or disks with inner opacity holes of at least \sim 5 AU radii .
Our disk frequency of 37 \pm 9 % is higher than what has been derived previously for K0-M5 stars in the same region ( on a 1.8 \sigma 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 \lesssim 10 ^ { 5 } years .
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 , which also provides indirect evidence for grain growth .
The silicate feature around 10 \mu m 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 .
By comparing SEDs of stars and brown dwarfs in UpSco , we find that dust settling is not a strong function of mass in this region .
Taken together , these results clearly demonstrate that we see disks in an advanced evolutionary state : Dust settling and grain growth are ubiquituous in circum-sub-stellar disks at ages of 5 Myr , arguing for planet forming processes in brown dwarf disks .
For almost all our targets , results from high-resolution spectroscopy and high-spatial resolution imaging have been published before , thus providing a large sample of brown dwarfs for which information about disks , accretion , and binarity is available .
We find that nine out of 13 objects with disks do not accrete significantly .
Hence , dusty disks can persist although the continuous accretion has stopped or dropped below measurable levels .
Four objects with disks are binaries , three of them with ( projected ) separations < 30 AU .
These objects likely harbour small disks truncated by the companion .