We have completed the first systematic survey for disks around spectroscopically identified young brown dwarfs and very low mass stars .
For a sample of 38 very cool objects in IC 348 and Taurus , we have obtained { L ^ { \prime } } -band ( 3.8 µm ) imaging with sufficient sensitivity to detect objects with and without disks .
The sample should be free of selection biases for our purposes .
Our targets span spectral types from M6 to M9.5 , corresponding to masses of \sim 15 to 100 M _ { Jup } and ages of \lesssim 5 Myr based on current models .
None appear to be binaries at 0 \farcs 4 resolution ( 55–120 AU ) .
Using the objects ’ measured spectral types and extinctions , we find that most of our sample ( 77 \% \pm 15 \% ) possess intrinsic IR excesses , indicative of circum ( sub ) stellar disks .
Because the excesses are modest , conventional analyses using only IR colors would have missed most of the sources with excesses .
Such analyses inevitably underestimate the disk fraction and will be less reliable for young brown dwarfs than for T Tauri stars .
The observed IR excesses are correlated with H \alpha emission , consistent with a common accretion disk origin .
In the same star-forming regions , we find that disks around brown dwarfs and T Tauri stars are contemporaneous ; assuming coevality , this demonstrates that the inner regions of substellar disks are at least as long-lived as stellar disks and evolve slowly for the first \sim 3 Myr .
The disk frequency appears to be independent of mass .
However , some objects in our sample , including the very coolest ( lowest mass ) ones , lack IR excesses and may be diskless .
The observed excesses can be explained by disk reprocessing of starlight alone ; the implied accretion rates are at least an order of magnitude below typical values for classical T Tauri stars .
The observed distribution of IR excesses suggests inner disk holes with radii of \gtrsim 2 R _ { \star } , consistent with the idea that such holes arise from disk-magnetosphere interactions .
Altogether , the frequency and properties of young circumstellar disks appear to be similar from the stellar regime down to the substellar and planetary-mass regime .
This provides prima facie evidence of a common origin for most stars and brown dwarfs .