A survey for circumstellar disks around young substellar objects

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'-band (3.8 mum) 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 similar to15-100 M-Jup and ages of less than or similar to5 Myr, based on current models. None appear to be binaries at 0."4 resolution (55-120 AU). Using the objects measured spectral types and extinctions, we find that most of our sample (77% +/- 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 Halpha 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 similar to3 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 greater than or similar to2 R-*, 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.