2008. The American Astronomical Society. All rights reserved. Printed in U.S.A. DUSTY DISKS AT THE BOTTOM OF THE INITIAL MASS FUNCTION

Isolated planetary-mass objects (IPMOs) have masses close to or below the deuterium-burning mass limit ( ∼)—at the bottom of the stellar initial mass function. We present an exploratory survey for disks in this15 MJup mass regime, based on a dedicated observing campaign with the Spitzer Space Telescope. Our targets include the full sample of spectroscopically confirmed IPMOs in the j Orionis cluster, a total of 18 sources. In the mass range, we identify four objects with 13 j color excess at a wavelength of 8.0 mm, interpreted as emission8–20 MJup from dusty disks. We thus establish that a substantial fraction of IPMOs harbor disks with lifetimes of at least 2– 4 Myr (the likely age of the cluster), indicating an origin from core collapse and fragmentation processes. The disk frequency in the IPMO sample is % at 8.0 mm, very similar to what has been found for stars and brown162913 dwarfs (∼30%). The object S Ori 70, a candidate object in this cluster, shows IRAC colors in excess of the3 MJup typical values for field T dwarfs (on a 2 j level), possibly due to disk emission or low gravity. This is a new indication for youth and thus an extremely low mass for S Ori 70. Subject headings: circumstellar matter — planetary systems — planetary systems: protoplanetary disks — stars: formation — stars: low-mass, brown dwarfs 1