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 (~15 M_Jup)—at the bottom of the stellar initial mass function. We present an exploratory survey for disks in this 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 σ Orionis cluster, a total of 18 sources. In the mass range 8-20 M_Jup, we identify four objects with >3 σ color excess at a wavelength of 8.0 μm, interpreted as emission 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 29+/- ¹⁶₁₃% at 8.0 μm, very similar to what has been found for stars and brown dwarfs (~30%). The object S Ori 70, a candidate 3 M_Jup object in this cluster, shows IRAC colors in excess of the typical values for field T dwarfs (on a 2 σ 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.