Dusty disks at the bottom of the IMF

Abstract

'Isolated planetary mass objects' (IPMOs) have masses close to or below the Deuterium-burning mass limit (~15 Jupiter masses) -- 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 Sigma Orionis cluster, a total of 18 sources. In the mass range 8... 20 MJup, we identify 4 objects with >3sigma colour excess at a wavelength of 8.0mu, interpreted as emission from dusty disks. We thus establish that a substantial fraction of IPMOs harbour 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% (16-45%) at 8.0mu, very similar to what has been found for stars and brown dwarfs (~30%). The object SOri70, a candidate 3 MJup object in this cluster, shows IRAC colours in excess of the typical values for field T dwarfs (on a 2sigma level), possibly due to disk emission or low gravity. This is a new indication for youth and thus an extremely low mass for SOri70.