New Stars on the Block

Groups of young stars located in the solar neighborhood have recently received a lot of attention. These stars, which are $\lesssim$ 10 million years old, can provide insights into the birth of stars and planetary systems. For example, the TW Hydrae Association is just at the age when planets are believed to form, while MBM12 represents an earlier stage. In this Perspective, I briefly discuss recent research in this field.Comment: 4 pages, "Perspective" article in Science, vol. 288, p. 6

Dusty disks at the bottom of the IMF

'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.Comment: 12 pages, 2 figures, accepted for publication in ApJ