The substellar population of the young cluster λ Orionis

By collecting optical and infrared photometry and low-resolution spectroscopy, we have identified a large number of low-mass stars and brown dwarf candidates belonging to the young cluster (similar to5 Myr) associated with the binary star lambda Orionis. The lowest mass object found is an M8.5 with an estimated mass of 0.02 M-circle dot (similar to0.01 M-circle dot for objects without spectroscopic confirmation). For those objects with spectroscopy, the measured strength of the Halpha emission line follows a distribution similar to other clusters with the same age range, with larger equivalent widths for cooler spectral types. Three of the brown dwarfs have Halpha emission equivalent widths of order 100 Angstrom, suggesting that they may have accretion disks and thus are the substellar equivalent of classical T Tauri stars. We have derived the initial mass function for the cluster. For the substellar regime, the index of the mass spectrum is alpha=0.60+/-0.06, very similar to other young associations.