2007A&A...470..903C

We investigate the mass function in the substellar domain down to a few Jupiter masses in the young σ Orionis open cluster (3±2Ma, d=360⁺⁷⁰_–60pc). We have performed a deep IJ-band search, covering an area of 790arcmin² close to the cluster centre. This survey was complemented with an infrared follow-up in the HK_s- and Spitzer 3.6-8.0µm-bands. Using colour-magnitude diagrams, we have selected 49 candidate cluster members in the magnitude interval 16.1mag < I < 23.0mag. Accounting for flux excesses at 8.0µm and previously known spectral features of youth, we identify 30 objects as bona fide cluster members. Four are first identified from our optical-near infrared data. Eleven have most probable masses below the deuterium burning limit which we therefore classify as candidate planetary-mass objects. The slope of the substellar mass spectrum (ΔN/ΔM≃aM^–α) in the mass interval 0.11M_☉<M<0.006M_☉ is α=+0.6±0.2. Any mass limit to formation via opacity-limited fragmentation must lie below 0.006M_☉. The frequency of σ Orionis brown dwarfs with circumsubstellar discs is 47±9%. The continuity in the mass function and in the frequency of discs suggests that very low-mass stars and substellar objects, even below the deuterium-burning mass limit, share the same formation mechanism.