The Substellar Mass Function In Σ Orionis Ii. Optical, Near-Infrared And Irac/Spitzer Photometry Of Young Cluster Brown Dwarfs And Planetary-Mass Objects

Abstract

Aims. We investigate the mass function in the substellar domain down to a few Jupiter masses in the young σ Orionis open cluster (3 ± 2 Ma, d = 360-60+70 pc). Methods. We have performed a deep IJ-band search, covering an area of 790 arcmin2 close to the cluster centre. This survey was complemented with an infrared follow-up in the HK s- and Spitzer 3.6-8.0 μm-bands. Using colour-magnitude diagrams, we have selected 49 candidate cluster members in the magnitude interval 16.1 mag \u3c I \u3c 23.0 mag. Results. Accounting for flux excesses at 8.0 μm 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 0.11 M⊙ \u3c M \u3c 0.006 M⊙ is α = +0.6 ± 0.2. Any mass limit to formation via opacity-limited fragmentation must lie below 0.006 M⊙. The frequency of σ Orionis brown dwarfs with circumsubstellar discs is 47±9 %. Conclusions. 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. © ESO 2007