The Substellar Mass Function in σ Orionis

We combine results from imaging searches for substellar objects in the σ Orionis cluster and follow-up photometric and spectroscopic observations to derive a census of the brown dwarf population in a region of 847 arcmin 2 . We identify 64 very low mass cluster member candidates in this region. We have available three-color ( I , Z , and J ) photometry for all of them, spectra for 24 objects, and K photometry for 27% of our sample. These data provide a well-defined sequence in the I versus I - J and I versus I - K color-magnitude diagrams and indicate that the cluster exhibits little reddening despite its young age (~5 Myr). Using state-of-the-art evolutionary models, we derive a mass function from the low-mass stars (0.2 M ☉ ) across the complete brown dwarf domain (0.075 to 0.013 M ☉ ) and into the realm of free-floating planetary-mass objects (≤0.013 M ☉ ). We find that the mass spectrum ( dN / dm ) ∝ m -α increases toward lower masses, with an exponent α = 0.8 ± 0.4. Our results suggest that planetary-mass isolated objects could be as common as brown dwarfs; both kinds of objects together would be as numerous as stars in the cluster. If the distribution of stellar and substellar masses in σ Orionis is representative of the Galactic disk, older and much lower luminosity free-floating planetary-mass objects with masses down to about 0.005 M ☉ should be abundant in the solar vicinity, with a density similar to M-type stars.