The substellar mass function in σ Orionis

We combine results from imaging searches for substellar objects in the sigma 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 (similar to5 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 (less than or equal to 0.013 M.). We find that the mass spectrum (dN/dm) proportional to m(-alpha) increases toward lower masses, with an exponent alpha = 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 sigma 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.