The Substellar Mass Function in σ Orionis

We combine results from imaging searches for substellar objects in the sigmaOrionis cluster and follow-up photometric and spectroscopic observations toderive a census of the brown dwarf population in a region of 847 arcmin^2. Weidentify 64 very low-mass cluster member candidates in this region. We haveavailable three color (IZJ) photometry for all of them, spectra for 9 objects,and K photometry for 27% of our sample. These data provide a well definedsequence in the I vs I-J, I-K color magnitude diagrams, and indicate that thecluster is affected by little reddening despite its young age (~5 Myr). Usingstate-of-the-art evolutionary models, we derive a mass function from thelow-mass stars (0.2 Msol) across the complete brown dwarf domain (0.075 Msol to0.013 Msol), and into the realm of free-floating planetary-mass objects (<0.013Msol). We find that the mass spectrum (dN/dm ~ m^{-alpha}) increases towardlower masses with an exponent alpha = 0.8+/-0.4. Our results suggest thatplanetary-mass isolated objects could be as common as brown dwarfs; both kindsof objects together would be as numerous as stars in the cluster. If thedistribution of stellar and substellar masses in sigma Orionis isrepresentative of the Galactic disk, older and much lower luminosityfree-floating planetary-mass objects with masses down to about 0.005 Msolshould be abundant in the solar vicinity, with a density similar to M-typestars.Comment: Accepted for publication in ApJ. 19 pages, 3 figures include