NEW ISOLATED PLANETARY-MASS OBJECTS AND THE STELLAR AND SUBSTELLAR MASS FUNCTION OF THE σ ORIONIS CLUSTER

We report on our analysis of the VISTA Orion ZY JHK(s) photometric data (completeness magnitudes of Z = 22.6 and J = 21.0 mag) focusing on a circular area of 2798.4 arcmin(2) around the young sigma Orionis star cluster (similar to 3 Myr, similar to 352 pc, and solar metallicity). The combination of the VISTA photometry with optical, WISE and Spitzer data allows us to identify a total of 210 sigma Orionis member candidates with masses in the interval 0.25-0.004M(circle dot), 23 of which are new planetary-mass object findings. These discoveries double the number of cluster planetary-mass candidates known so far. One object has colors compatible with a T spectral type. The sigma Orionis cluster harbors about as many brown dwarfs (69, 0.072-0.012 M-circle dot) and planetary-mass objects (37,0.012-0.004 M-circle dot) as very low mass stars (104, 0.25-0.072 M-circle dot). Based on Spitzer data, we derive a disk frequency of similar to 40% for very low mass stars, brown dwarfs, and planetary-mass objects in sigma Orionis. The radial density distributions of these three mass intervals are alike: all are spatially concentrated within an effective radius of 12' (1.2 pc) around the multiple star sigma Ori, and no obvious segregation between disk-bearing and diskless objects is observed. Using the VISTA data and the Mayrit catalog, we derive the cluster mass spectrum (Delta N/Delta M similar to M-alpha) from similar to 19 to 0.006 M-circle dot (VISTA ZJ completeness), which is reasonably described by two power-law expressions with indices of alpha = 1.7 +/- 0.2 for M > 0.35 M-circle dot, and alpha = 0.6 +/- 0.2 for M < 0.35 M-circle dot. The sigma Orionis mass spectrum smoothly extends into the planetary-mass regime down to 0.004 M-circle dot. Our findings of T-type sources (<0.004 M-circle dot) in the VISTA sigma Orionis exploration appear to be smaller than what is predicted by the extrapolation of the cluster mass spectrum down to the survey J-band completeness.