Optical and Near-infrared Spectra of σ Orionis Isolated Planetary-mass Objects

We have obtained low-resolution optical (0.7-0.98 mu m) and near-infrared (1.11-1.34 mu m and 0.8-2.5 mu m) spectra of 12 isolated planetary-mass candidates (J. =. 18.2-19.9 mag) of the 3 Myr sigma Orionis. star cluster with the aim of determining the spectroscopic properties of very young, substellar dwarfs and assembling a complete cluster mass function. We have classified our targets by visual comparison with high- and low-gravity standards and by measuring newly defined spectroscopic indices. We derived L0-L4.5 and M9-L2.5 using high-and low-gravity standards, respectively. Our targets reveal clear signposts of youth, thus corroborating their cluster membership and planetary masses (6-13 M-Jup). These observations complete the sigma Orionis. mass function by spectroscopically confirming the planetary-mass domain to a confidence level of similar to 75%. The comparison of our spectra with BTSettl solar metallicity model atmospheres yields a temperature scale of 2350-1800 K and a low surface gravity of log g approximate to 4.0 [cm s(-2)], as would be expected for young planetary-mass objects. We discuss the properties of the cluster's least-massive population as a function of spectral type. We have also obtained the first optical spectrum of S Ori 70, a T dwarf in the direction of sigma Orionis. Our data provide reference optical and near-infrared spectra of very young L dwarfs and a mass function that may be used as templates for future studies of low-mass substellar objects and exoplanets. The extrapolation of the sigma Orionis. mass function to the solar neighborhood may indicate that isolated planetary-mass objects with temperatures of similar to 200-300 K and masses in the interval 6-13M(Jup) may be as numerous as very low-mass stars.