Keck NIRC observations of planetary-mass candidate members in the sigma Orionis open cluster

We present K-band photometry and low-resolution near-infrared spectroscopy from 1.44 to 2.45 mum of isolated planetary-mass candidate members in the s igma Orionis cluster found by Zapatero Osorio et al. The new data have been obtained with NIRC at the Keck I telescope. All of our targets, except for one, are confirmed as likely cluster members. Hence, we also confirm that the planetary-mass domain in the cluster is well populated. Using our deep K-band images, we searched for companions to the targets in the separation range of 0."3-10" up to a maximum faint limit of K = 19.5 mag. One suspecte d companion seems to be an extremely red galaxy. The near-infrared colors o f the sigma Orionis substellar members indicate that dust grains condense a nd settle in their atmospheres. We estimate that the surface temperatures r ange from 2500 down to 1500 K. The spectroscopic sequence covers the full r ange of L subclasses, and the faintest object is tentatively classified as T0. These targets provide a sequence of substellar objects of known age, di stance, and metallicity, which can be used as a benchmark for understanding the spectral properties of ultracool dwarfs.