Activity at the deuterium-burning mass limit in Orion

We report very intense and variable H emission (pseudoequivalent widths of similar to180 and 410 Angstrom) of S Ori 55, a probable free-floating, M9-type substellar member of the young sigma Orionis open star cluster. After comparison with state-of-the-art evolutionary models, we infer that S Ori 55 is near or below the cluster deuterium-burning mass borderline, which separates brown dwarfs and planetary-mass objects. We find its mass to be 0.008-0.015 M-. for ages between 1 and 8 Myr, with similar to0.012 M-. the most likely value at the cluster age of 3 Myr. The largest Halpha intensity reached the saturation level of log (L-Halpha/L-bol) = -3. We discuss several possible scenarios for such a strong emission. We also show that sigma Ori M and L dwarfs have, in general, more H emission than their older field spectral counterparts. This could be due to a decline in the strength of the magnetic field with age in brown dwarfs and isolated planetary-mass objects or to a likely mass accretion from disks in the very young sigma Ori substellar members.