2010MNRAS.408.1095S

FUTau belongs to a rare class of young, wide brown dwarf binaries. We have resolved the system in a Chandra X-ray observation and detected only the primary, FUTauA. Hard X-ray emission, presumably from a corona, is present but, unexpectedly, we also detect a strong and unusually soft component from FUTauA. Its X-ray properties, so far unique among brown dwarfs, are very similar to those of the TTauri star TWHya. The analogy with TWHya suggests that the dominating soft X-ray component can be explained by emission from accretion shocks. However, the typical free-fall velocities of a brown dwarf are too low for an interpretation of the observed X-ray temperature as a post-shock region. On the other hand, velocities in excess of the free-fall speed are derived from archival optical spectroscopy, and independent pieces of evidence for strong accretion in FUTauA are found in optical photometry. The high X-ray luminosity of FUTauA coincides with a high bolometric luminosity confirming an unexplained trend among young brown dwarfs. In fact, FUTauA is overluminous with respect to evolutionary models while FUTauB is on the 1-Myr isochrone suggesting non-contemporaneous formation of the two components in the binary. The extreme youth of FUTauA could be responsible for its peculiar X-ray properties, in terms of atypical magnetic activity or accretion. Alternatively, rotation and magnetic field effects may reduce the efficiency of convection which in turn affects the effective temperature and radius of FUTauA, shifting its position in the Hertzsprung-Russell diagram. Although there is no direct proof of this latter scenario so far, we present arguments for its plausibility.