Discovery of a very cool object with extraordinarily strong H α emission ⋆

We report on the finding of the strongest Hα emission – pseudoequivalent width of 705 Å – known so far in a young, late type dwarf. This object, named as SOri 71, is a substellar candidate member of the 1–8Myr star cluster σOrionis. Due to its overluminous location in color-magnitude diagrams, SOri 71 might be younger than other cluster members, or a binary of similar components. Its mass is in the range 0.021–0.012M⊙, depending on evolutionary models and possible binarity. The broad Hα line of SOri 71 appears asymmetric, indicative of high velocity mass motions in the Hα forming region. The origin of this emission is unclear at the present time. We discuss three possible scenarios: accretion from a disk, mass exchange between the components of a binary system, and emission from a chromosphere.