Brown dwarf formation by gravitational fragmentation of massive, extended protostellar discs

Abstract

Abstract We suggest that low-mass hydrogen-burning stars like the Sun should sometimes form with massive extended discs, and we show, by means of radiation hydrodynamic simulations, that the outer parts of such discs (R≳ 100 au) are likely to fragment on a dynamical time-scale (103 to 104 yr), forming low-mass companions: principally brown dwarfs (BDs), but also very low-mass hydrogen-burning stars and planetary-mass objects. A few of the BDs formed in this way remain attached to the primary star, orbiting at large radii. The majority are released into the field by interactions amongst themselves; in so doing they acquire only a low velocity dispersion (≲2 km s−1), and therefore they usually retain small discs, capable of registering an infrared excess and sustaining accretion. Some BDs form close BD/BD binaries, and these binaries can survive ejection into the field. This BD formation mechanism appears to avoid some of the problems associated with the ‘embryo ejection’ scenario, and to answer some of the questions not yet answered by the ‘turbulent fragmentation’ scenario.