A Numerical Study of Brown Dwarf Formation via Encounters of Protostellar Disks

The formation of brown dwarfs (BDs) due to the fragmentation of early-stage proto-stellar disks undergoing pairwise encounters was investigated. High resolution allowed the use of realistic, flared initial disk models where both the vertical structure and the local Jeans mass were resolved, preventing numerical enhancement or suppression of fragmentation. The results show that objects with masses ranging from giant planets to low mass stars can form during such encounters from disks that were stable before the encounter. The parameter space of initial spin-orbit orientations and the azimuthal angles for each disk was explored with a series of 48 simulations. For the types of interactions studied, an upper limit on the initial Toomre Q value of ∼ 2 was found for fragmentation to occur. Depending on the initial configuration, shocks, tidal-tail structures and mass inflows were responsible for the condensation of disk gas into self-gravitating objects. In general, retrograde disks were more likely to fragment. It was also found that in fast encounters, where the interaction timescale was significantly shorter than the disks’ dynamical timescales, the proto-stellar disks tended to be truncated without forming objects. The newly-formed objects had masses ranging from 0.9 to 127 MJ , with the majority in the BD regime. These often resided in star-BD multiples and in some cases also formed hierarchical orbiting systems. Most of them had large angular momenta and highly flattened, disk-like shapes. With the inclusion of the appropriate physics these could reasonably be expected evolve into proto-BD disks with jets and subsequent planet formation around the BD. The objects had radii ranging from 0.1 to 10 AU at the time of formation. The disk gas was assumed to be locally isothermal, appropriate for the short cooling times in extended proto-stellar disks but not for condensed objects. An additional case with explicit cooling that reduced to zero for optically thick gas was simulated to test the extremes of cooling effectiveness and it was still possible to form objects in this case. Detailed radiative transfer (not studied here) is expected to lengthen the internal evolution timescale for these objects so that they spend considerable time as puffed-up, prolate ellipsoids, but not to alter the basic result that proto-BD disks can form during proto-stellar encounters.