A numerical study of brown dwarf formation via encounters of protostellar discs

The formation of brown dwarfs (BDs) due to the fragmentation of early-stage protostellar discs undergoing pairwise encounters was investigated. High resolution allowed the use of realistic, flared initial disc 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 discs that were stable before the encounter. The parameter space of initial spin–orbit orientations and the azimuthal angles for each disc were 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 disc gas into self-gravitating objects. In general, retrograde discs were more likely to fragment. It was also found that in fast encounters, where the interaction time-scale was significantly shorter than the discs' dynamical time-scales, the protostellar discs 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, disc-like shapes. With the inclusion of the appropriate physics, these could reasonably be expected to evolve into proto-BD discs 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 disc gas was assumed to be locally isothermal, appropriate for the short cooling times in extended protostellar discs 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 time-scale for these objects so that they spend considerable time as puffed-up, prolate ellipsoids, but not to alter the basic result that proto-BD discs can form during protostellar encounters.

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