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

Abstract

The formation of brown dwarfs (BDs) due to the fragmentation of proto-stellar disks undergoing pairwise encounters was investigated. High resolution allowed the use of realistic initial disk models where both the vertical structure and the local Jeans mass were resolved. The results show that objects with masses ranging from giant planets to low mass stars can form during such encounters from initially stable disks. The parameter space of initial spin-orbit orientations and the azimuthal angles for each disk was explored. 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. Retrograde disks were generally more likely to fragment. When 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 Jupiter masses, with the majority in the BD regime. They 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. The objects had radii ranging from 0.1 to 10 AU. 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 is expected to lengthen the internal evolution timescale for these objects, but not to alter our basic results.