Proto-Brown Dwarf Disks as Products of Protostellar Disk Encounters

The formation of brown dwarfs (BDs) via encounters between protostars has been confirmed with high-resolution numerical simulations with a restricted treatment of the thermal conditions. The new results indicate that young BDs formed this way are disklike and often reside in multiple systems. The newly formed proto-BD disks are up to 18 AU in size and spin rapidly, making small-scale bipolar outflows, fragmentation, and the possible formation of planetary companions likely, as have recently been observed for BDs. The object masses range from 2 to 73 Jupiter masses, distributed in a manner consistent with the observed substellar initial mass function. The simulations usually form multiple BDs on eccentric orbits about a star. One such system was hierarchical, a BD binary in orbit around a star, which may explain recently observed hierarchical systems. One-third of the BDs were unbound after a few thousand years, and interactions among orbiting BDs may eject more or add to the number of binaries. Improvements over prior work include resolution down to a Jupiter mass, self-consistent models of the vertical structure of the initial disks, and careful attention to avoid artificial fragmentation.