Date/Time
Date(s) - 20/06/2007
3:30 pm - 4:30 pm

Title: Proto-Brown Dwarf Disks as Products of Protostellar Disk Encounters

Speaker: Ms. Sijing Shen

Institute: McMaster University – Physics & Astronomy

Location: ABB 102

Description:

We present a high-resolution numerical study demonstrating the formation of brown dwarfs (BDs) via gravitational instability induced by dynamical encounters between protostellar disks in young star clusters. The objects so formed have properties consistent with the observed sub-stellar initial mass function (Barrado y Navascues et al. 2004), directly detected BD companions (Chauvin et al. 2005), disks around young BD objects (Mohanty 2005) and the newly discovered hierarchical multiple brown dwarf systems (Simon et al. 2006). A vertically resolved, flared protostellar disk model was adopted and the Jeans mass was resolved following the density growth in the simulations throughout the study. We found bound clumps with BD masses can be naturally produced in such encounters via fragmentation of protostellar disks, tidal structures or shock layers. The simulated BD fragments have rapidly spinning disks ranging from 0.3-18 AU in size. Adopting a moderate alpha viscosity, most of these proto-BD disks have observatiionally relevant lifetimes. In order to accrete despite the rotation, small-scale bipolar outflows and the formation of planetary companions are expected. The object masses range from 2 to 73 Jupiter masses at the end of the simulations. Further growth of these objects is limited due to gas dispersion induced by the encounter and ongoing tidal interactions with the original protostellar disks. Though the encounter-induced mechanism can operate alongside turbulent fragmentation models, it can more easily explain the observed close correlation between BD populations and cluster densities.