Abstract
A large fraction of the observed brown dwarfs may form by gravitational fragmentation of unstable discs. This model reproduces the brown dwarf desert, and provides an explanation the existence of planetary-mass objects and for the binary properties of low-mass objects. We have performed an ensemble of radiative hydrodynamic simulations and determined the statistical properties of the low-mass objects produced by gravitational fragmentation of discs. We suggest that there is a population of brown dwarfs loosely bound on wide orbits (100-5000 AU) around Sun-like stars that surveys of brown dwarf companions should target. Our simulations also indicate that planetary-mass companions to Sun-like stars are unlikely to form by disc fragmentation.
Highlights
The formation of brown dwarfs (BDs) is not well understood ([35])
A large fraction of the observed brown dwarfs may form by gravitational fragmentation of unstable discs
We suggest that there is a population of brown dwarfs loosely bound on wide orbits (100–5000 AU) around Sun-like stars that surveys of brown dwarf companions should target
Summary
The formation of brown dwarfs (BDs) is not well understood ([35]). Low-mass objects are difficult to form by gravitational fragmentation of unstable gas, as for masses in the BD regime ( 80 MJ, where MJ is the mass of Jupiter) a high density is required for the gas to be Jeans unstable. [24] and [14] suggest that these high density cores can be formed by colliding flows in a turbulent magnetic medium This model requires a large amount of turbulence, and has difficulty explaining the binary properties of BDs. the large number of brown-dwarf mass cores that the theory predicts have not been observed. Turbulent fragmentation can produce cores which collapse to form discs, the discs may fragment to produce BDs, and the BDs may avoid accreting additional mass by being ejected We discuss the properties of the BDs produced with this mechanism and the predictions of the model that can be tested by observations
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