SCYON Abstract

Received on April 2 2002

The Formation Mechanism of Brown Dwarfs

Authors	Matthew R. Bate^1,2, Ian A. Bonnell³, Volker Bromm^2,4
Affiliation	¹ School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, United Kingdom, ² Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, United Kingdom, ³ School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife, KY16 9SS, United Kingdom, ⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, U.S.A.
Accepted by	Monthly Notices of the Royal Astronomical Society
Contact	mbate@astro.ex.ac.uk
URL	http://www.astro.ex.ac.uk/people/mbate
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Abstract

We present results from the first hydrodynamical star formation calculation to demonstrate that brown dwarfs are a natural and frequent product of the collapse and fragmentation of a turbulent molecular cloud. The brown dwarfs form via the fragmentation of dense molecular gas in unstable multiple systems and are ejected from the dense gas before they have been able to accrete to stellar masses. Thus, they can be viewed as `failed stars'. Approximately three quarters of the brown dwarfs form in gravitationally-unstable circumstellar discs while the remainder form in collapsing filaments of molecular gas. These formation mechanisms are very efficient, producing roughly the same number of brown dwarfs as stars, in agreement with recent observations. However, because close dynamical interactions are involved in their formation, we find a very low frequency of binary brown dwarf systems (< 5%) and that those binary brown dwarf systems that do exist must be close < 10 AU. Similarly, we find that young brown dwarfs with large circumstellar discs (radii > 10 AU) are rare (~5%).