  Context: The principal mechanism by which brown dwarfs form, and its relation
to the formation of higher-mass (i.e. hydrogen-burning) stars, is poorly
understood. Aims: We advocate a new model for the formation of brown dwarfs.
Methods: In this model, brown dwarfs are initially binary companions, formed by
gravitational fragmentation of the outer parts (R > 100 AU) of protostellar
discs around low-mass hydrogen-burning stars. Most of these binaries are then
gently disrupted by passing stars to create a largely single population of
brown dwarfs and low-mass hydrogen-burning stars. Results: This idea is
consistent with the excess of binaries found in low-density pre-main sequence
populations, like that in Taurus, where they should survive longer than in
denser clusters. Conclusions: If brown dwarfs form in this way, as companions
to more massive stars, the difficulty of forming very low-mass prestellar cores
is avoided. Since the disrupted binaries will tend to be those involving
low-mass components and wide orbits, and since disruption will be due to the
gentle tides of passing stars (rather than violent N-body interactions in
small-N sub-clusters), the liberated brown dwarfs will have velocity
dispersions and spatial distributions very similar to higher-mass stars, and
they will be able to retain discs, and thereby to sustain accretion and
outflows. Thus the problems associated with the ejection and turbulence
mechanisms can be avoided. This model implies that most, possibly all, stars
and brown dwarfs form in binary or multiple systems.
