  (abridged) Sub-stellar mass objects have fundamentally different binary
properties than stars implying that they cannot have the same formation
history. They are rather unfinished stars having either been ejected by their
siblings from compact groups of accreting hydrostatic cores, or having lost
their accretion envelopes through photo-evaporation by nearby O stars or
through hyperbolic encounters with other accreting cores in their birth
clusters. All three scenarios are studied with a view to understanding the
observed smaller number of brown dwarfs (BDs) per star in the Taurus
star-forming region as opposed to the larger number of BDs per star seen in the
Orion Nebula cluster. It is found that both environments can produce the same
number of ejected BDs per star if the true production rate is about one BD per
four stars and the ejection velocity is about 2 km/s. This production rate is
much lower than the number of BDs per star inferred for the Galactic field.
Adding photo-evaporated and collisional BDs cannot reconcile this difference.
The only possible solution would be to postulate that tranquil star-formation
such as in Taurus is producing about one BD per star with ejection velocities
of about 3 km/s, while Orion-Nebula-type clusters produce at most about one BD
per three stars. Unless the Galactic-field census of BDs has been
overestimated, this then would imply that low-mass star-forming regions may be
the primary source of the field BDs, rather than rich clusters.
