We use numerical N-body simulations in order to test whether the
kinematics of stars and brown dwarfs at birth depend on mass. In
particular we examine how initial variations in velocity dispersion
can affect the spatial distribution of stellar and substellar objects
in clusters. We use 'toy' N-body models of a Pleiades-like cluster in
which brown dwarfs have their own velocity dispersion
sigma(VBD) which is k times larger than the stellar
one.
We find that in order to match the broad agreement between the brown
dwarf fraction in the field and in the Pleiades, the velocity
dispersion of brown dwarfs at birth has to be less than twice the
stellar velocity dispersion, i.e. cannot exceed a few km/s in the
Pleiades cluster. In order to discern more subtle differences between
the kinematics of brown dwarfs and stars at birth, our simulations
show that we need to look at clusters that are much less dynamically
evolved than the Pleiades. One might especially seek evidence of high
velocity brown dwarfs at birth by examining spatial distribution of
stars and brown darfs in clusters that are about a crossing timescale
old.