Kinematics of stars and brown dwarfs at birth

Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, UK e-mail: moraux@ast.cam.ac.uk

Received: 9 June 2004
Accepted: 15 September 2004

Abstract

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 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^-1 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.