  Hydrodynamical simulations of star formation indicate that the motions of
protostars through their natal molecular clouds may be crucial in determining
the properties of stars through competitive accretion and dynamical
interactions. Walsh, Myers & Burton recently investigated whether such motions
might be observable in the earliest stages of star formation by measuring the
relative shifts of line-centre velocities of low- and high-density tracers of
low-mass star-forming cores. They found very small (~0.1 km/s) relative
motions. In this paper, we analyse the hydrodynamical simulation of Bate,
Bonnell & Bromm and find that it also gives small relative velocities between
high-density cores and low-density envelopes, despite the fact that competitive
accretion and dynamical interactions occur between protostars in the
simulation. Thus, the simulation is consistent with the observations in this
respect. However, we also find some differences between the simulation and the
observations. Overall, we find that the high-density gas has a higher velocity
dispersion than that observed by Walsh et al. We explore this by examining the
dependence of the gas velocity dispersion on density and its evolution with
time during the simulation. We find that early in the simulation the gas
velocity dispersion decreases monotonically with increasing density, while
later in the simulation, when the dense cores have formed multiple objects, the
velocity dispersion of the high-density gas increases. Thus, the simulation is
in best agreement with the observations early on, before many objects have
formed in each dense core.
