  (abridged) Accretion in protoplanetary discs is thought to be driven by [...]
turbulence via the magnetorotational instability (MRI). Recent work has shown
that a planetesimal swarm embedded in a fully turbulent disc is subject to
strong excitation of the velocity dispersion, leading to collisional
destruction of bodies with radii R_p < 100 km. Significant diffusion of
planetesimal semimajor axes also arises, leading to large-scale spreading of
the planetesimal population throughout the inner regions of the protoplanetary
disc, in apparent contradiction of constraints provided by the distribution of
asteroids within the asteroid belt. In this paper, we examine the dynamics of
planetesimals embedded in vertically stratified turbulent discs, with and
without dead zones. Our main aims are to examine the turbulent excitation of
the velocity dispersion, and the radial diffusion, of planetesimals in these
discs. We employ three dimensional MHD simulations [...], along with an
equilibrium chemistry model [...] We find that planetesimals in fully turbulent
discs develop large random velocities that will lead to collisional
destruction/erosion for bodies with sizes below 100 km, and undergo radial
diffusion on a scale \sim 2.5 au over a 5 Myr disc life time. But planetesimals
in a dead zone experience a much reduced excitation of their random velocities,
and equilibrium velocity dispersions lie between the disruption thresholds for
weak and strong aggregates for sizes R_p < 100 km. We also find that radial
diffusion occurs over a much reduced length scale \sim 0.25 au over the disc
life time, this being consistent with solar system constraints. We conclude
that planetesimal growth via mutual collisions between smaller bodies cannot
occur in a fully turbulent disc. By contrast, a dead zone may provide a safe
haven in which km-sized planetesimals can avoid mutual destruction through
collisions.
