On the dynamics of planetesimals embedded in turbulent protoplanetary discs with dead zones

Accretion in protoplanetary discs is thought to be driven by magnetohydrodynamic (MHD) turbulence via the magnetorotational instability. 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 3D MHD simulations using the shearing box approximation, along with an equilibrium chemistry model that is used to calculate the ionization fraction of the disc gas as a function of time and position. Ionization is assumed to arise because of stellar X-rays, galactic cosmic rays and radioactive nuclei. In agreement with our previous study, 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 similar to 2.5 au over a 5 Myr disc lifetime. 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 similar to 0.25 au over the disc lifetime, 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.