(abridged) Angular momentum transport and accretion in protoplanetary discs
are generally believed to be driven by MHD turbulence via the
magneto-rotational instability (MRI). The dynamics of solid bodies embedded in
such discs (dust grains, boulders, planetesimals and planets) may be strongly
affected by the turbulence, such that the formation pathways for planetary
systems are determined in part by the strength and spatial distribution of the
turbulent flow.
We examine the dynamics of planetesimals, with radii between 1m \^a 10 km,
embedded in turbulent protoplanetary discs, using three dimensional MHD
simulations. The planetesimals experience gas drag and stochastic gravitational
forces due to the turbulent disc. We use, and compare the results from, local
shearing box simulations and global models in this study.
The main aims of this work are to examine: the growth, and possible
saturation, of the velocity dispersion of embedded planetesimals as a function
of their size and disc parameters; the rate of radial migration and diffusion
of planetesimals; the conditions under which the results from shearing box and
global simulations agree.
We find good agreement between local and global simulations when shearing
boxes of dimension 4H x 16H x 2H are used (H being the local scale height). The
magnitude of the density fluctuations obtained is sensitive to the box size,
due to the excitation and propagation of spiral density waves. This affects the
stochastic forcing experienced by planetesimals. [...]
Our models show that fully developed MHD turbulence in protoplanetary discs
would have a destructive effect on embedded planetesimals. Relatively low
levels of turbulence are required for traditional models of planetesimal
accretion to operate, this being consistent with the existence of a dead zone
in protoplanetary discs.Comment: 23 pages, 28 figures, 3 tables, accepted for publication in MNRA