Gravoturbulent Planetesimal Formation: The Positive Effect of long-lived   Zonal Flows
  Recent numerical simulations have shown long-lived axisymmetric sub- and super-Keplerian flows in protoplanetary disks. These zonal flows are found in local as well as global simulations of disks unstable to the magnetorotational instability. This paper covers our study of the strength and lifetime of zonal flows and the resulting long-lived gas over- and underdensities as functions of the azimuthal and radial size of the local shearing box. We further investigate dust particle concentrations without feedback on the gas and without self-gravity. Strength and lifetime of zonal flows increase with the radial extent of the simulation box, but decrease with the azimuthal box size. Our simulations support earlier results that zonal flows have a natural radial length scale of 5-7 gas pressure scale heights. This is the first study that combines three-dimensional MHD simulations of zonal flows and dust particles feeling the gas pressure. The pressure bumps trap particles with $\textrm{St} = 1$ very efficiently. We show that $\textrm{St} = 0.1$ particles (of some centimeters in size if at $5\textrm{AU}$ in an MMSN) reach a hundred-fold higher density than initially. This opens the path for particles of $\textrm{St} = 0.1$ and dust-to-gas ratio of 0.01 or for particles of $\textrm{St} \geq 0.5$ and dust-to-gas ratio $10^{-4}$ to still reach densities that potentially trigger the streaming instability and thus gravoturbulent formation of planetesimals. 
Karsten Dittrich, Hubert Klahr and Anders Johansen
astro-ph.EP