---
author:
- 'Anders Johansen, Hubert Klahr'
- Thomas Henning
title: |
    High-resolution simulations of planetesimal formation\
    in turbulent protoplanetary discs
---

Introduction
============

The formation of km-scale planetesimals from dust particles involves a complex interplay of physical processes, including most importantly collisional sticking [@Weidenschilling1984; @Weidenschilling1997; @DullemondDominik2005], the self-gravity of the particle mid-plane layer [@Safronov1969; @GoldreichWard1973; @Sekiya1998; @YoudinShu2002; @SchraeplerHenning2004; @Johansen+etal2007], and the motion and structure of the turbulent protoplanetary disc gas [@WeidenschillingCuzzi1993; @Johansen+etal2006; @Cuzzi+etal2008].

In the initial growth stages micrometer-sized silicate monomers readily stick to form larger dust aggregates [@Poppe+etal2000; @BlumWurm2008]. Further growth towards macroscopic sizes is hampered by collisional fragmentation and bouncing [@Zsom+etal2010], limiting the maximum particle size to a few cm or less [depending on the assumed velocity threshold for collisional fragmentation, see @Brauer+etal2008a; @Birnstiel+etal2009]. High-speed collisions between small impactors and a large target constitutes a path to net growth [@Wurm+etal2005], but the transport of small particles away from the mid-plane by turbulent diffusion limits the resulting growth rate dramatically [@Johansen+etal2008]. Material properties are also important. [@Wada+etal2009] demonstrated efficient sticking between ice aggregates consisting of $0.1$ $\mu$m monomers at speeds up to 50 m/s.

Turbulence can play a positive role for growth by concentrating mm-sized particles in convection 