ACCRETION AND DESTRUCTION OF PLANETESIMALS IN TURBULENT DISKS

We study the conditions for collisions between planetesimals to be accretional or disruptive in turbulent disks, through analytical arguments based on fluid dynamical simulations and orbital integrations. In turbulent disks, the velocity dispersion of planetesimals is pumped up by random gravitational perturbations from density fluctuations of the disk gas. When the velocity dispersion is larger than the planetesimals' surface escape velocity, collisions between planetesimals do not result in accretion and may even lead to their destruction. In disks with a surface density equal to that of the "minimum-mass solar nebula'' and with nominal magnetorotational instability (MRI) turbulence, we find that accretion proceeds only for planetesimals with sizes above similar to 300 km at 1 AU and similar to 1000 km at 5 AU. We find that accretion is facilitated in disks with smaller masses. However, at 5 AU and for nominal turbulence strength, km-sized planetesimals are in a highly erosive regime even for a disk mass as small as a fraction of the mass of Jupiter. The existence of giant planets implies that either turbulence was weaker than calculated by standard MRI models or some mechanism was capable of producing Ceres-mass planetesimals on very short timescales. In any case, our results show that in the presence of turbulence planetesimal accretion is most difficult in massive disks and at large orbital distances.