Accretion and destruction of planetesimals in turbulent disks

TL;DR

This paper investigates how turbulence in protoplanetary disks affects planetesimal collisions, finding that turbulence generally hinders accretion, especially for smaller bodies and at larger distances, challenging traditional planet formation models.

Contribution

It provides analytical and simulation-based analysis of collision outcomes in turbulent disks, identifying size and disk mass thresholds for accretion versus erosion.

Findings

Accretion occurs only for planetesimals larger than ~300 km at 1AU.

Turbulence inhibits accretion for km-sized planetesimals at 5AU.

Massive disks and larger distances favor erosive collisions.

Abstract

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 MRI turbulence, we find that accretion proceeds only for planetesimals with sizes above $\sim 300$ km at 1AU and $\sim 1000$ km at 5AU. We find that accretion is facilitated in disks with smaller masses. However, at 5AU 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 in 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.