Particle pileups and planetesimal formation

Solid particles in protoplanetary disks that are sufficiently supersolar in metallicity overcome turbulence generated by vertical shear to gravitationally condense into planetesimals. Supersolar metallicities result if solid particles pile up as they migrate starward as a result of aerodynamic drag. Previous analyses of aerodynamic drift rates that account for mean flow differences between gas and particles yield particle pileups. We improve on these studies not only by accounting for the collective inertia of solids relative to that of gas, but also by including the transport of angular momentum by turbulent stresses within the particle layer. These turbulent stresses are derived in a physically self-consistent manner from the structure of marginally Kelvin-Helmholtz turbulent flows. They are not calculated using the usual plate drag formulae, whose use we explain is inappropriate. Accounting for the relative inertia of solids to gas retards, but does not prevent, particle pileups and generates more spatially extended regions of metal enrichment. Turbulent transport hastens pileups. We conclude that particle pileup is a robust outcome in sufficiently passive protoplanetary disks. Connections to observations of circumstellar disks, including the Kuiper Belt, and the architectures of planetary systems are made.