Particle Pileups and Planetesimal Formation

Solid particles in protoplanetary disks that are sufficiently super-solar inmetallicity overcome turbulence generated by vertical shear to gravitationallycondense into planetesimals. Super-solar metallicities result if solidparticles pile up as they migrate starward as a result of aerodynamic drag.Previous analyses of aerodynamic drift rates that account for mean flowdifferences between gas and particles yield particle pile-ups. We improve onthese studies not only by accounting for the collective inertia of solidsrelative to that of gas, but also by including the transport of angularmomentum by turbulent stresses within the particle layer. These turbulentstresses are derived in a physically self-consistent manner from the structureof marginally Kelvin-Helmholtz turbulent flows. They are not calculated usingthe usual plate drag formulae, whose use we explain is inappropriate.Accounting for the relative inertia of solids to gas retards, but does notprevent, particle pile-ups, and generates more spatially extended regions ofmetal enrichment. Turbulent transport hastens pile-ups. We conclude thatparticle pile-up is a robust outcome in sufficiently passive protoplanetarydisks. Connections to observations of circumstellar disks, including the KuiperBelt, and the architectures of planetary systems are made.Comment: Final revised version, accepted to ApJ. Error corrected in density dependence of Epstein drift rate; correction caused quantitative changes, particularly in high particle density limit. Qualitative conclusion that particle pile-ups can trigger planetesimal formation within protostellar disk lifetime is unaffecte