PARTICLE CLUMPING AND PLANETESIMAL FORMATION DEPEND STRONGLY ON METALLICITY

Abstract

We present three-dimensional numerical simulations of particle clumping and\nplanetesimal formation in protoplanetary disks with varying amounts of solid\nmaterial. As centimeter-size pebbles settle to the mid-plane, turbulence\ndevelops through vertical shearing and streaming instabilities. We find that\nwhen the pebble-to-gas column density ratio is 0.01, corresponding roughly to\nsolar metallicity, clumping is weak, so the pebble density rarely exceeds the\ngas density. Doubling the column density ratio leads to a dramatic increase in\nclumping, with characteristic particle densities more than ten times the gas\ndensity and maximum densities reaching several thousand times the gas density.\nThis is consistent with unstratified simulations of the streaming instability\nthat show strong clumping in particle dominated flows. The clumps readily\ncontract gravitationally into interacting planetesimals of order 100 km in\nradius. Our results suggest that the correlation between host star metallicity\nand exoplanets may reflect the early stages of planet formation. We further\nspeculate that initially low metallicity disks can be particle enriched during\nthe gas dispersal phase, leading to a late burst of planetesimal formation.\n