Planetesimal formation near the snowline: in or out?

Context. The formation of planetesimals in protoplanetary disks is not well-understood. Streaming instability is a promising mechanism to directly form planetesimals from pebble-sized particles, provided a high enough solids-to-gas ratio. However, local enhancements of the solids-to-gas ratio are difficult to realize in a smooth disk, which motivates the consideration of special disk locations such as the snowline – the radial distance from the star beyond which water can condense into solid ice. Aims. In this article we investigate the viability of planetesimal formation by streaming instability near the snowline due to water diffusion and condensation. We aim to identify under what disk conditions streaming instability can be triggered near the snowline. Methods. To this end, we adopt a viscous disk model, and numerically solve the transport equations for vapor and solids on a cylindrical, 1D grid. We take into account radial drift of solids, gas accretion on to the central star, and turbulent diffusion. We study the importance of the back-reaction of solids on the gas and of the radial variation of the mean molecular weight of the gas. Different designs for the structure of pebbles are investigated, varying in the number and size of silicate grains. We also introduce a semi-analytical model that we employ to obtain results for different disk model parameters. Results. We find that water diffusion and condensation can locally enhance the ice surface density by a factor 3–5 outside the snowline. Assuming that icy pebbles contain many micron-sized silicate grains that are released during evaporation, the enhancement is increased by another factor ~2. In this “many-seeds” model, the solids-to-gas ratio interior to the snowline is enhanced as well, but not as much as just outside the snowline. In the context of a viscous disk, the diffusion-condensation mechanism is most effective for high values of the turbulence parameter α (10-3–10-2). Therefore, assuming young disks are more vigorously turbulent than older disks, planetesimals near the snowline can form in an early stage of the disk. In highly turbulent disks, tens of Earth masses can be stored in an annulus outside the snowline, which can be identified with recent ALMA observations.