# Planetesimal formation near the snowline: in or out?

## TL;DR

This study investigates how water diffusion and condensation near the snowline in protoplanetary disks can locally enhance solids-to-gas ratios, facilitating planetesimal formation via streaming instability, especially in turbulent, young disks.

## Contribution

It demonstrates that water diffusion and condensation can significantly increase solids density near the snowline, promoting planetesimal formation in turbulent disk conditions.

## Key findings

- Water diffusion and condensation can enhance ice surface density by 3-5 times outside the snowline.
- In a 'many-seeds' model, solids-to-gas ratio is increased inside the snowline due to evaporation.
- High turbulence levels ($$=10^{-3}--10^{-2}) favor early planetesimal formation near the snowline.

## Abstract

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. In this article we investigate the viability of planetesimal formation by streaming instability near the snowline due to water diffusion and condensation. 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. 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 $\sim$$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 $\alpha$ ($10^{-3}$--$10^{-2}$). 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.