Rapid Coagulation of Porous Dust Aggregates Outside the Snow Line: A Pathway to Successful Icy Planetesimal Formation

TL;DR

This study demonstrates that highly porous icy dust aggregates can rapidly grow outside the snow line, potentially overcoming the radial drift barrier and forming icy planetesimals through direct collisional growth.

Contribution

It introduces a porosity evolution model based on recent N-body simulations, showing porous aggregate growth can bypass the radial drift barrier outside the snow line.

Findings

Porous aggregates with densities less than 0.1 g/cm^3 can form.

High porosity accelerates collisional growth due to aerodynamical effects.

Porous icy dust can overcome the radial drift barrier within 10 AU.

Abstract

Rapid orbital drift of macroscopic dust particles is one of the major obstacles against planetesimal formation in protoplanetary disks. We reexamine this problem by considering porosity evolution of dust aggregates. We apply a porosity model based on recent N-body simulations of aggregate collisions, which allows us to study the porosity change upon collision for a wide range of impact energies. As a first step, we neglect collisional fragmentation and instead focus on dust evolution outside the snow line, where the fragmentation has been suggested to be less significant than inside the snow line because of a high sticking efficiency of icy particles. We show that dust particles can evolve into highly porous aggregates (with internal densities of much less than 0.1 g/cm^3) even if collisional compression is taken into account. We also show that the high porosity triggers significant acceleration in collisional growth. This acceleration is a natural consequence of particles' aerodynamical property at low Knudsen numbers, i.e., at particle radii larger than the mean free path of the gas molecules. Thanks to this rapid growth, the highly porous aggregates are found to overcome the radial drift barrier at orbital radii less than 10 AU (assuming the minimum-mass solar nebula model). This suggests that, if collisional fragmentation is truly insignificant, formation of icy planetesimals is possible via direct collisional growth of submicron-sized icy particles.