Fluffy dust forms icy planetesimals by static compression

TL;DR

This paper investigates how fluffy dust aggregates in protoplanetary disks are compressed by static forces like gas ram pressure and self-gravity, revealing a pathway from dust grains to icy planetesimals that overcomes key formation barriers.

Contribution

It introduces an analytical model showing the porosity evolution of dust aggregates under static compression, providing a new pathway for planetesimal formation beyond previous collisional models.

Findings

Dust aggregates are compressed to 10^{-3} g/cm^3 by disk gas.

Self gravity compresses aggregates to 10^{-1} g/cm^3 at 10 km radius.

The pathway allows rapid growth to avoid radial drift barrier beyond 6 AU.

Abstract

Context: In planetesimal formation theory, several barriers have been proposed, which are bouncing, fragmentation, and radial drift problems. To understand the structure evolution of dust aggregates is a key in the planetesimal formation. Dust grains become fluffy by coagulation in protoplanetary disks. However, once they become fluffy, they are not sufficiently compressed by collisional compression to form compact planetesimals. Aims: We aim to reveal the pathway of the dust structure evolution from dust grains to compact planetesimals. Methods: Using the compressive strength formula, we analytically investigate how fluffy dust aggregates are compressed by static compression due to ram pressure of the disk gas and self gravity of the aggregates in protoplanetary disks. Results: We reveal the pathway of the porosity evolution from dust grains via fluffy aggregates to form planetesimals, circumventing the barriers in planetesimal formation. The aggregates are compressed by the disk gas to the density of 10^{-3} g/cm^3 in coagulation, which is more compact than the case with collisional compression. Then, they are compressed more by self gravity to 10^{-1} g/cm^3 when the radius is 10 km. Although the gas compression decelerate the growth, they grow enough rapidly to avoid the radial drift barrier when the orbital radius is < 6 AU in a typical disk. Conclusions: We propose fluffy dust growth scenario from grains to planetesimals. It enables the icy planetesimal formation in a wide range beyond the snowline in protoplanetary disks. This result proposes a concrete initial condition of planetesimals for the later stages of the planet formation.