Dust density enhancements and the direct formation of planetary cores in gravitationally unstable discs

TL;DR

This study uses 3D simulations to show that spiral density waves in gravitationally unstable discs can significantly concentrate solids, enabling direct gravitational collapse into planetary cores, bypassing traditional growth barriers.

Contribution

It demonstrates that spiral density enhancements in self-gravitating discs can facilitate direct formation of planetary cores from dust, bypassing the collisional growth barriers.

Findings

Spiral density waves can increase local solid density by over an order of magnitude.

Solid particles can undergo gravitational collapse to form planetary cores of 1-10 Earth masses.

Dust concentration in self-gravitating discs can bypass the size barrier for planet formation.

Abstract

Planet formation via core accretion involves the growth of solids that can accumulate to form planetary cores. There are a number of barriers to the collisional growth of solids in protostellar discs, one of which is the drift, or metre, barrier. Solid particles experience a drag force that will tend to cause them to drift towards the central star in smooth, laminar discs, potentially removing particles before they grow large enough to decouple from the disc gas. Here we present 3-dimensional, shearing box simulations that explore the dynamical evolution of solids in a protostellar disc that is massive enough for the gravitational instability to manifest as spiral density waves. We expand on earlier work by considering a range of particle sizes and find that the spirals can still enhance the local solid density by more than an order of magnitude, potentially aiding grain growth. Furthermore, if solid particles have enough mass, and the particle size distribution extends to sufficiently large particle sizes, the solid component of the disc can undergo direct gravitational collapse to form bound clumps with masses typically between $1$ and $10$ M$_\oplus$. Thus, the concentration of dust in a self-gravitating disc could bypass the size barrier for collisional growth and directly form planetary cores early in the lifetime of the disc.