Considering contact forces during the formation of planetesimals by gravitational collapse: mutual orbits, spin states, and shapes

TL;DR

This study uses an advanced simulation method to model planetesimal formation via gravitational collapse, revealing detailed shapes, spin states, and binary systems, improving understanding beyond previous perfect-merger models.

Contribution

It introduces the SSDEM within PKDGRAV for more realistic modeling of planetesimal formation, capturing shapes, spins, and binary orbits that previous models could not.

Findings

Planetesimals have 10-hour average rotation periods.

Variety of shapes observed, including spherical, oblate, and egg-shaped.

Formation of binary systems from single collapsing clouds.

Abstract

In this work, we apply a soft-sphere discrete element method (SSDEM) within the PKDGRAV N-body integrator to investigate the formation of planetesimal systems through the gravitational collapse of clouds of super-particles. Previously published numerical models have demonstrated that the gravitational collapse of pebble clouds is an efficient pathway to produce binary planetesimal systems. However, such investigations were limited by their use of a perfect-merger and inflated-radii super-particle approach, which inhibits any analysis of planetesimal shapes and spin states, precludes the formation of the tightest binary orbits, and produces significantly under-dense planetesimals. The SSDEM enables super-particles to rest upon each other through mutual surface penetration and by simulating contact physics. Super-particles do not need to be inflated and collisions are not treated as perfect mergers; we can thus track the evolution of planetesimal shapes, spins, and tight binary orbits. We demonstrate that the SSDEM is an excellent method to model the collapse process, and is capable of producing many binary planetesimal systems from a single cloud. Our results confirm the findings of previously published perfect-merging models while also producing novel results about planetesimal spin and shape properties. Newly-formed planetesimals exhibit 10-hr rotation periods on average and can be characterized by a wide variety of shapes (spherical, oblate, top-shaped, flattened, egg-shaped, or prolate), with the most-massive planetesimals primarily forming as spheres and oblate-spheroids.