Binary Planetesimal Formation from Gravitationally Collapsing Pebble Clouds

TL;DR

This study investigates how pebble clouds formed by streaming instability gravitationally collapse into planetesimals, producing various binary and multiple systems, with properties influenced by initial angular momentum, and compares simulation results to observed Kuiper belt binaries.

Contribution

The paper combines ATHENA and PKDGRAV simulations to analyze the collapse of pebble clouds and links initial angular momentum to the resulting planetesimal and binary properties, providing insights into Kuiper belt binaries.

Findings

Pebble clouds rapidly collapse into short-lived disk structures.

Low angular momentum clouds produce single planetesimals or tight binaries.

High angular momentum clouds form binary planetesimals resembling Kuiper belt binaries.

Abstract

Planetesimals are compact astrophysical objects roughly 1-1000 km in size, massive enough to be held together by gravity. They can grow by accreting material to become full-size planets. Planetesimals themselves are thought to form by complex physical processes from small grains in protoplanetary disks. The streaming instability (SI) model states that mm/cm-size particles (pebbles) are aerodynamically collected into self-gravitating clouds which then directly collapse into planetesimals. Here we analyze ATHENA simulations of the SI to characterize the initial properties (e.g., rotation) of pebble clouds. Their gravitational collapse is followed with the PKDGRAV N-body code, which has been modified to realistically account for pebble collisions. We find that pebble clouds rapidly collapse into short-lived disk structures from which planetesimals form. The planetesimal properties depend on the cloud's scaled angular momentum, l=L/(M R_H^2 Omega, where L and M are the angular momentum and mass, R_H is the Hill radius, and Omega is the orbital frequency. Low-l pebble clouds produce tight (or contact) binaries and single planetesimals. Compact high-l clouds give birth to binary planetesimals with attributes that closely resemble the equal-size binaries found in the Kuiper belt. Significantly, the SI-triggered gravitational collapse can explain the angular momentum distribution of known equal-size binaries -- a result pending verification from studies with improved resolution. About 10% of collapse simulations produce hierarchical systems with two or more large moons. These systems should be found in the Kuiper belt when observations reach the threshold sensitivity.