The angular momentum of two collided rarefied preplanetesimals and the formation of binaries

TL;DR

This study models the angular momentum in collisions of rarefied preplanetesimals to explain binary formation and observed orbital characteristics, highlighting the importance of initial conditions and contraction processes.

Contribution

It introduces new models for binary formation from RPP collisions, accounting for angular momentum and explaining the distribution of binaries farther from the Sun.

Findings

Binary systems can form via mergers or disc accretion from RPPs.

Observed angular momenta are lower than expected, indicating prior contraction of RPPs.

Formation scenarios explain wide separations like in 2001 QW322.

Abstract

This paper studies the mean angular momentum associated with the collision of two celestial objects in the earliest stages of planet formation. Of primary concern is the scenario of two rarefied preplanetesimals (RPPs) in circular heliocentric orbits. The theoretical results are used to develop models of binary or multiple system formation from RPPs, and explain the observation that a greater fraction of binaries originated farther from the Sun. At the stage of RPPs, small-body satellites can form in two ways: a merger between RPPs can have two centers of contraction, or the formation of satellites from a disc around the primary or the secondary. Formation of the disc can be caused by that the angular momentum of the RPP formed by the merger is greater than the critical angular momentum for a solid body. One or several satellites of the primary (moving mainly in low-eccentricity orbits) can be formed from this disc at any separation less than the Hill radius. The first scenario can explain a system such as 2001 QW322 where the two components have similar masses but are separated by a great distance. In general, any values for the eccentricity and inclination of the mutual orbit are possible. Among discovered binaries, the observed angular momenta are smaller than the typical angular momenta expected for identical RPPs having the same total mass as the discovered binary and encountering each other in circular heliocentric orbits. This suggests that the population of RPPs underwent some contraction before mergers became common.