A new condition for the transition from runaway to oligarchic growth

TL;DR

This paper revises the understanding of the transition from runaway to oligarchic growth in planetary formation, proposing a new criterion based on timescale analysis that suggests larger transition sizes than previously thought.

Contribution

It introduces a new criterion for the runaway-oligarchy transition based on timescale arguments, challenging the adequacy of previous models and criteria.

Findings

Transition size ranges from hundreds to ~1000 km depending on disk location.

Runaway growth likely explains the size distribution of Kuiper belt objects.

The new criterion provides better initial conditions for oligarchic growth models.

Abstract

Accretion among macroscopic bodies of ~km size or larger is enhanced significantly due to gravitational focusing. Two regimes can be distinguished. Initially, the system experiences runaway growth, in which the gravitational focusing factors increase, and bodies at the high-mass tail of the distribution grow fastest. However, at some point the runaway body dynamically heats its environment, gravitational focusing factors decrease, and runaway growth passes into oligarchic growth. Based on the results of recent simulations, we reconsider the runaway growth-oligarchy transition. In contrast to oligarchy, we find that runaway growth cannot be approximated with a two component model (of small and large bodies) and that the criterion of Ida & Makino (1993), which is frequently adopted as the start of oligarchy, is not a sufficient condition to signify the transition. Instead, we propose a new criterion based on timescale arguments. We then find a larger value for the runaway growth-oligarchy transition: from several hundreds of km in the inner disk regions up to ~10^3 km for the outer disk. These findings are consistent with the view that runaway growth has been responsible for the size distribution of the present day Kuiper belt objects. Our finding furthermore outlines the proper initial conditions at the start of the oligarchy stage.