Collisional fragmentation and bulk composition tracking in REBOUND

TL;DR

This paper introduces a fragmentation and composition tracking module for the REBOUND n-body simulation code, examining how collision modeling and expansion factors influence planetary formation, composition, and radial mixing.

Contribution

It presents a new fragmentation model and composition tracking method integrated into REBOUND, and analyzes the effects of expansion factors on collision outcomes and planetary system properties.

Findings

Higher expansion factors increase merger rates and produce larger, more distant planets.

Fragmentation enhances radial mixing and increases planetary water content.

Radial mixing decreases with larger expansion factors due to earlier collisions.

Abstract

We present a fragmentation module and a composition tracking code for the $n$-body code REBOUND. Our fragmentation code utilises previous semi-analytic models and follows an implementation method similar to fragmentation for the $n$-body code MERCURY. In our $n$-body simulations with fragmentation, we decrease the collision and planet formation timescales by inflating the particle radii by an expansion factor $f$ and experiment with various values of $f$ to understand how expansion factors affect the collision history and final planetary system. As the expansion factor increases, so do the rate of mergers which produces planetary systems with more planets and planets at larger orbits. Additionally, we present a composition tracking code which follows the compositional change of homogeneous bodies as a function of mass exchange and use it to study how fragmentation and the use of an expansion factor affects volatile delivery to the inner terrestrial disc. We find that fragmentation enhances radial mixing relative to perfect merging and that on average, as $f$ increases so does the average water mass fraction of the planets. Radial mixing decreases with increasing $f$ as collisions happen early on, before the bodies have time to grow to excited orbits and move away from their original location