The "Breaking The Chains" migration model for super-Earths formation: the effect of collisional fragmentation

TL;DR

This study investigates how collisional fragmentation affects the formation and final configuration of super-Earth systems, finding that perfect accretion models are generally sufficient despite some fragmentation events.

Contribution

It introduces a detailed N-body simulation approach including fragmentation effects, showing that imperfect accretion does not significantly alter super-Earth system evolution.

Findings

Fragmentation events are common but involve only ~10% of system mass.

Reaccretion of fragments by planets is frequent, limiting impact on system dynamics.

Perfect accretion approximation remains valid for modeling super-Earth formation.

Abstract

Planets between 1-4 Earth radii with orbital periods <100 days are strikingly common. The migration model proposes that super-Earths migrate inwards and pile up at the disk inner edge in chains of mean motion resonances. After gas disk dispersal, simulations show that super-Earth's gravitational interactions can naturally break their resonant configuration leading to a late phase of giant impacts. The instability phase is key to matching the orbital spacing of observed systems. Yet, most previous simulations have modelled collisions as perfect accretion events, ignoring fragmentation. In this work, we investigate the impact of imperfect accretion on the breaking the chains scenario. We performed N-body simulations starting from distributions of planetary embryos and modelling the effects of pebble accretion and migration in the gas disk. Our simulations also follow the long-term dynamical evolution of super-Earths after the gas disk dissipation. We compared the results of simulations where collisions are treated as perfect merging events with those where imperfect accretion and fragmentation are allowed. We concluded that the perfect accretion is a suitable approximation in this regime, from a dynamical point of view. Although fragmentation events are common, only ~10% of the system mass is fragmented during a typical "late instability phase", with fragments being mostly reacreted by surviving planets. This limited total mass in fragments proved to be insufficient to alter qualitatively the final system dynamical configuration -- e.g. promote strong dynamical friction or residual migration -- compared to simulations where fragmentation is neglected.