The Dynamical Fate of Self-Gravitating Disc Fragments After Tidal Downsizing

TL;DR

This study investigates the long-term dynamical evolution of self-gravitating disc fragments, showing that scattering and cluster interactions can produce free-floating substellar objects, with implications for understanding planet and brown dwarf populations.

Contribution

It provides the first detailed N-body simulations of post-formation dynamical evolution of disc fragments in various environments, linking formation models to observed free-floating objects.

Findings

25% ejection rate of fragments due to scattering

Ejecta velocity dispersion matches hydrodynamic simulations

13% of objects ejected from planetary systems in clusters

Abstract

The gravitational instability model of planet/brown dwarf formation proposes that protostellar discs can fragment into objects with masses above a few Jupiter masses at large semimajor axis. Tidal downsizing may reduce both the object mass and semimajor axis. However, most studies of tidal downsizing end when the protostellar disc disperses, while the system is embedded in its parent star-forming region. To compare disc fragment descendants with exoplanet and brown dwarf observations, the subsequent dynamical evolution must be explored. We carry out N-Body integrations of fragment-fragment scattering in multi-object star systems, and star systems embedded in substructured clusters. In both cases, we use initial conditions generated by population synthesis models of tidal downsizing. The scattering simulations produce a wide range of eccentricities. The ejection rate is around 25%. The ejecta mass distribution is similar to that for all objects, with a velocity dispersion consistent with those produced by full hydrodynamic simulations. The semimajor axis distribution after scattering extends to parsec scales. In the cluster simulations, 13% of objects are ejected from their planetary system, and around 10% experience significant orbit modification. A small number of objects are recaptured on high eccentricity, high inclination orbits. The velocity distribution of ejecta is similar to that produced by fragment-fragment scattering. If fragment-fragment scattering and cluster stripping act together, then disc fragmentation should be efficient at producing free-floating substellar objects, and hence characterising the free-floating planet population will provide strong constraints on the frequency of disc fragmentation.