Towards a Population Synthesis Model of Objects formed by Self-Gravitating Disc Fragmentation and Tidal Downsizing

TL;DR

This paper introduces a population synthesis model for objects formed through gravitational instability and tidal downsizing, exploring their formation pathways and potential to produce a wide range of celestial bodies.

Contribution

It presents a semi-analytic population synthesis model coupling disc evolution with fragmentation and embryo evolution processes, advancing the understanding of GI and tidal downsizing in star and planet formation.

Findings

GI plus tidal downsizing can produce objects from terrestrial planets to brown dwarfs.

The model suggests GI is not the main planet formation mode but forms gas giants at large distances.

Varying parameters affects the types and distributions of formed objects.

Abstract

Recently, the gravitational instability (GI) model of giant planet and brown dwarf formation has been revisited and recast into what is often referred to as the "tidal downsizing" hypothesis. The fragmentation of self-gravitating protostellar discs into gravitationally bound embryos - with masses of a few to tens of Jupiter masses, at semi major axes above 30 - 40 au - is followed by grain sedimentation inside the embryo, radial migration towards the central star and tidal disruption of the embryo's upper layers. The properties of the resultant object depends sensitively on the timescales upon which each process occurs. Therefore, GI followed by tidal downsizing can theoretically produce objects spanning a large mass range, from terrestrial planets to giant planets and brown dwarfs. Whether such objects can be formed in practice, and what proportions of the observed population they would represent, requires a more involved statistical analysis. We present a simple population synthesis model of star and planet formation via GI and tidal downsizing. We couple a semi-analytic model of protostellar disc evolution to analytic calculations of fragmentation, initial embryo mass, grain growth and sedimentation, embryo migration and tidal disruption. While there are key pieces of physics yet to be incorporated, it represents a first step towards a mature statistical model of GI and tidal downsizing as a mode of star and planet formation. We show results from four runs of the population synthesis model, varying the opacity law and the strength of migration, as well as investigating the effect of disc truncation during the fragmentation process. Our early results suggest that GI plus tidal downsizing is not the principal mode of planet formation, but remains an excellent means of forming gas giant planets, brown dwarfs and low mass stars at large semimajor axes. (Abridged)