DIPSY: A new Disc Instability Population SYnthesis, II. The Populations of Companions Formed Through Disc Instability

TL;DR

This study uses a comprehensive model to predict the formation and characteristics of companions via disc instability across various stellar masses, highlighting its role in forming distant, massive companions but limited planetary objects within 100 AU.

Contribution

It provides the first quantitative population synthesis of disc instability outcomes, considering a wide range of initial conditions and analyzing the survival and properties of formed companions.

Findings

Approximately 10% of discs fragment via DI.

About half of the fragments survive after 100 Myr.

Most companions are brown dwarfs or low-mass stars, with few planetary-mass objects inside 100 AU.

Abstract

We applied the global end-to-end model described in Paper~I of this series to perform a population synthesis of companions formed via disc instability (DI). By using initial conditions compatible with both observations and hydrodynamical simulations, and by studying a large range of primary masses (0.05-5 Msol), we can provide quantitative predictions of the outcome of DI. In the baseline population, we find that ~10 % of the discs fragment, and about half of these end up with a surviving companion after 100 Myr. 75\% of the companions are in the brown dwarf regime, 15 % are low-mass stars, and 10 % planets. At distances larger than ~100 au, DI produces planetary-mass companions on a low percent level. Inside of 100 AU, however, planetary-mass companions are very rare (low per mill level). The average companion mass is ~30 Mj scaling weakly with stellar mass. Most of the initial fragments do not survive on a Myr timescale; they either collide with other fragments or are ejected, resulting in a population of free-floating objects (about 1-2 per star). We also quantify several variant populations to critically assess some of our assumptions used in the baseline population. DI appears to be a key mechanism in the formation of distant companions with masses ranging from low-mass stars down to the planetary regime, contributing, however, only marginally to planetary mass objects inside of 100 AU. Our results are sensitive to a number of physical processes, which are not completely understood. Two of them, gas accretion and clump-clump collisions, are particularly important and need to be investigated further. Magnetic fields and heavy-element accretion have not been considered in our study, although they are also expected to affect the inferred population. We suggest acknowledging the importance of the gravito-turbulent phase, which most protoplanetary discs experience.