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

TL;DR

The study delivers a comprehensive DI population synthesis (DIPSY) by linking early infall-driven disc formation, fragmentation, clump evolution, gas accretion, and N-body dynamics to 100 Myr evolution across 0.05–5 M⊙ hosts. The baseline DI population yields ~10% disc fragmentation with about half of fragments surviving; surviving companions are predominantly brown dwarfs, with a minority of planetary-mass objects, and planets inside 100 AU are rare. The mass–distance distribution forms an inverted rotated ‘L’ with few planetary-mass objects inside 100–1000 AU, a result driven by internal clump–clump interactions and gas accretion; ejections are common, leading to ~1–2 free-floating objects per star. Comparisons to observations (e.g., SHINE, BEAST, Bowler) show qualitative agreement for brown dwarfs but ongoing tension for planets, underscoring missing physics (magnetic fields, solid accretion) and the need for further observational diagnostics to constrain disc instability as a planet/companion formation channel.

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.

Figures (14)

• Figure 1: Occurrence of the different possible fragment fates for all fragments formed in the baseline population. From left to right: Collision and merger with another clump, ejection from the system, thermal disruption, accretion on the primary, survival of the fragment, tidal disruption.
• Figure 2: Mass vs semi-major axis for all the surviving companions of the baseline population. The colour code of each companion gives the (final) mass of the primary (host star). Differences in occurrence rate due to the different weights of stellar masses according to the IMF are not reflected in this figure; instead, for each of the 100 stellar mass bins, $\approx$ 1000.0 systems containing a total of 7400 companions are shown. The number of systems ($N_{\rm syst}$), the number of surviving companions ($N_{\rm comp}$), the number of objects accreted on the primary ($N_{\rm acc}$), and the number of ejected objects ($N_{\rm ejec}$) are given in the bottom left corner. The horizontal background colours indicate how the companions might be classified as stars, BDs, or planets according to their mass. Clearly, the distinction between these classes is not as clear-cut as these limits may suggest. Objects at very large separations ($\gtrapprox1000au)$ may not survive long enough to be observed, as the grey area suggests, as they can become unbound.
• Figure 3: Initial mass function 2005ASSL..327...41C for the range of host star masses studied. The objects at the lower mass end are BDs, and the stellar spectral types from M to B are coloured. Bins in final host star mass used in our study are shown as vertical dashed lines (only every tenth bin is shown for visibility). This demonstrates the range in primary mass covered in our study as well as the logarithmic binning.
• Figure 4: Mass-semi-major axis diagram, weighted according to $\xi(\log m)$ (Chabrier-2005 IMF, see text Sect. \ref{['sec:imf']}). With this weighting, the population is completely dominated by M-dwarfs (note the difference in Fig. \ref{['fig:am']}).
• Figure 5: Distributions of four key properties for the companions of the baseline population DIPSY-0, separately for M/K-, G/F-, and A/B host stars, weighted according to $\xi(\log m)$ within the three bins, for systems with $M_\mathrm{*,final} > 0.2 M_\odot$. Top left: CMF. For comparison, $\xi_\mathrm{sys}(\log m$) is shown as a thin black line (dashed part indicates extrapolation). Top right: Distribution of semi-major axes. Bottom left: Distribution of eccentricities. Bottom right: Orbital inclinations.