Context. Dust grains embedded in a gas flow give rise to a class of hydrodynamic instabilities that can occur whenever there exists a relative velocity between gas and dust. These instabilities have predominantly been studied for single grain sizes, for which a strong interaction can be found between drifting dust and a travelling gas wave leading to fast-growing perturbations (growth rates ∝√μ) even at small dust-to-gas ratios μ. They are called resonant drag instabilities. We focus on the acoustic resonant drag instability, which is potentially important in AGB star outflows, around supernova remnants, and star clusters in starburst galaxies. Aims. We study the acoustic resonant drag instability, taking into account a continuous spectrum of grain sizes, to determine whether it survives in the polydisperse regime and how the resulting growth rates compare to the monodisperse case. Methods. We solved the linear equations for a polydisperse fluid for the acoustic drag instability, focusing on small dust-to-gas ratios. Results. Size distributions that have a realistic width turn the fast-growing perturbations ∝√μ of the monodisperse limit into slower growing perturbations ∝ μ due to the fact that the backreaction on the gas involves an integration over the resonance. Furthermore, the large wave numbers that grow fastest in the monodisperse regime are stabilised by a size distribution, severely limiting the growth rates in the polydisperse regime. Conclusions. The acoustic resonant drag instability turns from a singularly perturbed problem in μ in the monodisperse limit into a regular perturbation for a sufficiently wide size distribution. It can still grow exponentially in the polydisperse regime, but at a slower pace compared to the single size case.

Context. Dust grains embedded in gas flow give rise to a class of hydrodynamic instabilities called resonant drag instabilities, some of which are thought to be important during the process of planet formation. These instabilities have predominantly been studied for single grain sizes, in which case they are found to grow fast. Non-linear simulations indicate that strong dust overdensities can form, aiding the formation of planetesimals. In reality, however, there is going to be a distribution of dust sizes, which may have significant consequences. Aims. We aim to study two different resonant drag instabilities - the streaming instability and the settling instability - taking into account a continuous spectrum of grain sizes, to determine whether these instabilities survive in the polydisperse regime and how the resulting growth rates compare to the monodisperse case. Methods. We solved the linear equations for a polydisperse fluid in an unstratified shearing box to recover the streaming instability and, for approximate stratification, the settling instability, in all cases focusing on low dust-to-gas ratios. Results. Size distributions of realistic widths turn the singular perturbation of the monodisperse limit into a regular perturbation due to the fact that the back-reaction on the gas involves an integration over the resonance. The contribution of the resonance to the integral can be negative, as in the case of the streaming instability, which as a result does not survive in the polydisperse regime; or positive, which is the case in the settling instability. The latter therefore has a polydisperse counterpart, with growth rates that can be comparable to the monodisperse case. Conclusions. Wide size distributions in almost all cases remove the resonant nature of drag instabilities. This can lead to reduced growth, as is the case in large parts of parameter space for the settling instability, or complete stabilisation, as is the case for the streaming instability.

The IRAS01425+2902 wide binary system was recently reported to have both a young planet and a puzzling geometric arrangement, where the planet and binary both orbit edge-on, but misaligned by 60^◦ to the circumprimary disc. This is the youngest transiting planet yet to be detected but its misalignment to the disc is difficult to explain. In this paper we explore the dissolution of an unstable triple system as a potential mechanism to produce this system. We simulate the effects of an ejection interaction in models using a highly inclined, retrograde flyby centred on the primary star of IRAS01425. The escaping star of ∼ 0.35 M_☉ inclines both the disc and binary orbits such that they have a relative misalignment of ≳ 60^◦, as inferred from observations. The planet orbit also becomes inclined relative to the disc, and our interpretation predicts that the binary should have a highly eccentric orbit (e ≳ 0.5 from our simulations). We additionally demonstrate that despite the high relative misalignment of the disc it is unlikely to be vulnerable to von Zeipel-Kozai-Lidov oscillations.

We carry out three-dimensional smoothed particle hydrodynamics simulations to study the role of gravitational and drag forces on the concentration of large dust grains (St > 1) in the spiral arms of gravitationally unstable protoplanetary discs, and the resulting implications for planet formation. We find that both drag and gravity play an important role in the evolution of large dust grains. If we include both, grains that would otherwise be partially decoupled will become well coupled and trace the spirals. For the dust grains most influenced by drag (with Stokes numbers near unity), the dust disc quickly becomes gravitationally unstable and rapidly forms clumps with masses between 0.15–6M⨁. A large fraction of clumps are below the threshold where runaway gas accretion can occur. However, if dust self-gravity is neglected, the dust is unable to form clumps, despite still becoming trapped in the gas spirals. When large dust grains are unable to feel either gas gravity or drag, the dust is unable to trace the gas spirals. Hence, full physics is needed to properly simulate dust in gravitationally unstable discs. Dust trapping of large grains in spiral arms of discs stable to gas fragmentation could explain planet formation in very young discs by a population of planetesimals formed due to the combined roles of drag and gravity in the earliest stages of a disc’s evolution. Furthermore, it highlights that gravitationally unstable discs are not just important for forming gas giants quickly, it can also rapidly form Earth mass bodies.

We identify a new dust instability that occurs in warped discs. The instability is caused by the oscillatory gas motions induced by the warp in the bending wave regime. We first demonstrate the instability using a local 1D (vertical) toy model based on the warped shearing box coordinates and investigate the effects of the warp magnitude and dust Stokes number on the growth of the instability. We then run 3D smoothed particle hydrodynamics (SPH) simulations and show that the instability is manifested globally by producing unique dust structures that do not correspond to gas pressure maxima. The 1D and SPH analysis suggest that the instability grows on dynamical time-scales and hence is potentially significant for planet formation.

We investigate the formation of dust traffic jams in polar-aligning circumbinary discs. In our first paper, we found as the circumbinary disc evolves towards a polar configuration perpendicular to the binary orbital plane, the differential precession between the gas and dust components leads to multiple dust traffic jams. These dust traffic jams evolve to form a coherent dust ring. In part two, we use 3D smoothed particle hydrodynamical simulations of gas and dust to model an initially highly misaligned circumbinary disc around the 99 Herculis (99 Her) binary system. Our results reveal that the formation of these dust rings is observed across various disc parameters, including the disc aspect ratio, viscosity, surface density power-law index, and temperature power-law index. The dust traffic jams are long-lived and persist even when the disc is fully aligned polar. The midplane dust-to-gas ratio within the rings can surpass unity, which may be a favourable environment for planetesimal formation. Using 2D inviscid shearing box calculations with parameters from our 3D simulations, we find streaming instability modes with significant growth rates. The streaming instability growth time-scale is less than the tilt oscillation time-scale during the alignment process. Therefore, the dust ring will survive once the gas disc aligns polar, suggesting that the streaming instability may aid in forming polar planets around 99 Her.

We investigate the formation of dust traffic jams in polar-aligning circumbinary discs. We use 3D smoothed particle hydrodynamical simulations of both gas and dust to model an initially highly misaligned circumbinary disc around an eccentric binary. As the circumbinary disc evolves to a polar configuration (perpendicular to the binary orbital plane), the difference in the precession between the gas and dust produces dust traffic jams, which become dense dust rings. We find the formation of dust rings exists for different Stokes number, binary eccentricity, and initial disc tilt. Dust rings are only produced while the circumbinary disc is misaligned to the binary orbital plane. When the disc becomes polar aligned, the dust rings are still present and long-lived. Once these dust rings are formed, they drift inward. The drift time-scale depends on the Stokes number. The lower the Stokes number, the faster the dust ring drifts near the inner edge of the disc. The dust rings will have an increased mid-plane dust-to-go ratio, which may be a favourable environment for the steaming instability to operate.

Misaligned circumbinary disks will produce dust traffic jams during alignment or antialignment to the binary orbital plane. We conduct a hydrodynamical simulation of an initially misaligned circumbinary disk undergoing polar alignment with multiple dust species. Due to differential precession between the gas and dust components, multiple dust traffic jams are produced within the disk during polar alignment. The radial locations of the dust traffic jams depend on the Stokes number of the grains, which depends on grain size. We compute the dust temperature structure using postprocessing radiative transfer to produce continuum images at centimeter wavelengths. Multiple distinct rings emerge in the continuum images, corresponding to the dust traffic jams. The angular resolution of upcoming observations from the Square Kilometre Array and the next-generation Very Large Array will be sufficient to detect centimeter-sized grains in protoplanetary disks and resolve the widths of dust traffic jams. Therefore, dust traffic jams resulting from the differential precession of gas and dust in misaligned circumbinary disks will be a prime target for more extended wavelength observations.

Context. GG Tau is one of the most studied young multiple stellar systems: GG Tau A is a hierarchical triple surrounded by a massive disc and its companion, GG Tau B, is also a binary. Despite numerous observational attempts, a comprehensive understanding of the geometry of the GG Tau A system is still elusive. Given the significant role of dynamical interactions in shaping the evolution of these systems, it is relevant to characterise the stellar orbits and the discs' properties. Aims. To determine the best orbital configuration of the GG Tau A system and its circumtriple disc, we provide new astrometric measures of the system and we run a set of hydrodynamical simulations with two representative orbits to test how they impact a disc composed of dust and gas. Methods. We tested the dynamical evolution of the two scenarios on short and long timescales. We obtained synthetic flux emission from our simulations at different timescales and we compared them with multi-wavelength observations of 1300 μm ALMA dust continuum emission and 1.67 μm SPHERE dust scattering to infer the most likely orbital arrangement. Results. We extend the analysis of the binary orbital parameters using six new epochs from archival data, showing that the current measurements alone (and future observations coming in the next 5- 10 yr) are not capable of fully breaking the degeneracy between families of coplanar and misaligned orbits, but finding that a modest misalignment is probable. We find that the timescale for the onset of the disc eccentricity growth, τ _ecc, is a fundamental timescale for the morphology of the system. Results from the numerical simulations obtained using the representative coplanar and misaligned (Δ θ = 30) orbits show that the best match between the position of the stars, the cavity size, and the dust ring size of GG Tau A is obtained with the misaligned configuration on timescales shorter than τ _ecc. The results exhibit an almost circular cavity and dust ring, favouring slightly misaligned (Δ θ ∼ 10- 30) low-eccentricity (e ∼ 0.2- 0.4) orbits. However, for both scenarios, the cavity size and its eccentricity quickly grow for timescales longer than τ _ecc and the models do not reproduce the observed morphology anymore. This implies that either the age of the system is shorter than τ _ecc or that the disc eccentricity growth is not triggered or dissipated in the system. This finding raises questions about the future evolution of the GG Tau A system and, more generally, the time evolution of eccentric binaries and their circumbinary discs.