Evolution of dust in a protoplanetary disc driven by stellar flybys: implications for the streaming instability

TL;DR

Stellar flybys in young clusters can dramatically reshape protoplanetary discs and alter dust evolution. We-use 3D SPH simulations with gas+dust, exploring parabolic flybys of varying mass and inclination to quantify how dust with $St>1$ responds and how this affects streaming-instability-driven clumping. The study finds that low-mass flybys tend to suppress dust concentration below the clumping threshold, while equal-mass flybys can trigger substantial dust accumulation that surpasses the critical abundance, promoting rapid clumping. These results imply that flyby-driven spirals and rings can create transient or lasting environments conducive to planetesimal formation, with clear observational signatures in dust morphology and SI activity.

Abstract

Stellar flybys are a common dynamical process in young stellar clusters and can significantly reshape protoplanetary discs. However, their impact on dust dynamics remains poorly understood, particularly in the weakly coupled regime (St$\gg$1). We present three-dimensional hydrodynamical simulations of parabolic stellar flybys-both coplanar and inclined-interacting with a gaseous and dusty protoplanetary disc. Dust species with Stokes numbers ranging from 15 to 100, corresponding to four grain sizes under a uniform initial gas surface density, are included. Perturber masses of 0.1 and 1$\mathrm{M}_{\odot}$ are considered. The induced spiral structures exhibit distinct dynamical behaviours in gas and dust: dust spirals retain a nearly constant pattern speed, while gas spirals gradually decelerate. The pitch angles of both components decrease over time, with dust evolving more rapidly. In the weakly coupled regime, gas and dust spirals are spatially offset, facilitating dust accumulation around both structures. Equal-mass flybys truncate the disc at approximately $\sim$0.55$r_{\mathrm{Hill}}$, producing tightly wound, ring-like spirals that promote dust concentration. By mapping the streaming instability growth rates in the solid abundance-Stokes number space across three evolutionary phases, we find that a low-mass flyby suppresses dust concentration below the critical clumping threshold after periastron and maintains this suppression over time, indicating long-lasting inhibition of dust clumping. An equal-mass flyby raises local solid abundance well above the threshold, suggesting that such encounters may foster conditions favourable for dust clumping. Flyby-induced spirals play a central role in shaping dust evolution, leading to distinct spatial and temporal behaviours in weakly coupled discs.

Figures (21)

• Figure 1: Evolution of the column density of gaseous and dusty components, with various initial Stokes numbers, during a low-mass coplanar flyby. We display only the dynamics of the gas disc from simulation ID st15co, as the gas disc behavior is nearly identical across simulations of the same flyby passage. The remaining rows show dust discs with various initial Stokes numbers. Blue dots mark the location of the perturbing companion, and green denotes the central star.
• Figure 2: Evolution of the azimuthally-averaged gas and dust column densities ($\Sigma_{\rm g/d}$, left) and eccentricity ($e$, right) profiles in a radial-time grid during a coplanar low-mass flyby. White dashed lines mark the time of flyby perihelion. White regions indicate areas of zero density at the corresponding radius. The first panel in both profiles shows the gaseous disc, while subsequent panels depict dust discs with varying initial Stokes numbers.
• Figure 3: The evolution of the density-weighted eccentricity of the disc is shown. The vertical black line represents the time that flyby hit the perihelion for each simulation. The other non-vertical dashed lines represent the evolution of eccentricity for the gaseous disc, while the solid lines depict that for the dusty disc. Three panels show the evolution of averaged eccentricity weighted by column density encountered by low-mass coplanar, low-mass inclined and equal-mass flyby, respectively. Due to the similarity of gas disc evolution among the simulations with same central star and flyby passage, we only show the evolution from the simulations with St$\sim 15$ for each flyby setup.
• Figure 4: The polar projection of column density distributions for gas ($\Sigma_{\rm g}$, shown in blue) and dust ($\Sigma_{\rm d}$, shown in red) in the st15co simulation at $t = 8743$ yr highlights the differences between the two components. This visualization reveals an offset between the dust and gas spirals, indicating that the dust spirals do not coincide with the gas spirals.
• Figure 5: An azimuthal cut of low-mass coplanar case with initial $\rm St\sim 15$. We fix the radius at 75 au and calculate the column density of both gas and dust for every angle at each time step. The upper panel represents the azimuthal cut of the gas disc, while the lower panel represents the dust disc. The black line indicates the azimuthal position of the flyby.