Dust trapping in protoplanetary discs after stellar flybys

TL;DR

This study investigates how coplanar stellar flybys perturb protoplanetary discs and influence dust dynamics with 3D SPH simulations. By modeling a two-fluid dust population in a vertically isothermal disc and tracking dust particles within flyby-induced substructures, the work shows that flybys create long-lived dust traps, particularly in prograde encounters, where dust-to-gas ratios rise significantly in dust substructures. The authors connect these overdensities to the streaming instability by converting local dust densities to a surface ratio $Z$ and comparing with the SI threshold $Z_{\rm crit}$ as a function of St; in many cases, the threshold is exceeded for tens of dynamical times after the flyby, implying potential planetesimal formation. Observationally, these results suggest that flyby-induced substructures could be identified via combined dust continuum and gas measurements, providing a mechanism to trigger SI in young discs and influence early planet formation. Overall, the work highlights stellar flybys as a plausible driver of dust trapping and planetesimal formation in dense star-forming environments, with the trapping efficiency depending on encounter geometry, periastron distance, and dust grain size.

Abstract

Stellar flybys are likely to be common in young star-forming regions and could be responsible for substructures observed in protoplanetary discs. Using three-dimensional smoothed particle hydrodynamics simulations, we study dust trapping in discs perturbed by parabolic coplanar flybys. We find that spiral structures are induced in the gas and dust discs for both prograde and retrograde encounters. By tracking individual dust particles within the flyby-induced substructures, we determine that they have a highly enhanced dust to gas ratio compared to particles in an unperturbed disc. We further find that the local dust to gas ratios in flyby-induced substructures are sufficiently high to trigger the streaming instability and hence facilitate planetesimal formation in young discs.

Figures (23)

• Figure 1: Face-on view of the gas column density of the primary disc for (top to bottom): the standard_run, intermediate, far, half, retro and int_retro simulations. Columns (left to right) are at times $t=0.5$ (periastron), $t=0.67, t=0.83$ and $t=1$. Sink particles (in red) are large for visualisation purposes only. In each of the prograde encounters we observe a bridge of material connecting the two stars, and (in some cases) the induction of a two-armed spiral structure which then dissipates. In the retrograde encounters the spiral structures persist for longer.
• Figure 2: Face-on view of the dust column density of the primary disc for (top to bottom): the standard_run, intermediate, far, half, retro and int_retro simulations. Columns (left to right) are at times $t=0.5$ (periastron), $t=0.67, t=0.83$ and $t=1$. Sink particles (in blue) are large for visualisation purposes only. In the prograde encounters the dust disc is significantly truncated. Ring-like substructures of higher dust density appear when the periastron distance is larger than 175 AU, as well as in both the retrograde encounters.
• Figure 3: Mass of the gas and dust components of the primary disc over the course of each flyby in scaled time units. Perturbers on prograde orbits capture significantly more material from the disc than retrograde perturbers.
• Figure 4: Total dust to gas ratio of the primary disc over the course of each flyby in scaled time units, defined as the ratio of the gas mass to the dust mass in the primary disc.
• Figure 5: Radius of the gas and dust discs over the flyby in scaled time units, calculated as the radius at which 63.2% of the total (\ref{['fig:gas_r63']}) gas or (\ref{['fig:dust_r63']}) dust mass of the primary disc is enclosed. Dust discs undergo more truncation than gas discs in each encounter. The large peak in radius at periastron for the retro simulation can be attributed to the disc "puffing up" as the perturber passes through it.