Warps survive beyond fly-by encounters in protoplanetary disks. RW Aur A as a case study

TL;DR

The paper investigates how stellar fly-bys reshape protoplanetary disks, focusing on warps and spirals across different encounter geometries and applying the results to RW Aur A. Using grid-based 3D hydrodynamics with a low-viscosity disk and subsequent radiative-transfer post-processing, the authors show that inclined fly-bys induce warps of a few degrees that can persist long after the perturber has moved away, while tidally induced spirals are short-lived. When mapped onto RW Aur A with constrained orbital parameters, the model yields a warp of about $5^\ ightcirc$ and a final mean tilt near $2.6^\ ightcirc$, with dust continuum morphologies matching ALMA observations and CO kinematics revealing potential remnant structures. The results support fly-by scenarios as plausible origins for moderate warps in young disks and highlight detectable, time-dependent kinematic and shadow features that could be observed with current and future facilities.

Abstract

Stellar fly-bys can have multiple dynamical effects on protoplanetary disks, including warping and the excitation of spiral arms. Since observations indicate that warps are common, we aim to investigate these effects for different fly-by trajectories. We further link our models to observations by applying them to the RW Aur system, which is a fly-by candidate with a relatively well constrained trajectory. We investigate the disk dynamics in grid-based hydrodynamical simulations, which allow for a lower disk viscosity than commonly used SPH models. We post-process our simulations of the RW Aur system with radiative transfer models to create synthetic images of the dust continuum and gas kinematics. Fly-bys inclined with respect to the original disk plane can excite warps of a few degrees: the exact outcome depends on the specific geometry of the encounter. Specifically, we find that the position of the periastron with respect to the initial disk plane plays a role for the resulting warp strength. Within our parameter set, the strongest warp is excited for a retrograde fly-by with a periastron that is not in the same plane as the disk. Our models show that the warp can persist even after the perturber can no longer be clearly linked to the system, implying that past fly-bys are a possible origin of observed warps. Excited spirals arms, on the other hand, are much more short-lived than the warp. The RW Aur system presents a perfect opportunity to apply these results: we find that a warp of about 5° can be excited, and that the strong spiral arms have already disappeared at the current time of observation 300 years after periastron). This compares well with existing continuum observations, and our synthetic kinematic evaluations hint at remnant structures in the gas density that may be detectable.

Figures (30)

• Figure 1: Schematics of the definition for the geometry of the fly-by trajectory with respect to the disk plane. Indicated are the inclination of the trajectory $\theta$, the longitude of ascending node $\Omega$, and the argument of periapsis $\omega$. The distance of the closest approach, i.e. the periapsis, is $r_\mathrm{p}$.
• Figure 2: Trajectory configurations for our fly-by simulations. The disk lies in the $x$-$y$-plane at the origin of the coordinate system with a counter-clockwise rotation. Configuration 1 is shown in blue on the left, where the trajectory is rotated about the $x$-axis with a periapsis in the same plane as the disk. Configuration 2 (orange, right) is rotated about the $y$-axis with a periapsis out of the disk plane.
• Figure 3: Inclination evolution of the fly-by simulations with prograde and retrograde fly-by of Configuration 1 (periastron in the same plane as the disk) in the top two panels and Configuration 2 (periastron out of disk plane) in the bottom 2 panels. Time $t=0$ indicates the moment of closest approach. The color corresponds to time and the vertical blue and orange dashed lines indicate the times when the perturber crosses the (initial) disk midplane. We note that we plot the inclination profile only up to the outer radius of the disk $r_\mathrm{out}=26\,\mathrm{au}$, but the computational domain extends further out.
• Figure 4: Mean tilt defined as the angle between the total angular momentum vector of the disk and the $z$-axis for all six trajectory configurations.
• Figure 5: Evolution of the orbital plane at $r=23.9\,\mathrm{au}$ in the retrograde simulation in Configuration 2 (see Figure \ref{['fig:trajectories']}, right). We start the fit according to Equation \ref{['eq:fit']} (blue dashed line) at the first maximum of the curve (grey dashed line) and stop the fit at $t=1.58 \times 10^4\,\mathrm{yr}$, where the period seems to change. We note that this figure shows the evolution over a longer time than Figure \ref{['fig:flyby-incl']}.