Warps survive beyond fly-by encounters in protoplanetary disks

Aims. 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 well-constrained trajectory. Methods. We investigated the disk dynamics in grid-based hydrodynamical simulations, which allow for a lower disk viscosity than in commonly used smoothed particle hydrodynamics models. We post-processed our simulations of the RW Aur system with radiative transfer models to create synthetic images of the dust continuum and gas kinematics. Results. Fly-bys inclined with respect to the original disk plane can excite warps of a few degrees, but 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 in 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 a warp. We performed a simulation of the recent close encounter in the observed disk around RW Aur A, one of the few systems with a well-constrained trajectory. We find that a warp of about 5 degrees can be excited and that the strong spiral arms have already disappeared at the current time of observation (300 yr 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.