Scattered light signatures of flyby-induced warps in protoplanetary discs

Abstract

We explore the observational signatures of flybys in scattered light images of protostellar discs. The warps are modelled using 1D warp propagation theory coupled to a fast radiative transfer code that simulates the shadows induced. We consider two scenarios, namely a flyby in a plane orthogonal to, and at an angle with, the disc plane. In both models the outer disc becomes warped (leading to a broad shadow in the outer disc) and the warp wave propagates back and forth (causing the shadow to oscillate). We find that the inner disc, although tilted, is not warped and is therefore not shadowed. For a low viscosity disc ($\alpha=10^{-4}$) the warp lasts for most of the disc's lifetime ($\tau \sim 10^6\,$years), and for $50\%$ of the time the azimuthal variance of the surface brightness from the scattered light images, $\sigma^2$, is above $0.01$, meaning that the shadow in the disc is significant. We find that a significant fraction of discs in nearby star forming regions should have undergone a flyby sufficient to induce an observable warp, and that surveys of shadowed discs could provide a valuable probe of disc viscosity.