Spiral waves triggered by shadows in transition disks

TL;DR

Shadow-induced spiral arms in transition disks can form due to variable illumination from inner disk shadows, leading to observable structures that resemble those caused by planetary influences.

Contribution

This study demonstrates that shadows cast by inner disk warps can generate spiral arms in outer disks, providing an alternative explanation to planet-induced spirals.

Findings

Shadowing causes spiral density waves with specific pitch angles.

Self-gravity enhances the density contrast of these spirals.

Predicted observable signatures in scattered light match some observed disk features.

Abstract

Circumstellar asymmetries such as central warps have recently been shown to cast shadows on outer disks. We investigate the hydrodynamical consequences of such variable illumination on the outer regions of a transition disk, and the development of spiral arms. Using 2D simulations, we follow the evolution of a gaseous disk passively heated by the central star, under the periodic forcing of shadows with an opening angle of $\sim$28$^\circ$. With a lower pressure under the shadows, each crossing results in a variable azimuthal acceleration, which in time develops into spiral density waves. Their pitch angles evolve from $\Pi \sim 15^\circ-22^\circ$ at the onset, to $\sim$11$^\circ$-14$^\circ$, over $\sim$65~AU to 150~AU. Self-gravity enhances the density contrast of the spiral waves, as also reported previously for spirals launched by planets. Our control simulations with unshadowed irradiation do not develop structures, except for a different form of spiral waves seen at later times only in the gravitationally unstable control case. Scattered light predictions in the $H$-band show that such illumination spirals should be observable. We suggest that spiral arms in the case-study transition disk HD~142527 could be explained as a result of shadowing from the tilted inner disk.