Heatmaps of the perturbed gas density ΔΣ_g/Σ_g, 0 (panel b), temperature, T (c), and mm-grain dust density, Σ_d (d), after 100 kyr of evolution in our model with external irradiation, showing prominent spiral structures in gas, along with the disk’s colder, shadowed farside and rings in the dust distribution. Gas density perturbations are also shown at an earlier state (10 kyr) in panel a. A black-and-white dotted circle marks the exponential tapering radius of the disk at 100 au. The dashed line in the middle panel indicates the τ = 1 surface towards the external source (here at a distance of 0.1 pc towards the right, with a luminosity of 2 × 10⁵ L_⊙), albeit computed with several underlying simplifications in Appendix A.1.