Spiral excitation in protoplanetary disks through gap-edge illumination

Distinctive kinematic signatures in CO isotopologues

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, Heidelberg 69117, Germany
² Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, Garching bei München 85748, Germany
³ Institut für Theoretische Astrophysik, Ruprecht-Karls Universität Heidelberg, Albert-Ueberle-Straße 2, Heidelberg 69120, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 2 November 2025
Accepted: 27 November 2025

Abstract

High-resolution near-infrared observations have revealed prominent two-armed spirals in a multitude of systems, such as MWC 758, SAO 206462, and V1247 Ori. Alongside the classical theory of disk–companion interaction, shadow-based driving has come into vogue as a potential explanation for such large-scale substructures. This raises the question of how these two mechanisms might be distinguished from one another in observations. To investigate this question, we ran a pair of hydrodynamical simulations with PLUTO. The first, with full radiation hydrodynamics and gas-grain collision, was designed to develop shadow-driven spirals at the outer gap edge of a subthermal Saturn-mass planet. The second simulation, with parameterized β-cooling, was set up to capture the more standard view of spiral wave excitation by a super-thermal, multi-Jupiter-mass, exterior planetary companion. Post-processing of these simulations with the Monte Carlo radiative transfer (MCRT) code RADMC3D revealed that strong vertical velocities in the shadow-driven case create a prominent two-armed feature in the moment-1 CO maps, particularly when the disk is viewed face-on in optically thicker isotopologues; this feature is not seen in the standard planet-driven case. Conversely, the presence or absence of such signatures in two-armed spiral systems would distinguish those potentially driven by exterior multi-Jupiter-mass companions, and thus help identify promising targets for future direct-imaging campaigns.