Spiral excitation in protoplanetary disks through gap-edge illumination: Distinctive kinematic signatures in CO isotopologues

Citation: Muley, D.; Hühn, L.-A.; Jiang, H.-C.; Melon Fuksman, J. D.; 2025; Accepted for publication in Astronomy & Astrophysics; arXiv:2511.22527

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. How might these two mechanisms be distinguished from one another in observations? To investigate this question, we ran a pair of hydrodynamical simulations with PLUTO. One, with full radiation hydrodynamics and gas-grain collision, was designed to develop shadow-driven spirals at the outer gap edge of a sub-thermal, Saturn-mass planet. The other, with parametrized $\beta$-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; such a 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.