Spiral excitation in protoplanetary disks through gap-edge illumination: Three-temperature radiation hydrodynamics and NIR image modeling

The advent of high-resolution, near-infrared (NIR) instruments such as VLT/SPHERE and Gemini/GPI has helped uncover a wealth of substructure in planet-forming disks, including large, prominent spiral arms in MWC 758, SAO 206462, and V1247 Ori. In the classical theory of disk-planet interaction, these arms are consistent with Lindblad-resonance driving by companions of multiple Jupiter masses. Despite improved detection limits, evidence for massive bodies like this in connection with spiral substructure has been inconclusive. In search of an alternative explanation, we used the PLUTO code to run 3D hydrodynamical simulations with two comparatively low planet masses (Saturn mass and Jupiter mass) and two thermodynamic prescriptions (three-temperature radiation hydrodynamics, and the more traditional beta -cooling) in a low-mass disk. In the radiative cases, an m = 2 mode, potentially attributable to the interaction of stellar radiation with gap-edge asymmetries, creates an azimuthal pressure gradient, which in turn gives rise to prominent spiral arms in the upper layers of the disk. Monte Carlo radiative transfer post-processing with RADMC3D revealed that in NIR scattered light, these gap-edge spirals are significantly more prominent than the traditional Lindblad spirals for planets in the mass range we tested. Our results demonstrate that even intermediate-mass protoplanets, which are less detectable, but more ubiquitous than super-Jupiters, are capable of indirectly inducing large-scale spiral disk features, and underscore the importance of including radiation physics in any efforts to reproduce observations.