The Appearance of Vortices in Protoplanetary Disks in Near-infrared Scattered Light

Azimuthally asymmetric structures have been discovered in millimeter continuum emission from many protoplanetary disks. One hypothesis is that they are vortices produced by the Rossby wave instability, for example at the edges of planet-opened gaps or dead zones. Confirming the vortex nature of these structures will have profound implications to planet formation. One way to test the hypothesis is to compare the observed morphology of vortex candidates in near-infrared scattered light with theoretical expectations. To this end, we synthesize the appearance of vortices in H-band polarized light by combining hydrodynamic and radiative transfer simulations of the Rossby wave instability at a dead-zone edge. In a disk at 140 pc, at the peak in its evolution a vortex at 65 au may appear as a radially narrow arc 50%-70% brighter compared with an axisymmetric disk model. The contrast depends on the inclination of the disk and the position angle of the vortex only weakly. Such contrast levels are well detectable in imaging observations of bright disks using instruments such as the Very Large Telescope/SPHERE, Subaru/SCExAO, and Gemini/GPI. A vortex also casts a shadow in the outer disk, which may aid its identification. Finally, at modest-to-high inclinations (e.g., 60°) a vortex may mimic a one-armed spiral. In the HD 34282 disk, such a one-armed spiral with a shadowed region on the outside has been found in scattered light. This feature roughly coincides with an azimuthal asymmetry in millimeter continuum emission, signifying the presence of a vortex.