Analysis of the arm-like structure in the outer disk of PDS 70. Spiral density wave or vortex?

Context. Observing dynamical interactions between planets and disks is key to understanding their formation and evolution. Two protoplanets have recently been discovered within the PDS 70 protoplanetary disk, along with an arm-like structure toward the northwest of the star.
Aims: Our aim is to constrain the morphology and origin of this arm-like structure, and to assess whether it could trace a spiral density wave caused by the dynamical interaction between the planet PDS 70c and the disk.
Methods: We analyzed polarized and angular differential imaging (PDI and ADI) data taken with VLT/SPHERE, spanning six years of observations. The PDI data sets were reduced using the irdap polarimetric data reduction pipeline, while the ADI data sets were processed using mustard, a novel algorithm based on an inverse problem approach to tackle the geometrical biases spoiling the images previously used for the analysis of this disk.
Results: We confirm the presence of the arm-like structure in all PDI and ADI data sets, and extract its trace by identifying local radial maxima in azimuthal slices of the disk in each data set. We do not observe a southeast symmetric arm with respect to the disk minor axis, which seems to disfavor the previous hypothesis that the arm is the footprint of a double-ring structure. If the structure traces a spiral density wave following the motion of PDS 70c, we would expect 11°.28_−0°.86^+2°.20 rotation for the spiral in six years. However, we do not measure any significant movement of the structure.
Conclusions: If the arm-like structure is a planet-driven spiral arm, the observed lack of rotation would suggest that the assumption of rigid-body rotation may be inappropriate for spirals induced by planets. We suggest that the arm-like structure may instead trace a vortex appearing as a one-armed spiral in scattered light due to projection effects. The vortex hypothesis accounts for both the lack of observed rotation and the presence of a nearby sub-millimeter continuum asymmetry detected with ALMA. Additional follow-up observations and dedicated hydrodynamical simulations could confirm this hypothesis.