Analysis of the arm-like structure in the outer disk of PDS 70 : Planet-driven spiral arm or vortex ?

We present our attempt to constrain the morphology and origin of an arm-like structure in the north-west side of the PDS 70 outer disk, and to assess whether it could trace a spiral density wave caused by the dynamical interaction between the planet PDS 70c and the disk. We re-analyze polarized and angular differential imaging (PDI and ADI) data taken with VLT/SPHERE, spanning six years of observations. PDI data sets are reduced using the IRDAP polarimetric data reduction pipeline, while ADI data sets are processed using MUSTARD, a novel inverse problem algorithm to tackle the geometrical biases spoiling the images previously used for the analysis of this disk. We confirm the presence of the arm-like structure in all PDI and ADI datasets. We do not observe a south-east symmetric locus with respect to the disk minor axis, which seems to reject 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° rotation for the spiral in six years. However, we do not measure any significant movement of the structure. We suggest that the arm-like structure may rather trace a vortex, appearing as a one-armed spiral in scattered light due to projection effects. This hypothesis is corroborated by the presence of an asymmetry in the sub-mm continuum detected with ALMA. Additional follow-up observations and dedicated hydrodynamical simulations could confirm this hypothesi