Vortices and Spirals in the HD135344B Transition Disk

In recent years, spiral structures have been seen in scattered light observations and signs of vortices in millimeter images of protoplanetary disks, both probably linked with the presence of planets. We present Atacama Large Millimeter/submillimeter Array Band 7 (335 GHz or 0.89 mm) continuum observations of the transition disk HD 135344B at an unprecedented spatial resolution of 0.″16, using superuniform weighting. The data show that the asymmetric millimeter-dust ring seen in previous work actually consists of an inner ring and an outer asymmetric structure. The outer feature is cospatial with the end of one of the spiral arms seen in scattered light, but the feature itself is not consistent with a spiral arm due to its coradiance. We propose a new possible scenario to explain the observed structures at both wavelengths. Hydrodynamical simulations show that a massive planet can generate a primary vortex (which dissipates at longer timescales, becoming an axisymmetric ring) and trigger the formation of a second generation vortex further out. Within this scenario, the two spiral arms observed at scattered light originate from a planet at ∼30 au and from the secondary vortex at ∼75 au rather than a planet further out as previously reported.