Possible Evidence of Ongoing Planet Formation in AB Aurigae. A Showcase of the SPHERE/ALMA Synergy

Context. Planet formation is expected to take place in the first millionyears of a planetary system through various processes, which remain to betested through observations. Aims. With the recent discovery, using ALMA, oftwo gaseous spiral arms inside the 120 au cavity and connected to dustyspirals, the famous protoplanetary disk around AB Aurigae presents a strongincentive for investigating the mechanisms that lead to giant planet formation.A candidate protoplanet located inside a spiral arm has already been claimed inan earlier study based on the same ALMA data. Methods. We used SPHERE at theVery Large Telescope (VLT) to perform near-infrared (IR) high-contrast imagingof AB Aur in polarized and unpolarized light in order to study the morphologyof the disk and search for signs of planet formation. Results. SPHERE hasdelivered the deepest images ever obtained for AB Aur in scattered light. Amongthe many structures that are yet to be understood, we identified not only theinner spiral arms, but we also resolved a feature in the form of a twist in theeastern spiral at a separation of about 30 au. The twist of the spiral isperfectly reproduced with a planet-driven density wave model when projectioneffects are accounted for. We measured an azimuthal displacement with respectto the counterpart of this feature in the ALMA data, which is consistent withKeplerian motion on a 4-yr baseline. Another point sxce is detected near theedge of the inner ring, which is likely the result of scattering as opposed tothe direct emission from a planet photosphere. We tentatively derived massconstraints for these two features. Conclusions. The twist and its apparentorbital motion could well be the first direct evidence of a connection betweena protoplanet candidate and its manifestation as a spiral imprinted in the gasand dust distributions.