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 million
years of a planetary system through various processes, which remain to be
tested through observations. Aims. With the recent discovery, using ALMA, of
two gaseous spiral arms inside the 120 au cavity and connected to dusty
spirals, the famous protoplanetary disk around AB Aurigae presents a strong
incentive for investigating the mechanisms that lead to giant planet formation.
A candidate protoplanet located inside a spiral arm has already been claimed in
an earlier study based on the same ALMA data. Methods. We used SPHERE at the
Very Large Telescope (VLT) to perform near-infrared (IR) high-contrast imaging
of AB Aur in polarized and unpolarized light in order to study the morphology
of the disk and search for signs of planet formation. Results. SPHERE has
delivered the deepest images ever obtained for AB Aur in scattered light. Among
the many structures that are yet to be understood, we identified not only the
inner spiral arms, but we also resolved a feature in the form of a twist in the
eastern spiral at a separation of about 30 au. The twist of the spiral is
perfectly reproduced with a planet-driven density wave model when projection
effects are accounted for. We measured an azimuthal displacement with respect
to the counterpart of this feature in the ALMA data, which is consistent with
Keplerian motion on a 4-yr baseline. Another point sxce is detected near the
edge of the inner ring, which is likely the result of scattering as opposed to
the direct emission from a planet photosphere. We tentatively derived mass
constraints for these two features. Conclusions. The twist and its apparent
orbital motion could well be the first direct evidence of a connection between
a protoplanet candidate and its manifestation as a spiral imprinted in the gas
and dust distributions.
