  One serious challenge for planet formation is the rapid inward drift of
pebble-sized dust particles in protoplanetary disks. Dust trapping at local
maxima in the disk gas pressure has received much theoretical attention but
still lacks observational support. The cold dust emission in the AB Aur disk
forms an asymmetric ring at a radius of about 120 au, which is suggestive of
dust trapping in a gas vortex. We present high spatial resolution (0".58x0".78
~ 80x110 au) NOEMA observations of the 1.12 mm and 2.22 mm dust continuum
emission from the AB Aur disk. Significant azimuthal variations of the flux
ratio at both wavelengths indicate a size segregation of the large dust
particles along the ring. Our continuum images also show that the intensity
variations along the ring are smaller at 2.22 mm than at 1.12 mm, contrary to
what dust trapping models with a gas vortex have predicted. Our two-fluid
(gas+dust) hydrodynamical simulations demonstrate that this feature is well
explained if the gas vortex has started to decay due to turbulent diffusion,
and dust particles are thus losing the azimuthal trapping on different
timescales depending on their size. The comparison between our observations and
simulations allows us to constrain the size distribution and the total mass of
solid particles in the ring, which we find to be of the order of 30 Earth
masses, enough to form future rocky planets.
