"Ash-fall" induced by molecular outflow in protostar evolution
  Dust growth and its associated dynamics play key roles in the first phase of
planet formation in young stellar objects (YSOs). Observations have detected
signs of dust growth in very young protoplanetary disks. Furthermore, signs of
planet formation, gaps in the disk at a distance of several 10 astronomical
units (AU) from the central protostar are also reported. From a theoretical
point of view, however, it is not clear how planet form at the outer region of
a disk despite the difficulty due to rapid inward drift of dust so called
radial drift barrier. Here, on the basis of three-dimensional
magneto-hydrodynamical simulations of disk evolution with the dust growth, we
propose a mechanism named "ash-fall" phenomenon induced by powerful molecular
outflow driven by magnetic field which may circumvent the radial drift barrier.
We found that the large dust which grows to a size of $\sim \cm$ in the inner
region of a disk is entrained by an outflow from the disk. Then large dust
decoupled from gas is ejected from the outflow due to centrifugal force,
enriching the grown dust in the envelope and is eventually fall onto the outer
edge of the disk. The overall process is similar to behaviour of ash-fall from
volcanic eruptions. In the ash-fall phenomenon, the Stokes number of dust
increases by reaccreting to the less dense disk outer edge. This may make the
dust grains overcome the radial drift barrier. Consequently, the ash-fall
phenomenon can provide a crucial assist for making the formation of the
planetesimals in outer region of the disk possible, and hence the formation of
wide-orbit planets and the formation of the gaps.
