Late encounter-events as a source of disks and spiral structures --
  Forming second generation disks
  Both observations of arc-like structures and luminosity bursts of stars > 1
Myr in age indicate that at least some stars undergo late infall events. We
investigate scenarios of replenishing the mass reservoir around a star via
capturing and infalling events of cloudlets. We carry out altogether 24
three-dimensional hydrodynamical simulations of cloudlet encounters with a
Herbig star of mass 2.5 solar mass using the moving mesh code AREPO. To account
for the two possibilities of a star or a cloudlet traveling through the
interstellar medium (ISM), we put either the star or the cloudlet at rest with
respect to the background gas. For absent cooling in the adiabatic runs, almost
none of the cloudlet gas is captured due to high thermal pressure. However,
second-generation disks easily form when accounting for cooling of the gas. The
disk radii range from several 100 au to about 1000 au and associated arc-like
structures up to 10 000 au in length form around the star for runs with and
without stellar irradiation. Consistent with angular momentum conservation, the
arcs and disks are larger for larger impact parameters. Accounting for
turbulence in the cloudlet only mildly changes the model outcome. In the case
of the star being at rest with the background gas, the disk formation and mass
replenishment process is more pronounced and the associated arc-shaped
streamers are longer-lived. The results of our models confirm that late
encounter events lead to the formation of transitional disks associated with
arc-shaped structures such as observed for AB Aurigae or HD 100546. In
addition, we find that second-generation disks and their associated filamentary
arms are longer lived (>100 000 yrs) in infall events, when the star is at rest
with the background gas.
