The origin of tail-like structures around protoplanetary disks
  We study the origin of tail-like structures recently detected around the disk
of SU Aurigae and several FU~Orionis-type stars. Dynamic protostellar disks
featuring ejections of gaseous clumps and quiescent protoplanetary disks
experiencing a close encounter with an intruder star were modeled using the
numerical hydrodynamics code FEOSAD. Both the gas and dust dynamics were taken
into account, including dust growth and mutual friction between the gas and
dust components. Only plane-of-the-disk encounters were considered. Ejected
clumps produce a unique type of tail that is characterized by a bow-shock
shape. Such tails originate from the supersonic motion of ejected clumps
through the dense envelope that often surrounds young gravitationally unstable
protostellar disks. The ejected clumps either sit at the head of the tail-like
structure or disperse if their mass is insufficient to withstand the head wind
of the envelope. On the other hand, close encounters with quiescent
protoplanetary disks produce three types of the tail-like structure; we define
these as pre-collisional, post-collisional, and spiral tails. These tails can
in principle be distinguished from one another by particular features of the
gas and dust flow in and around them. We find that the brown-dwarf-mass
intruders do not capture circumintruder disks during the encounter, while the
subsolar-mass intruders can acquire appreciable circumintruder disks with
elevated dust-to-gas ratios, which can ease their observational detection.
However, this is true only for prograde collisions; the retrograde intruders
fail to collect appreciable amounts of gas or dust from the disk of the target.
The predicted mass of dust in the model tail-like structures is higher than
what was inferred for similar structures in SU~Aur, FU~Ori, and Z~CMa, making
their observational detection feasible. Abridged.
