The effect of external environment on the evolution of protostellar
  disks
  Using numerical hydrodynamics simulations we studied the gravitational
collapse of pre-stellar cores of sub-solar mass embedded into a low-density
external environment. Four models with different magnitude and direction of
rotation of the external environment with respect to the central core were
studied and compared with an isolated model. We found that the infall of matter
from the external environment can significantly alter the disk properties as
compared to those seen in the isolated model. Depending on the magnitude and
direction of rotation of the external environment, a variety of disks can form
including compact (<= 200 AU) ones shrinking in size due to infall of external
matter with low angular momentum, as well as extended disks forming due to
infall of external matter with high angular momentum. The former are usually
stable against gravitational fragmentation, while the latter are prone to
fragmentation and formation of stellar systems with sub-stellar/very-low-mass
companions. In the case of counterrotating external environment, very compact
(< 5 AU) and short-lived (<= a few * 10^5 yr) disks can form when infalling
material has low angular momentum. The most interesting case is found for the
infall of counterrotating external material with high angular momentum, leading
to the formation of counterrotating inner and outer disks separated by a deep
gap at a few tens AU. The gap migrates inward due to accretion of the inner
disk onto the protostar, turns into a central hole, and finally disappears
giving way to the outer strongly gravitationally unstable disk. This model may
lead to the emergence of a transient stellar system with
sub-stellar/very-low-mass components counterrotating with respect to that of
the star.
