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Using numerical hydrodynamics simulations we studied the gravitationalcollapse of pre-stellar cores of sub-solar mass embedded into a low-densityexternal environment. Four models with different magnitude and direction ofrotation of the external environment with respect to the central core werestudied and compared with an isolated model. We found that the infall of matterfrom the external environment can significantly alter the disk properties ascompared to those seen in the isolated model. Depending on the magnitude anddirection of rotation of the external environment, a variety of disks can formincluding compact (<= 200 AU) ones shrinking in size due to infall of externalmatter with low angular momentum, as well as extended disks forming due toinfall of external matter with high angular momentum. The former are usuallystable against gravitational fragmentation, while the latter are prone tofragmentation and formation of stellar systems with sub-stellar/very-low-masscompanions. In the case of counterrotating external environment, very compact(< 5 AU) and short-lived (<= a few * 10^5 yr) disks can form when infallingmaterial has low angular momentum. The most interesting case is found for theinfall of counterrotating external material with high angular momentum, leadingto the formation of counterrotating inner and outer disks separated by a deepgap at a few tens AU. The gap migrates inward due to accretion of the innerdisk onto the protostar, turns into a central hole, and finally disappearsgiving way to the outer strongly gravitationally unstable disk. This model maylead to the emergence of a transient stellar system withsub-stellar/very-low-mass components counterrotating with respect to that ofthe star.

The effect of external environment on the evolution of protostellar disks