The Bridge: a transient phenomenon of forming stellar multiples
  Observations with modern instruments such as Herschel reveal that stars form
clustered inside filamentary arms of ~1 pc length embedded in Giant Molecular
Clouds. On smaller scales (~1000 au), observations of, e.g., IRAS 16293--2422
show signs of filamentary `bridge' structures connecting young protostars in
their birth environment. We investigate the formation of a deeply embedded
triple protostellar system, and characterize its connection to the filamentary
structure present on GMC scales. Using the magnetohydrodynamical code RAMSES,
we carry out zoom-in simulations of low-mass star formation starting from
Giant-Molecular-Cloud-scales. Colliding flows of gas in the filamentary arms
induce the formation of two protostellar companions at distances of ~1000 au
from the primary. After their birth, the stellar companions quickly (~10 kyr)
approach and orbit the primary on eccentric orbits with separations of ~100 au.
The colliding flows induce transient structures lasting for up to a few 10 kyr
connecting two forming protostellar objects that are kinematically quiescent
along the line-of-sight. Colliding flows compress the gas and trigger the
formation of stellar companions via turbulent fragmentation. Our results
suggest that protostellar companions form with a wide separation of ~1000 au.
Smaller separations (~100 au or less) are a consequence of subsequent migration
and capturing. Associated with the formation phase of the companion, the
turbulent environment induces arc- and bridge-like structures. These bridges
can become kinematically quiescent, when the velocity components of the
colliding flows eliminate each other. However, the gas in bridges still
contributes to stellar accretion later. Our results demonstrate: bridge-like
structures are a transient phenomenon of stellar multiple formation.
