  Studies of jets from young stellar objects (YSO's) suggest that material is
launched from a small central region at wide opening angles and collimated by
an interaction with the surrounding environment. Using time-dependent,
numerical magnetohydrodynamic simulations, we follow the detailed launching of
a central wind via the coupling of a stellar dipole field to the inner edge of
an accretion disc. Our method employs a series of nested computational grids,
which allows the simulations to follow the central wind out to scales of tens
of AU, where it may interact with its surroundings. The coupling between the
stellar magnetosphere and disc inner edge has been known to produce an outflow
containing both a highly collimated jet plus a wide-angle flow. The jet and
wide-angle wind flow at roughly the same speed (100--200 km/s), and most of the
energy and mass is carried off at relatively wide angles. We show that the
addition of a weak disc-associated field (<< 0.1 Gauss) has little effect on
the wind launching, but it collimates the entire flow (jet + wide wind) at a
distance of several AU. The collimation is inevitable, regardless of the
relative polarity of the disc field and stellar dipole, and the result is a
more powerful and physically broader collimated flow than from the star-disc
interaction alone. Within the collimation region, the morphology of the
large-scale flow resembles a pitchfork, in projection. We compare these results
with observations of outflows from YSO's and discuss the possible origin of the
disc-associated field.
