Collimation of a central wind by a disc-associated magnetic field

Studies of jets from young stellar objects (YSOs) 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 followthe central wind out to scales of tens of astronomical units (au), where it may interact with its surroundings. The coupling between the stellar magnetosphere and the inner edge of the disc 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(-1)), 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 G) 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 YSOs and discuss the possible origin of the disc-associated field.