2003ApJ...595..631K

We numerically follow the time evolution of axisymmetric outflows magnetocentrifugally driven from the inner portion of accretion disks from their launching surface to large observable distances. Special attention is paid to the collimation of part of the outflow into a dense, narrow jet around the rotation axis after a steady state has been reached. For parameters typical of T Tauri stars, we define a fiducial ``jet'' as that outlined by the contour of constant density at 10⁴ cm^–3. We find that the jet, so defined, appears nearly cylindrical well above the disk, in agreement with previous asymptotic analyses. Closer to the equatorial plane, the density contour can either bulge outward or pinch inward, depending on the conditions at the launching surface, particularly the mass flux distribution. We find that even though a dense, jetlike feature is always formed around the axis, there is no guarantee that the high-density axial jet would dominate the more tenuous, wide-angle part of the wind. Specifically, on the 100 AU scale, resolvable by the Hubble Space Telescope and ground-based adaptive optics for nearby T Tauri winds, the fraction of the wind mass flux enclosed by the fiducial jet can vary substantially, again depending on the launching conditions. We show two examples in which the fraction is ∼20% and ∼45%. These dependences may provide a way to constrain the conditions at the launching surface, which are poorly known at present.