It is widely believed that outflows (or winds) from many astrophysical systems, ranging from young stellar objects to active galactic nuclei, are driven magneto-centrifugally from rapidly rotating central objects. A natural consequence of rotation and flux-freezing in such winds is that the magnetic field becomes predominantly toroidal once the flow speed exceeds the fast magnetosonic speed. We demonstrate that, because of this predominantly toroidal field, narrow jetlike density features can form spontaneously around the rotation axis of magnetohydrodynamic (MHD) winds even when their densities are initially distributed spherically. We limit our demonstration to the supermagnetosonic region where self-consistent solutions can be found by the well-known "method of characteristics". It is shown that, for nonrelativistic and modestly relativistic winds, the initially spherical isodensity contours become more and more elongated along the rotation axis, and thus more and more jetlike, on increasingly larger scales. This elongation is associated with collimation of wind streamlines by toroidal magnetic fields, although isodensity contours appear more jetlike than streamlines in general, as first noted by Shu et al. in 1995. Our concrete numerical examples support their asymptotic results that well-collimated "jets" are always surrounded by wide-angle winds and that isodensity contours could become more or less parallel to the rotation axis at large distances. We also show that formation of jetlike density features is more difficult in the supermagnetosonic region of highly relativistic MHD winds.
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