The Structure of Magnetocentrifugal Winds. I. Steady Mass Loading

We present the results of a series of time-dependent numerical simulations ofcold, magnetocentrifugally launched winds from accretion disks. Our simulationsspan four and half decades of mass loading; in the context of a disk with alaunching region from $0.1\AU$ to $1.0\AU$ around a $1\solarmass$ star and afield strength of about $20\gauss$ at the inner disk edge, this amounts to massloss rates of $1\times 10^{-9}$ -- $3\times 10^{-5}\solarmassyear$ from eachside of the disk. We find that the degree of collimation of the wind increaseswith mass loading; however even the ``lightest'' wind simulated issignificantly collimated compared with the force-free magnetic configuration ofthe same magnetic flux distribution. The implication is that for flows fromyoung stellar objects a radial field approximation is inappropriate.Surprisingly, the terminal velocity of the wind and the magnetic lever arm arestill well-described by the analytical solutions for a radial field geometry.We also find that the isodensity contours and Alfv\'en surface are very nearlyself-similar in mass loading. The wind becomes unsteady above some criticalmass loading rate. For a small enough injection speed, we are able to obtainthe first examples of a class of heavily-loaded magnetocentrifugal winds withmagnetic fields completely dominated by the toroidal component all the way tothe launching surface.Comment: 36 pages, including 19 figures, LaTeX, revised and accepted by Ap