The Structure of Magnetocentrifugal Winds I. Steady Mass Loading

Abstract

We present the results of a series of time-dependent numerical simulations of cold, magnetocentrifugally launched winds from accretion disks. Our simulations span four and half decades of mass loading; in the context of a disk with a launching region from $0.1\AU$ to $1.0\AU$ around a $1\solarmass$ star and a field strength of about $20\gauss$ at the inner disk edge, this amounts to mass loss rates of $1\times 10^{-9}$ -- $3\times 10^{-5}\solarmassyear$ from each side of the disk. We find that the degree of collimation of the wind increases with mass loading; however even the ``lightest'' wind simulated is significantly collimated compared with the force-free magnetic configuration of the same magnetic flux distribution. The implication is that for flows from young stellar objects a radial field approximation is inappropriate. Surprisingly, the terminal velocity of the wind and the magnetic lever arm are still well-described by the analytical solutions for a radial field geometry. We also find that the isodensity contours and Alfvén surface are very nearly self-similar in mass loading. The wind becomes unsteady above some critical mass loading rate. For a small enough injection speed, we are able to obtain the first examples of a class of heavily-loaded magnetocentrifugal winds with magnetic fields completely dominated by the toroidal component all the way to the launching surface.