Controlling the collimation and rotation of hydromagnetic disc winds

We present a comprehensive set of axisymmetric, time-dependent simulations of jets from Keplerian discs whose mass loading as a function of disc radius is systematically changed. For a reasonable model for the density structure and injection speed of the underlying accretion disc, mass loading is determined by the radial structure of the magnetic field of the disc. We vary this structure by using four different magnetic field configurations, ranging from the Ouyed-Pudritz 'potential' configuration, to the increasingly more steeply falling Blandford-Payne and Pelletier-Pudritz models, and ending with a quite steeply raked configuration that bears similarities to the Shu X-wind model. We find that the radial distribution of the mass load has a profound effect on the rotational profile of the underlying jet as well as the degree of collimation of its outflow velocity and magnetic field lines. These four models have systematic differences in the power-law rotation profiles of jet material far from the source: v Φ(r) oc r a ranging over -0.46 ≥ a ≥ -0.76. We show analytically and confirm by our simulations that the collimation of a jet depends on its radial current distribution, which in turn is prescribed by the mass load. Models with steeply descending mass loads have strong toroidal fields, and these collimate to cylinders (this includes the Ouyed-Pudritz and Blandford-Payne outflows). On the other hand, the more gradually descending mass load profiles (the PP92 and monopolar distributions) have weaker toroidal fields and these result in wide-angle outflows with parabolic collimation. We also present detailed structural information about jets such as their radial profiles of jet density, toroidal magnetic field and poloidal jet speed, as well as an analysis of the bulk energetics of our different simulations. Our results are in excellent agreement with the predictions of asymptotic collimation for axisymmetric, stationary jets.