Controlling the collimation and rotation of hydromagnetic disc winds
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abstract
(Abriged) We present a comprehensive set of axisymmetric, time-dependent
simulations of jets from Keplerian disks whose mass loading as a function of
disk radius is systematically changed. For a reasonable model for the density
structure and injection speed of the underlying accretion disk, mass loading is
determined by the radial structure of the disk's magnetic field structure. We
vary this structure by using four different magnetic field configurations,
ranging from the "potential" configuration (Ouyed&Pudritz 1997), to the
increasingly more steeply falling Blandford&Payne (1982) and Pelletier&Pudritz
(1992) 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 both the rotational profile of the
underlying jet as well as the degree of collimation of its outflow velocity and
magnetic field lines. 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.
