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Scientific interests
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Astrophysical jets are defined as highly collimated beams of matter moving with a high velocity. It is now generally accepted that magnetic fields are responsible for both acceleration and collimation of the flow. As a general property observed in astrophysical jet sources, there is the additional signature for the existence of an accretion disk. This holds for jets sources of all different scales of energy or spatial dimension: Active galactic nuclei, neutron stars and protostellar objects. We numerically calculate the structure of the jet magnetosphere and the dynamics of the plasma dynamics in the collimation region of magnetised jets. Some recent results cover the time dependent MHD simulation of a protostellar jet from a Keplerian disk using the ZEUS-3D code. As a test we aplied the boundary conditions of the model of Ouyed and Pudritz.
Model scenario of a magnetised star-disk system
Using the ZEUS 3D code in 2.5 dimensions we calculated the jet propagation from an accretion disk into a hydrostatic disk corona. We apply the model of Ouyed and Pudritz (1997) and use it as a test example for our simulations of dipolar-type magnetic fields interacting with an accretion disk.
The jet runs from left (accretion disk) to the right (hydrostatic corona). Shown are density (colors) and poloidal field lines (black). The jet ACCELERATES and COLLIMATES. Movie of the jet simulation (~1MB)
Using the ZEUS 3D code in 2.5 dimensions the evolution of a stellar dipolar-type magnetic field interacting with an accretion disk is calculated. Boundary condition is an inflow from a Keplerian disk (as in the model of Ouyed and Pudritz 1997). The accretion disk is at the lower boundary. A stellar surface is prescribed between the axis and the disk inner radius. The size of the box is 20x20 inner disk radii. Shown are density (colors) and poloidal field lines (black).
In the first simulation, the star is at rest. 100 Keplerian periods of the inner disk are calculated. A bubble forms disrupting the dipole. A disk wind accelerates and slowly collimates, indicating a possible final stationary state (Fendt & Elstner, 1999, A&A 349, L61, pdf-file ).
Dipolar field, star at rest (~2MB) In the second example the star rotates with a corotation radius at the inner disk radius. A two-component outflow (disk wind and stelalr wind) is formed which is uncollimated. More than 2500 Keplerian periods of the inner disk are calculated in order to obtain a quasi-stationary final state (run S2). A larger stellar wind mass flow rate stabilizes the flow along the axis (run L5) (see Fendt & Elstner, 2000, A&A 363, 208 pdf-file). The following gif-animations show the long-term evolution of the flow (use xanim).
In another paper we investigated the relation between the magnitude of
jet magnetic diffusivity and the degree of jet collimation.
(see Fendt & Cemeljic, 2002, A&A 395, 1045,
pdf-file).
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Small frames, long run (~11MB) 
Large frames, final time steps (~9MB) 
Medium size frames, long run (~25MB) 
Large frames, long run (~35MB) 
