Astronomy and Astrophysics – Astrophysics
Scientific paper
Sep 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...313..591f&link_type=abstract
Astronomy and Astrophysics, v.313, p.591-604
Astronomy and Astrophysics
Astrophysics
56
Magnetohydrodynamics, Ism: Jets And Outflows, Stars: Pre-Main-Sequence, Stars: Magnetic Field, Stars: Mass Loss, Stars: Neutron
Scientific paper
Outflows driven by rapidly rotating magnetospheres occur in various astrophysical systems. Of particular importance are stellar magnetospheres in close contact with surrounding accretion disks. This work continues the investigations of paper I, where the overall structure of the resulting magnetic surfaces has been calculated. Here, we investigate the dynamics of the wind flow in such collimated magnetospheres. Since for most configurations the plasma pressure is unimportant, the relativistic cold wind equation is solved along various magnetic flux surfaces, providing in this way a two-dimensional mapping of the plasma velocity, density and energy. These parameters are discussed as a function of the magnetization σ that is a measure of the mass load in the magnetosphere. High magnetization means low-density magnetospheres, low magnetization high-density outflows. For a magnetization constant across the magnetic surfaces, the asymptotic jet velocity increases and the particle density decreases with increasing distance from the jet axis. An increasing magnetization, on the other hand, leads to a plasma velocity decreasing and a density increasing towards the jet boundary. Collimated outflows are therefore expected to have inhomogeneous density and velocity profiles. In the cold wind approximation the initial magnetization of the flow is the only parameter that determines the asymptotic velocity and density. For low magnetizations, σ<1, velocity and density are found to behave as a power law in σ, while the fast magnetosonic Mach-number virtually stays constant at a value of 2.5. Flows, which are collimated into a cylindrical shape, convert twice as much of the Poynting flux into kinetic energy when compared to a monopole type outflow. In distinction to previous models we find that the Alfven surface and the surface of the fast magnetosonic points converge asymptotically to cylindrical surfaces parallel to the jet axis. This divides up the collimated jet into three separate regions: in the innermost part, the plasma flow remains sub-Alfvenic, in the middle super-Alfvenic, but sub-magnetosonic and, only in the outer part, jet plasma flows super-magnetosonically. Applications for the jet structure of protostellar outflows are discussed.
Camenzind Max
Fendt Ch.
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