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Magnetized accretion-ejection structures
IV. Magnetically-driven jets from resistive, viscous, Keplerian discs
Fabien Casse and
Received 26 July 1999 / Accepted 10 November 1999
We present steady-state calculations of self-similar magnetized accretion discs driving cold, adiabatic, non-relativistic jets. For the first time, both the magnetic torque due to the jets and a turbulent "viscous" torque are taken into account. This latter torque allows a dissipation of the accretion power as radiation at the disc surfaces, while the former predominantly provides jets with power.
The parameter space of these structures has been explored. It is characterized by four free parameters, namely the disc aspect ratio and three MHD turbulence parameters, related to the anomalous magnetic diffusivities and viscosity. It turns out that launching cold jets from thin, dissipative discs implies anisotropic turbulent dissipation. Jets that asymptotically reach a high Alfvénic Mach number are only produced by weakly dissipative discs.
We obtained general analytical relations between disc and jet quantities that must be fulfilled by any steady-state model of cold jets, launched from a large radial extension of thin discs. We also show that such discs cannot have a dominant viscous torque. This is because of the chosen geometry, imposing the locus of the Alfvén surface.
Some observational consequences of these cold magnetized accretion-ejection structures are also briefly discussed.
Key words: accretion, accretion disks Magnetohydrodynamics (MHD) stars: formation ISM: jets and outflows galaxies: nuclei galaxies: jets
Send offprint requests to: Fabien.Casse@obs.ujf-grenoble.fr
Online publication: January 18, 2000