Magnetized accretion-ejection structures. III. Stellar and extragalactic jets as weakly dissipative disk outflows.

Details Magnetized accretion-ejection structures. III. Stellar and extragalactic jets as weakly dissipative disk outflows. Magnetized accretion-ejection structures. III. Stellar and extragalactic jets as weakly dissipative disk outflows.

Mar 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...295..807f&link_type=abstract

Astronomy and Astrophysics, v.295, p.807

Astronomy and Astrophysics

Astrophysics

144

Accretion, Accretion Disks, Magnetohydrodynamics (Mhd), Stars: Formation, Ism: Jets And Outflows, Galaxies: Jets

Scientific paper

The production of jets from a Keplerian accretion disk, thread by a bipolar magnetic field, is considered as a successful paradigm for young stellar objects (YSOs) and Radio Loud active galactic nuclei (AGN). However, while the acceleration of the jet is understood, the issue of the expell of a part of the matter from the disk is still an enigma. In this paper, we elucidate the physical process that leads to ejection, exhibit the conditions allowing the steady state launch of jets and expound the properties of these magnetized accretion-ejection structures. As in a huge Barlow wheel, the magnetic field lines extract all the angular momentum and the mechanical power from the disk, while a turbulent magnetic diffusivity allows the matter to pass through the field lines and thus, to be accreted. Because of the disk differential rotation, the radial current decreases as one goes upwards in the disk atmosphere, leading then to a change of sign of the magnetic torque. From this point on, both angular momentum and energy are transfered back to the matter. Magnetic azimuthal acceleration leads to the increase of the angular velocity which drives an outwards radial velocity, while the magnetic pressure associated to the horizontal field develops a vertical acceleration. Hence, magnetically driven jets can be seen as being both centrifugally and magnetic pressure driven. At the still resistive base of the jet, the magnetic force pushes matter towards the magnetic surfaces. A transition between the resistive disk and the ideal MHD jet is then naturally achieved. The intrinsic two-dimensionality of the problem arises as a regularity condition that must be satisfied, in order to smoothly break through a slow magnetosonic critical point. We derive the observational signatures of such a disk, as well as the global energy budget and its consequences on the jets, depending on the mass ejection rate. Magnetized accretion disks supply their jets with almost all the available gravitational power, being then weakly dissipative. We discuss the possibility for the disk to be either convective or radiative, in regard to its settlement around a young stellar object, or within an active galactic nuclei. Self-consistent solutions are displayed in the context of AGN and YSOs.

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