Other
Scientific paper
Sep 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994phdt.......100f&link_type=abstract
PhD thesis, Paris VII University, (1994)
Other
2
Scientific paper
For both active galactic nuclei (AGN) and young stellar objects (YSO), the common belief is growing that there is an interdependency between accretion of mass onto a central object and the highly collimated jets. This thesis deals with the investigation of the physical mechanism that leads to the formation of jets from a magnetized accretion disk. This has been done by solving the set of magnetohydrodynamical (MHD) equations in the case of an isothermal disk, using a self-similar approach. All the dynamical terms are included, so that the main results are independant of the modelling and thus, completely general. Indeed, a different temperature vertical profile only slightly modifies the parameters required for stationarity. A resistive thin accretion disk is thread by open magnetic field lines, sheared by its differential rotation. The field lines brake the disk and extract both angular momentum and mechanical energy from it. Because of the large magnetic "lever arm" acting on the disk, the magnetic braking is always dominant and the viscous torque is negligible. An equipartition magnetic field is enough, without significantly perturbing the Keplerian rotation. Thus, jets carry away all the angular momentum of the underlying accretion disk. Steady state accretion is achieved in the disk due to an anomalous magnetic diffusivity that allows the matter to slip across the field lines. This anomalous transport coefficient should arise from the saturation of a strong magnetic instability triggered in the disk. Ambipolar diffusion, which could have been used without losing the generality of the present results, remains however smaller than this anomalous diffusivity in the inner parts of a circumstellar disk. It has been found that steady state ejection can be achieved only if the magnetic torque changes its sign at the disk surface. From this point on, the field lines accelerate azimuthaly the matter transfering it both angular momentum and energy. This requires a balance between the differential rotation effect (that tends to provide a negative radial current inside the disk) and the "Barlow Wheel" effect (that gives rise to a positive radial current at the disk midplane, hence allowing magnetic braking), leading to the decrease on a disk scale height of the radial current. This means that the field lines are less sheared as one goes upwards to the disk surface. A natural transition between the accretion disk and the jet arises because, as the radial current decreases vertically, the vertical magnetic compression drops and the plasma pressure gradient gives rise to an ascendant motion. Thus, it is the plasma pressure that first drives ejection. The magnetic tension forces then the plasma to be ejected outwardly, being then more and more attached to the poloidal field lines. In the ideal MHD region above the disk, the jet velocity becomes super slow-magnetosonic (first critical point encountered by the flow) and increases due to both magnetic and centrifugal forces. The overall structure is complex, with an equipartition between magnetic and thermal energy densities as well as components of the magnetic field of the same order of magnitude at the disk surface. The full parameter space of such a structure will be fixed by two additional regularity conditions, at the Alfvénic and fast magnetosonic critical points of the jet. We derive, for both AGN and YSO, the observational signatures of optically thick MHD disks driving jets, as well as the global energy budget and its consequences on jets.
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