Computer Science
Scientific paper
Sep 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999phdt.........7t&link_type=abstract
Thesis (PhD). UNIVERSITY OF CALIFORNIA, SANTA CRUZ, Source DAI-B 60/03, p. 1127, Sep 1999, 143 pages.
Computer Science
1
Disks
Scientific paper
We consider the central 1 AU of an accretion disk around a Solar-mass protostar, and examine how an initially weak magnetic field may be regenerated, what effect the field may have on the evolution of the disk, and what rôle it may play in driving jets and winds. Portions of the disk have Ohmic resistivities large enough to damp modes of the major local linear instabilities present, the magneto-rotational instability (MRI) and Parker buoyancy instability. Using three- dimensional magnetohydrodynamic (MHD) calculations with and without resistivity, we confirm that on a toroidal magnetic field at a given vertical wavelength, the azimuthal wavelength of the MRI which grows fastest in ideal MHD is prevented from growing when resistivity is increased above the level predicted by Papaloizou & Terquem 1997. Within a range of resistivities, modes of the Parker buoyancy instability nevertheless grow at near their ideal-MHD rates. We construct a simple model of the disk in this régime, in which radial magnetic field is regenerated from toroidal field by Coriolis twist acting on rising buoyant elements. Vertical field is also produced by buoyancy, while toroidal field is created by the action of differential rotation on radial field. The model has a steady state in which the largest component of the field is toroidal. The evidence available suggests that the innermost 0.1-1 AU of the disk will generate rising, magnetized fluid elements. We follow the evolution of such an element ejected through a nozzle in the disk surface into an unmagnetized, low-density ambient medium, using axisymmetric MHD calculations. Transfer of orbital angular momentum causes part of the element to spiral towards the central body, where it is driven upwards by gradients in thermal plus toroidal magnetic pressure and forms a jet collimated by toroidal field. After Lorentz forces slow the differential rotation in the material at the base of the jet, the jet ceases. If the mass flux per field line is sufficiently low, material subsequently ejected through the nozzle forms a magneto-centrifugal wind. The speeds and mass flow rates obtained in the jet lie in the ranges observed in protostellar jets and high- velocity outflows from T Tauri stars.
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