Bipolar outflows from stars and galaxies as a tornado phenomenon |
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| Authors: | P F Browne |
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| Institution: | (1) Physics Department, UMIST, PO Box 88, M60 1QD Manchester, UK |
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| Abstract: | At an early stage in the lives of stars and galaxies when they are surrounded by discs, vorticity in the disc concentrates into a central vortex, thus converting a Keplerian velocity fieldu
ø  r
–1/2 into an irrotational velocity fieldu
ør
–1, which implies inward transfer of angular momentum. Centrifugal forces due to spin-up of the inner region and gravity dominant in the outer region then squeeze gas at intermediate layers, increasing pressure gradient in the axial direction sufficiently to drive a wide-angle low-velocity bipolar outflow from the disc. A logarithmic singularity of vorticity at the axis implies strong centrifugal forces which expand plasma to radiusR where pressure gradient balances centrifugal force density of ions; the much weaker centrifugal force density of electrons cannot balance pressure gradient, so that electrons are driven inwards relative to ions until charge separation limits the relative displacement. Now the radial gradient ofu
øcauses ions to rotate at a different rate to electrons, generating an azimuthal current densityj
øwhich is the source of an axial magnetic fieldB
zin the core of the central vortex. Centrifuging carries lines of B to the core wall, where they are wound into helical force-free configuration with B j. An annular channel of radiusR and thickness  R into which parallel helical lines ofj andB are compressed constitutes a  magnetic vortex tube (MVT). An MVT separates an inner high-velocity highly collimated outflow from the outer low-velocity wide-angle outflow, and is responsible for jets. Magnetic pinches in the MVT may constrict the core flow at HH objects. |
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| Keywords: | Outflows Jets |
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