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Astron. Astrophys. 325, 1099-1114


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Analytical studies of collimated winds

IV. Rotating and collimated MHD outflows

E. Trussoni1, K. Tsinganos2, and C. Sauty3

1Osservatorio Astronomico di Torino, Strada dell'Osservatorio 20, I-10025 Pino Torinese (TO), Italy
2Department of Physics, University of Crete and Research Center of Crete, P.O. Box 2208, GR-710 03 Heraklion, Crete, Greece
3Observatoire de Paris, DAEC, URA 173, F-92195 Meudon Cedex, France

Received 13 December 1996 / Accepted 24 Febraury 1997

Abstract

This paper continues the study of the initial acceleration and final collimation of magnetized and rotating astrophysical winds, via analytical and exact steady MHD solutions, self-similar in the meridional direction. By prescribing the shape of the streamlines on the poloidal plane for a nonspherically symmetric gas pressure, related a posteriori to the density via a nonconstant polytropic index FORMULA relationship ( FORMULA ), the main physical features of the outflowing plasma are deduced. Simple analytical relations show that cylindrical collimation and superAlfvénic terminal velocities can be attained asymptotically which depend on the rotation rate, the collimation distance from the base and the pressure gradient. If the plasma is overpressured at the flow axis, the pinching magnetic field can confine the jet, while if the gas is underpressured at its axis, the centrifugal force cannot always counterbalance the pinching magnetic stress and inwards pressure gradient. Physically acceptable solutions are obtained by a numerical integration of the radial dependence of the MHD system from the subAlfvénic to the asymptotically collimated regions and by a smooth crossing of the Alfvén critical surface. Two classes of solutions are found where either the flow speed increases monotonically to an asymptotic value, or it reaches a maximum value at an intermediate region. In the last case it is the toroidal magnetic field that collimates asymptotically the wind, while in the former the outflow of a slow rotator (respectively fast rotator) is collimated by the gas pressure (respectively by the magnetic field). The possible implications of these results on the modelling of astrophysical winds from slow and fast magnetic rotators are shortly discussed.

Key words: MHD - plasmas - Sun: atmosphere - solar wind - stars: atmosphere - ISM: jets and outflows

Send offprint requests to: E. Trussoni (Torino)


© European Southern Observatory (ESO) 1997

Online publication: September 9, 1997
Last change: April 28, 1998
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