How can outflows such as astrophysical winds and jets form and collimate ? What differences exhibit these two types of plasma flows ? This work makes an attempt to answer those questions thanks to a simple model that deals with the time-independent and axisymmetric MHD equations. The geometry of magnetic surfaces has been assumed to be conical up to the fast magnetosonic critical point. The transversal force balance is calculated along the Alfven surface, and the critically conditions are derived from the Bernoulli equation at the two other critical points. They are used to calculate the specific energy, the angular momentum and the mass loss rate, that are constant for each flux surface. This allows to deal with the asymptotic structure in pressure equilibrium with the surrounding medium. The choice of boundary conditions is unrestricted. It is found that rigid slow rotators, associated to stellar winds, carry a diffused poloidal current, and are gas pressure dominated. On the other hand, rigid fast rotators, corresponding to jets, carry a current concentrated around the polar axis, show strongly distorted critical surfaces and are magnetic pressure dominated at the border of the jet. Regardless the class of rotator, the angular velocity is bounded from above for a given mass loss rate, and regardless an external confining pressure, the collimation of magnetic rotators is asymptotically cylindrical.
|Low Mass Star Formation - from Infall to Outflow, Poster proceedings of IAU Symposium No. 182 on Herbig-Haro Objects and the Birth of Low Mass Stars. 20-24 January 1997, Chamonix, France, Edited by F. Malbet and A. Castets
|Published - 2003