Nonradial and nonpolytropic astrophysical outflows

V. Acceleration and collimation of self-similar winds

¹ Université de Paris 7 – Observatoire de Paris, LUTH, unité CNRS FRE 2462, 92190 Meudon, France
² Istituto Nazionale di Astrofisica (INAF) – Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese (TO), Italy
³ Section of Astrophysics, Department of Physics, University of Athens, Panepistimiopolis, 157 84 Zografos, Greece

Corresponding author: C. Sauty, christophe.sauty@obspm.fr

Received: 6 December 2001
Accepted: 16 April 2002

Abstract

An exact model for magnetized and rotating outflows, underpressured at their axis, is analysed by means of a nonlinear separation of the variables in the two-dimensional governing magnetohydrodynamic (MHD) equations for axisymmetric plasmas. The outflow starts subsonically and subAlfvénically from the central gravitating source and its surrounding accretion disk and after crossing the MHD critical points, high values of the Alfvén Mach number may be reached. Three broad types of solutions are found: (a) collimated jet-type outflows from efficient magnetic rotators where the outflow is confined by the magnetic hoop stress; (b) collimated outflows from inefficient magnetic rotators where the outflow is cylindrically confined by thermal pressure gradients; and (c) radially expanding wind-type outflows analogous to the solar wind. In most of the cases examined cylindrically collimated (jet-type) outflows are naturally emerging with thermal and magnetic effects competing in the acceleration and the confinement of the jet. The interplay of all MHD volumetric forces in accelerating and confining the jet is displayed along all its length and for several parameters. The solutions may be used for a physical understanding of astrophysical outflows, such as those associated with young stellar objects, planetary nebulae, extragalactic jets, etc.