FOrmation of protostellar jets as two-component outflows from star-disk magnetospheres

Abstract

Axisymmetric magnetohydrodynamic simulations have been applied to investigate (1) the interrelation between a central stellar magnetosphere and stellar wind with a surrounding magnetized disk outflow, and (2) how the overall formation of a large scale jet is affected by that. The initial magnetic field distribution applied is a superposition of two components - the stellar dipole and the surrounding disk magnetic field - in either parallel or antiparallel alignment. Correspondingly, the mass outflow is launched as stellar wind plus disk wind. Our simulations evolve from an initial state in hydrostatic equilibrium with an initially force-free magnetic field configuration. Due to differential rotation between star and disk, a strong toroidal magnetic field component is induced. The stellar dipole inflates and opens up on large scale. Stellar wind and disk wind may evolve in a pair of collimated outflows. However, the existence of a reasonably strong disk wind component is essential for collimation. The classical disk jet, as known from previous numerical studies, becomes less collimated due to the pressure of the central stellar wind. In some simulations we observe the generation of strong flares triggering a sudden change in the outflow mass loss rate (or velocity) by a factor of two, accompanied by a redistribution in the radial profile of momentum flux and jet velocity across the jet. We discuss the hypothesis that these flares may trigger internal shocks in the asymptotic jets which are observed as knots. © 2009. The American Astronomical Society. All rights reserved.