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

TL;DR

This study uses axisymmetric MHD simulations to explore how star-disk magnetic interactions influence the formation, collimation, and variability of protostellar jets, revealing the importance of disk winds and flare-induced shocks.

Contribution

It introduces a model with combined stellar dipole and disk magnetic fields, demonstrating their roles in jet collimation and variability, including flare effects and internal shock formation.

Findings

Stellar and disk winds can form collimated outflows.

A strong disk wind component is crucial for jet collimation.

Flares can induce internal shocks and knot formation in jets.

Abstract

Axisymmetric magnetohydrodynamic (MHD) simulations have been applied to investigate the interrelation of a central stellar magnetosphere and stellar wind with a surrounding magnetized disk outflow and how the overall formation of a large scale jet is affected. The initial magnetic field distribution applied is a superposition of two components - a stellar dipole and a surrounding disk magnetic field, in both either parallel or anti-parallel alignment. Correspondingly, the mass outflow is launched as stellar wind plus a disk wind. Our simulations evolve from an initial state in hydrostatic equilibrium and an initially force-free magnetic field configuration. Due to initial differential rotation and induction of a strong toroidal magnetic field the stellar dipolar field inflates and is disrupted on large scale. Stellar and disk wind may evolve in a pair of collimated outflows. The existence of a reasonably strong disk wind component is essential for collimation. A disk jet as known from previous numerical studies will become de-collimated by the stellar wind. In some simulations we observe the generation of strong flares triggering a sudden change in the outflow mass loss rate by a factor of two and also a re-distribution 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.