Magnetic launching and collimation of jets from the disk-magnetosphere boundary: 2.5D MHD simulations

摘要

We use axisymmetric magnetohydrodynamic (MHD) simulations to investigate thelaunching and collimation of jets emerging from the disk-magnetosphere boundaryof accreting magnetized stars. Our analysis shows that the matter flows intothe jet from the inner edge of the accretion disk. It is magneticallyaccelerated along field lines extending up from the disk and simultaneouslycollimated by the magnetic pinch force. In the reference run which we use foranalysis, the matter in the jet crosses the Alfv\'en surface a few R_* abovethe disk and the fast magnetosonic surface ~13 R_* above the disk. At largerdistances, the magnetic pressure is a few times smaller than the total matterpressure but the magnetic force continues to accelerate and collimate the jet.In steady state, we observe a matter ejection-to-accretion ratio of ~0.2.Across different simulation runs, we measure a range of half-opening anglesbetween {\Theta} \approx 4{\deg} and 20{\deg} at the top of the simulationregion, depending on the degree of magnetization in the outflow. We considerthe case of stars undergoing epochs of high accretion (such as EXors, FUORs,and CTTSs) where the stellar magnetosphere is strongly compressed by theincoming accretion disk. For a typical EXor (mass 0.8 M_sun, radius 2 R_sun)accreting at ~10^-5 M_sun/yr, we measure poloidal velocities in the jet rangingfrom 30 km/s on the outer edge of the jet to more than 260 km/s on the inneredge. In general, the models can be applied to a variety of magnetizedstars--white dwarfs, neutron stars, and brown dwarfs--which exhibit periods ofhigh accretion.