Patrick S Lii, Marina M. Romanova, Richard V. Lovelace
We use axisymmetric magnetohydrodynamic (MHD) simulations to investigate the
launching and collimation of jets emerging from the disk-magnetosphere boundary
of accreting magnetized stars. Our analysis shows that the matter flows into
the jet from the inner edge of the accretion disk. It is magnetically
accelerated along field lines extending up from the disk and simultaneously
collimated by the magnetic pinch force. In the reference run which we use for
analysis, the matter in the jet crosses the Alfv\'en surface a few R_* above
the disk and the fast magnetosonic surface ~13 R_* above the disk. At larger
distances, the magnetic pressure is a few times smaller than the total matter
pressure 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 angles
between {\Theta} \approx 4{\deg} and 20{\deg} at the top of the simulation
region, depending on the degree of magnetization in the outflow. We consider
the case of stars undergoing epochs of high accretion (such as EXors, FUORs,
and CTTSs) where the stellar magnetosphere is strongly compressed by the
incoming 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 ranging
from 30 km/s on the outer edge of the jet to more than 260 km/s on the inner
edge. In general, the models can be applied to a variety of magnetized
stars--white dwarfs, neutron stars, and brown dwarfs--which exhibit periods of
high accretion.
View original:
http://arxiv.org/abs/1104.4374
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