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| Theory
& Computational AP — Picture 4 Simulations
of Magnetic Collapse |
Image Credit: Winny
Chuang & Tony
Allen
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| These
are moment 0, 1, and 2 plots for MHD collapse
calculations of magnetized, rotating, singular
isothermal toroids. Stars are believed to
have formed from the collapse of molecular
clouds. These clouds are observed to be cold,
with weak magnetic fields, and small initial
rotation. Realistic models of these clouds
have been proposed by Frank Shu (Tsing-Hua
U.) and Zhi-yun Li (UVa). These models have
been used as initial states for magnetohydrodynamic
collapse calculations to study the physics
of the early stages of star formation. The
moment-0 plot shows the false-color column
density an observer might see if viewing the
cloud from a side perpendicular to the axis
of rotation. A high density, magnetically
supported pseudodisk is present in the mid-plane,
but is hard to see with this false-color scheme.
The moment-1 plot shows the velocity profile
of the collapsing cloud. Far from the center,
the pre-collapse, rotational effects are visible.
Closer to the center, one can see slower,
magnetically braked material as well as a
angular-momentum-carrying outflow. The outflow
is driven by the same magneto-centrifugal
mechanism believed to be responsible for jets,
although it is at much lower velocity than
typical jets. This braking mechanism allows
for efficient transport of angular momentum
from a forming protostar to the outer cloud
and is a possible explanation of the angular
momentum removal problem in star formation;
with magnetic braking, central accretion proceeds
at a constant rate allowing growth to stellar
mass-scales. The moment-2 plot hows velocity
dispersion in the region. |
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