922 research outputs found
Protostellar Accretion Flows Destabilized by Magnetic Flux Redistribution
Magnetic flux redistribution lies at the heart of the problem of star
formation in dense cores of molecular clouds that are magnetized to a realistic
level. If all of the magnetic flux of a typical core were to be dragged into
the central star, the stellar field strength would be orders of magnitude
higher than the observed values. This well-known "magnetic flux problem" can in
principle be resolved through non-ideal MHD effects. Two dimensional
(axisymmetric) calculations have shown that ambipolar diffusion, in particular,
can transport magnetic flux outward relative to matter, allowing material to
enter the central object without dragging the field lines along. We show
through simulations that such axisymmetric protostellar accretion flows are
unstable in three dimensions to magnetic interchange instability in the
azimuthal direction. The instability is driven by the magnetic flux
redistributed from the matter that enters the central object. It typically
starts to develop during the transition from the prestellar phase of star
formation to the protostellar mass accretion phase. In the latter phase, the
magnetic flux is transported outward mainly through advection, by strongly
magnetized low-density regions that expand against the collapsing inflow. The
tussle between the gravity-driven infall and magnetically driven expansion
leads to a filamentary inner accretion flow, more disordered than previously
pictured. The efficient outward transport of magnetic flux by advection lowers
the field strength at small radii, making the magnetic braking less efficient
and the formation of rotationally supported disks easier in principle. However,
we find no evidence for such disks in any of our rotating collapse simulations.
We conclude that the inner protostellar accretion flow is shaped to a large
extent by this magnetic interchange instability. How disks form in such an
environment is unclear.Comment: 14 pages, 8 figures, submitted to Ap
Magneto-Centrifugal Launching of Jets from Accretion Disks. I: Cold Axisymmetric Flows
The magneto-centrifugal model for jet formation is studied by time-dependent
simulations reaching steady state in a cold gas with negligible fluid pressure,
in an axisymmetric geometry, using a modification of the Zeus3D code adapted to
parallel computers. The number of boundary conditions imposed at the coronal
base takes into account the existence of the fast and Alfvenic critical
surfaces, avoiding over-determination of the flow. The size and shape of the
computational box is chosen to include these critical surfaces, reducing the
influence of the outer boundary conditions. As there is a region, near the
origin, where the inclination of field lines to the axis is too small to drive
a centrifugal wind, we inject a thin, axial jet, expected to form
electromagnetically near black holes. Acceleration and collimation appear for
wide generic conditions. A reference run is shown in detail, with a wind
leaving the computational volume in the axial direction with a poloidal
velocity equal to 4 times the poloidal Alfven speed, collimated inside 11
degrees. Finally, the critical surfaces, fieldlines, thrust, energy, torque and
mass discharge of the outgoing wind are shown for simulations with various
profiles of mass and magnetic flux at the base of the corona.Comment: 27 pages, including 10 figures and 2 tables. To appear in ApJ (Dec
1999). Revised version clarifies the abstract, section 3.2.4, conclusions and
appendix, adds a simulation to section 4.2, and updates the reference
Structure of Magnetocentrifugal Disk-Winds: From the Launching Surface to Large Distances
Protostellar jets and winds are probably driven magnetocentrifugally from the
surface of accretion disks close to the central stellar objects. The exact
launching conditions on the disk, such as the distributions of magnetic flux
and mass ejection rate, are poorly unknown. They could be constrained from
observations at large distances, provided that a robust model is available to
link the observable properties of the jets and winds at the large distances to
the conditions at the base of the flow. We discuss the difficulties in
constructing such large-scale wind models, and describe a novel technique which
enables us to numerically follow the acceleration and propagation of the wind
from the disk surface to arbitrarily large distances and the collimation of
part of the wind into a dense, narrow ``jet'' around the rotation axis. Special
attention is paid to the shape of the jet and its mass flux relative to that of
the whole wind. The mass flux ratio is a measure of the jet formation
efficiency.Comment: 6 pages, figures included. To appear in "The Origins of Stars and
Planets: The VLT View". J. Alves and M. McCaughrean, editor
Magnetic Flux Expulsion in Star Formation
Stars form in dense cores of magnetized molecular clouds. If the magnetic
flux threading the cores is dragged into the stars, the stellar field would be
orders of magnitude stronger than observed. This well-known "magnetic flux
problem" demands that most of the core magnetic flux be decoupled from the
matter that enters the star. We carry out the first exploration of what happens
to the decoupled magnetic flux in 3D, using an MHD version of the ENZO adaptive
mesh refinement code. The field-matter decoupling is achieved through a sink
particle treatment, which is needed to follow the protostellar accretion phase
of star formation. We find that the accumulation of the decoupled flux near the
accreting protostar leads to a magnetic pressure buildup. The high pressure is
released anisotropically, along the path of least resistance. It drives a
low-density expanding region in which the decoupled magnetic flux is expelled.
This decoupling-enabled magnetic structure has never been seen before in 3D MHD
simulations of star formation. It generates a strong asymmetry in the
protostellar accretion flow, potentially giving a kick to the star. In the
presence of an initial core rotation, the structure presents an obstacle to the
formation of a rotationally supported disk, in addition to magnetic braking, by
acting as a rigid magnetic wall that prevents the rotating gas from completing
a full orbit around the central object. We conclude that the decoupled magnetic
flux from the stellar matter can strongly affect the protostellar collapse
dynamics
Using Machine Learning for Model Physics: an Overview
In the overview, a generic mathematical object (mapping) is introduced, and
its relation to model physics parameterization is explained. Machine learning
(ML) tools that can be used to emulate and/or approximate mappings are
introduced. Applications of ML to emulate existing parameterizations, to
develop new parameterizations, to ensure physical constraints, and control the
accuracy of developed applications are described. Some ML approaches that allow
developers to go beyond the standard parameterization paradigm are discussed.Comment: 50 pages, 3 figures, 1 tabl
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