52 research outputs found
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
Nonlinear Criterion for the Stability of Molecular Clouds
Dynamically significant magnetic fields are routinely observed in molecular
clouds, with mass-to-flux ratio lambda = (2 pi sqrt{G}) (Sigma/B) ~ 1 (here
Sigma is the total column density and B is the field strength). It is widely
believed that ``subcritical'' clouds with lambda < 1 cannot collapse, based on
virial arguments by Mestel and Spitzer and a linear stability analysis by
Nakano and Nakamura. Here we confirm, using high resolution numerical models
that begin with a strongly supersonic velocity dispersion, that this criterion
is a fully nonlinear stability condition. All the high-resolution models with
lambda <= 0.95 form ``Spitzer sheets'' but collapse no further. All models with
lambda >= 1.02 collapse to the maximum numerically resolvable density. We also
investigate other factors determining the collapse time for supercritical
models. We show that there is a strong stochastic element in the collapse time:
models that differ only in details of their initial conditions can have
collapse times that vary by as much as a factor of 3. The collapse time cannot
be determined from just the velocity dispersion; it depends also on its
distribution. Finally, we discuss the astrophysical implications of our
results.Comment: 11 pages, 5 figure
A Unified Model for Bipolar Outflows from Young Stars: Apparent Magnetic Jet Acceleration
We explore a new, efficient mechanism that can power toroidally magnetized
jets up to two to three times their original terminal velocity after they enter
a self-similar phase of magnetic acceleration. Underneath the elongated outflow
lobe formed by a magnetized bubble, a wide-angle free wind, through the
interplay with its ambient toroid, is compressed and accelerated around its
axial jet. The extremely magnetic bubble can inflate over its original size,
depending on the initial Alfv\'en Mach number of the launched flow. The
shape-independent slope is a salient feature
of the self-similarity in the acceleration phase. Peculiar kinematic signatures
are observable in the position--velocity (PV) diagrams and can combine with
other morphological signatures as probes for the density-collimated jets
arising in toroidally dominated magnetized winds. The apparent second
acceleration is powered by the decrease of the toroidal magnetic field but
operates far beyond the scales of the primary magnetocentrifugal launch region
and the free asymptotic terminal state. Rich implications may connect the jets
arising from the youngest protostellar outflows such as HH 211 and HH 212 and
similar systems with parsec-scale jets across the mass and evolutionary
spectra.Comment: 21 pages, 8 figures, 1 table, to appear in Astrophysical Journal
Letters (2023
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