206 research outputs found
Outflows and Jets from Collapsing Magnetized Cloud Cores
Star formation is usually accompanied by outflow phenomena. There is strong
evidence that these outflows and jets are launched from the protostellar disk
by magneto-rotational processes. Here, we report on our three dimensional,
adaptive mesh, magneto-hydrodynamic simulations of collapsing, rotating,
magnetized Bonnor-Ebert-Spheres whose properties are taken directly from
observations. In contrast to the pure hydro case where no outflows are seen,
our present simulations show an outflow from the protodisk surface at ~ AU and
a jet at ~ 0.07 AU after a strong toroidal magnetic field build up. The large
scale outflow, which extends up to ~ AU at the end of our simulation, is driven
by toroidal magnetic pressure (spring), whereas the jet is powered by
magneto-centrifugal force (fling). At the final stage of our simulation these
winds are still confined within two respective shock fronts. Furthermore, we
find that the jet-wind and the disk-anchored magnetic field extracts a
considerable amount of angular momentum from the protostellar disk. The initial
spin of our cloud core was chosen high enough to produce a binary system. We
indeed find a close binary system (separation ~ 3 R_sol) which results from the
fragmentation of an earlier formed ring structure. The magnetic field strength
in these protostars reaches ~ 3 kG and becomes about 3 G at 1 AU from the
center in agreement with recent observational results.Comment: revised version, accepted for publication in ApJ, a higher resolution
version of this paper can be downloaded at
http://www.physics.mcmaster.ca/~banerjee/outflows.pd
Accretion and magnetic field morphology around Class 0 stage protostellar discs
We analyse simulations of turbulent, magnetised molecular cloud cores
focussing on the formation of Class 0 stage protostellar discs and the physical
conditions in their surroundings. We show that for a wide range of initial
conditions Keplerian discs are formed in the Class 0 stage already. In
particular, we show that even subsonic turbulent motions reduce the magnetic
braking efficiency sufficiently in order to allow rotationally supported discs
to form. We therefore suggest that already during the Class 0 stage the
fraction of Keplerian discs is significantly higher than 50%, consistent with
recent observational trends but significantly higher than predictions based on
simulations with misaligned magnetic fields, demonstrating the importance of
turbulent motions for the formation of Keplerian discs. We show that the
accretion of mass and angular momentum in the surroundings of protostellar
discs occurs in a highly anisotropic manner, by means of a few narrow accretion
channels. The magnetic field structure in the vicinity of the discs is highly
disordered, revealing field reversals up to distances of 1000 AU. These
findings demonstrate that as soon as even mild turbulent motions are included,
the classical disc formation scenario of a coherently rotating environment and
a well-ordered magnetic field breaks down. Hence, it is highly questionable to
assess the magnetic braking efficiency based on non-turbulent collapse
simulation. We strongly suggest that, in addition to the global magnetic field
properties, the small-scale accretion flow and detailed magnetic field
structure have to be considered in order to assess the likelihood of Keplerian
discs to be present.Comment: 14 pages, 6 figures, accepted for publication in MNRAS, updated to
final versio
The Formation of Star Clusters II: 3D Simulations of Magnetohydrodynamic Turbulence in Molecular Clouds
(Abridged) We present a series of decaying turbulence simulations that
represent a cluster-forming clump within a molecular cloud, investigating the
role of magnetic fields on the formation of potential star-forming cores. We
present an exhaustive analysis of numerical data from these simulations that
includes a compilation of all of the distributions of physical properties that
characterize bound cores - including their masses, radii, mean densities,
angular momenta, spins, magnetizations, and mass-to-flux ratios. We also
present line maps of our models that can be compared with observations. Our
simulations range between 5-30 Jeans masses of gas, and are representative of
molecular cloud clumps with masses between 100-1000 solar masses. The cores
have mass-to-flux ratios that are generally less than that of the original
cloud, and so a cloud that is initially highly supercritical can produce cores
that are slightly supercritical, similar to that seen by Zeeman measurements of
molecular cloud cores. Clouds that are initially only slightly supercritical
will instead collapse along the field lines into sheets, and the cores that
form as these sheets fragment have a different mass spectrum than what is
observed. The spin rates of these cores suggests that subsequent fragmentation
into multiple systems is likely. The sizes of the bound cores that are produced
are typically 0.02-0.2 pc and have densities in the range 10^4-10^5 cm^{-3} in
agreement with observational surveys. Finally, our numerical data allow us to
test theoretical models of the mass spectrum of cores, such as the turbulent
fragmentation picture of Padoan-Nordlund. We find that while this model gets
the shape of the core mass spectrum reasonably well, it fails to predict the
peak mass in the core mass spectrum.Comment: Accepted by MNRAS. 28 pages, 16 figures. Substantial revision since
last versio
Saving Planetary Systems: Dead Zones & Planetary Migration
The tidal interaction between a disk and a planet leads to the planet's
migration. A long-standing question regarding this mechanism is how to stop the
migration before planets plunge into their central stars. In this paper, we
propose a new, simple mechanism to significantly slow down planet migration,
and test the possibility by using a hybrid numerical integrator to simulate the
disk-planet interaction. The key component of the scenario is the role of low
viscosity regions in protostellar disks known as dead zones, which affect
planetary migration in two ways. First of all, it allows a smaller-mass planet
to open a gap, and hence switch the faster type I migration to the slower type
II migration. Secondly, a low viscosity slows down type II migration itself,
because type II migration is directly proportional to the viscosity. We present
numerical simulations of planetary migration by using a hybrid symplectic
integrator-gas dynamics code. Assuming that the disk viscosity parameter inside
the dead zone is (alpha=1e-4-1e-5), we find that, when a low-mass planet (e.g.
1-10 Earth masses) migrates from outside the dead zone, its migration is
stopped due to the mass accumulation inside the dead zone. When a low-mass
planet migrates from inside the dead zone, it opens a gap and slows down its
migration. A massive planet like Jupiter, on the other hand, opens a gap and
slows down inside the dead zone, independent of its initial orbital radius. The
final orbital radius of a Jupiter mass planet depends on the dead zone's
viscosity. For the range of alpha's noted above, this can vary anywhere from 7
AU, to an orbital radius of 0.1 AU that is characteristic of the hot Jupiters.Comment: 38 pages, 14 figures, some changes in text and figures, accepted for
publication in Ap
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