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## Nonaxisymmetric magnetorotational instability in ideal and viscous plasmas

Fonte: AMER INST PHYSICS
Publicador: AMER INST PHYSICS

Tipo: Artigo de Revista Científica

ENG

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36.06%

#WEAKLY MAGNETIZED DISKS#LOCAL SHEAR INSTABILITY#CYLINDRICAL PLASMA#ACCRETION DISKS#STABILITY#FLOWS#EQUILIBRIUM#GEOMETRY#Physics, Fluids & Plasmas

The excitation of magnetorotational instability (MRI) in rotating laboratory plasmas is investigated. In contrast to astrophysical plasmas, in which gravitation plays an important role, in laboratory plasmas it can be neglected and the plasma rotation is equilibrated by the pressure gradient. The analysis is restricted to the simple model of a magnetic confinement configuration with cylindrical symmetry, in which nonaxisymmetric perturbations are investigated using the local approximation. Starting from the simplest case of an ideal plasma, the corresponding dispersion relations are derived for more complicated models including the physical effects of parallel and perpendicular viscosities. The Friemann-Rotenberg approach used for ideal plasmas is generalized for the viscous model and an analytical expression for the instability boundary is obtained. It is shown that, in addition to the standard effect of radial derivative of the rotation frequency (the Velikhov effect), which can be destabilizing or stabilizing depending on the sign of this derivative in the ideal plasma, there is a destabilizing effect proportional to the fourth power of the rotation frequency, or, what is the same, to the square of the plasma pressure gradient, and to the square of the azimuthal mode number of the perturbations. It is shown that the instability boundary also depends on the product of the plasma pressure and density gradients...

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## Contributions to the theory of magnetorotational instability and waves in a rotating plasma

Fonte: MAIK NAUKA/INTERPERIODICA/SPRINGER
Publicador: MAIK NAUKA/INTERPERIODICA/SPRINGER

Tipo: Artigo de Revista Científica

ENG

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36.06%

#WEAKLY MAGNETIZED DISKS#LOCAL SHEAR INSTABILITY#3-DIMENSIONAL MAGNETOHYDRODYNAMIC SIMULATIONS#STELLAR RADIATIVE ZONES#KINETIC ALFVEN WAVES#ACCRETION DISKS#INTERCHANGE INSTABILITY#GLOBAL STABILITY#LIQUID-METAL#COUETTE-FLOW#Physics, Multidisciplinary

The one-fluid magnetohydrodynamic (MHD) theory of magnetorotational instability (MRI) in an ideal plasma is presented. The theory predicts the possibility of MRI for arbitrary 0, where 0 is the ratio of the plasma pressure to the magnetic field pressure. The kinetic theory of MRI in a collisionless plasma is developed. It is demonstrated that as in the ideal MHD, MRI can occur in such a plasma for arbitrary P. The mechanism of MRI is discussed; it is shown that the instability appears because of a perturbed parallel electric field. The electrodynamic description of MRI is formulated under the assumption that the dispersion relation is expressed in terms of the permittivity tensor; general properties of this tensor are analyzed. It is shown to be separated into the nonrotational and rotational parts. With this in mind, the first step for incorporation of MRI into the general theory of plasma instabilities is taken. The rotation effects on Alfven waves are considered.

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## Angular Momentum Transport and Variability in Boundary Layers of Accretion Disks Driven by Global Acoustic Modes

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

36.08%

Disk accretion onto a weakly magnetized central object, e.g. a star, is
inevitably accompanied by the formation of a boundary layer near the surface,
in which matter slows down from the highly supersonic orbital velocity of the
disk to the rotational velocity of the star. We perform high resolution 2D
hydrodynamical simulations in the equatorial plane of an astrophysical boundary
layer with the goal of exploring the dynamics of non-axisymmetric structures
that form there. We generically find that the supersonic shear in the boundary
layer excites non-axisymmetric quasi-stationary acoustic modes that are trapped
between the surface of the star and a Lindblad resonance in the disk. These
modes rotate in a prograde fashion, are stable for hundreds of orbital periods,
and have a pattern speed that is less than and of order the rotational velocity
at the inner edge of the disk. The origin of these intrinsically global modes
is intimately related to the operation of a corotation amplifier in the system.
Dissipation of acoustic modes in weak shocks provides a universal mechanism for
angular momentum and mass transport even in purely hydrodynamic (i.e.
non-magnetized) boundary layers. We discuss the possible implications of these
trapped modes for explaining the variability seen in accreting compact objects.; Comment: 41 pages...

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## Waves and Instabilities in Accretion Disks: MHD Spectroscopic Analysis

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 14/03/2002

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26.29%

A complete analytical and numerical treatment of all magnetohydrodynamic
waves and instabilities for radially stratified, magnetized accretion disks is
presented. The instabilities are a possible source of anomalous transport.
While recovering results on known hydrodynamicand both weak- and strong-field
magnetohydrodynamic perturbations, the full magnetohydrodynamic spectra for a
realistic accretion disk model demonstrates a much richer variety of
instabilities accessible to the plasma than previously realized. We show that
both weakly and strongly magnetized accretion disks are prone to strong
non-axisymmetric instabilities.The ability to characterize all waves arising in
accretion disks holds great promise for magnetohydrodynamic spectroscopic
analysis.; Comment: FOM-Institute for plasma physics "Rijnhuizen", Nieuwegein, the
Netherlands 12 pages, 3 figures, Accepted for publication in ApJL

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## The Formation and Structure of a Strongly Magnetized Corona above Weakly Magnetized Accretion Disks

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 07/12/1999

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46.54%

We use three-dimensional magnetohydrodynamical (MHD) simulations to study the
formation of a corona above an initially weakly magnetized, isothermal
accretion disk. We also describe a modification to time-explicit numerical
algorithms for MHD which enables us to evolve highly stratified disks for many
orbital times. We find that MHD turbulence driven by the magnetorotational
instability (MRI) produces strong amplification of weak fields within two scale
heights of the disk midplane in a few orbital times. About 25 % of the magnetic
energy generated by the MRI within two scale heights escapes due to buoyancy,
producing a strongly magnetized corona above the disk. Most of the buoyantly
rising magnetic energy is dissipated between 3 and 5 scale heights, suggesting
the corona will also be hot. The average vertical disk structure consists of a
weakly magnetized turbulent core below a strongly magnetized corona which is
stable to the MRI. The largescale field structure in both the disk and corona
is toroidal. The functional form of the stress is flat within two scale
heights, but proportional to the density above two scale heights.
For initially weak uniform vertical fields, we find the exponential growth of
magnetic field via axisymmetric vertical modes of the MRI produces strongly
buoyant sheets of magnetic energy which break the disk apart into horizontal
channels. These channels rise several scale heights vertically before the onset
of the Parker instability distorts the sheets and allows matter to flow back
towards the midplane and reform a disk. We suggest this evolution may be
relevant to the dynamical processes which disrupt the inner regions of a disk
when it interacts with a strongly magnetized central object.; Comment: 30 pages...

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## Magneto-rotational overstability in accretion disks

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

36.34%

We present analytical and numerical studies of magnetorotational
instabilities occuring in magnetized accretion disks. In these studies we make
use of the linearised compressible MHD equations. These calculations are
performed for general radially stratified disks in the cylindrical limit. In
particular, we investigate the influence of nonvanishing toroidal magnetic
field component on the growth rate and oscillation frequency of
magnetorotational instabilities in Keplerian disks. We find the persistence of
these instabilities in accretion disks close to equipartition. Our calculations
show that these eigenmodes become overstable (complex eigenvalue), due to the
presence of a toroidal magnetic field component, while their growth rate
reduces slightly. Furthermore, we demonstrate the presence of
magneto-rotational overstabilities in weakly magnetized sub-Keplerian rotating
disks. We show that the growth rate scales with the rotation frequency of the
disk. These eigenmodes also have a nonzero oscillation frequency, due to the
presence of the dominant toroidal magnetic field component. The overstable
character of the MRI increases as the rotation frequency of the disk decreases.; Comment: 11 pager, 18 Postscript figures, accepted for publication in
Astronomy & Astrophysics

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## Convective magneto-rotational instabilities in accretion disks

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 18/04/2005

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36.32%

We present a study of instabilities occuring in thick magnetized accretion
disks. We calculate the growth rates of these instabilities and characterise
precisely the contribution of the magneto-rotational and the convective
mechanism. All our calculations are performed in radially stratified disks in
the cylindrical limit. The numerical calculations are performed using the
appropriate local dispersion equation solver discussed in Blokland et al.
(2005). A comparison with recent results by Narayan et al. (2002) shows
excellent agreement with their approximate growth rates only if the disks are
weakly magnetized. However, for disks close to equipartition, the dispersion
equation from Narayan et al. (2002) loses its validity. Our calculations allow
for a quantitative determination of the increase of the growth rate due to the
magneto-rotational mechanism. We find that the increase of the growth rate for
long wavelength convective modes caused by this mechanism is almost neglible.
On the other hand, the growth rate of short wavelength instabilities can be
significantly increased by this mechanism, reaching values up to 60%.; Comment: 10 pages, 9 figures, Accepted for publication in Astronomy &
Astrophysics

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## Transonic Magnetic Slim Accretion Disks and kilo-Hertz Quasi-Periodic Oscillations in Low-Mass X-Ray Binaries

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

36.06%

The inner regions of accretion disks of weakly magnetized neutron stars are
affected by general relativity and stellar magnetic fields. Even for field
strengths sufficiently small so that there is no well-defined magnetosphere
surrounding the neutron star, there is still a region in the disk where
magnetic field stress plays an important dynamical role. We construct magnetic
slim disk models appropriate for neutron stars in low-mass X-ray binaries
(LMXBs) which incorporate both effects (GR and magnetic fields). The
B-field--disk interaction is treated in a phenomenological manner, allowing for
both closed and open field configurations. We show that even for surface
magnetic fields as weak as $10^7-10^8$ G, the sonic point of the accretion flow
can be significantly modified from the pure GR value (near $6M$). We derive an
approximate analytical expression for the sonic radius and show that it mainly
depends on the surface field strength $B_0$ and mass accretion rate $\dot M$
through the ratio $b^2\propto B_0^2/\dot M$. The sonic radius thus obtained
approaches the usual Alfven radius for high $b^2$, and asymptotes to $6M$ as
$b^2\to 0$. We therefore suggest that for neutron stars in LMXBs, the
distinction between the disk sonic radius and the magnetospheric radius may not
exist. We apply our theoretical results to the kHz QPOs observed in the X-ray
fluxes of LMXBs. If these QPOs are associated with the orbital frequency at the
inner radius of the disk...

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## Formation of Spiral-Arm Spurs and Bound Clouds in Vertically Stratified Galactic Gas Disks

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 28/03/2006

Relevância na Pesquisa

36.36%

(Abridged) We investigate the growth of spiral-arm substructure in vertically
stratified, self-gravitating, galactic gas disks, using local numerical MHD
simulations. Our new models extend our previous 2D studies (Kim & Ostriker
2002), which showed that a magnetized spiral shock in a thin disk can undergo
magneto-Jeans instability (MJI), resulting in interarm spur structures and
massive fragments. Similar spur features have recently been seen in
high-resolution observations of several galaxies. Here, we consider two sets of
numerical models: 2D models that use a thick-disk gravitational kernel, and 3D
runs with explicit vertical stratification. When disks are sufficiently
magnetized and self-gravitating, the result in both sorts of models is the
growth of spiral-arm substructure similar to that in our previous razor-thin
models. Reduced self-gravity in thick disks increases the spur spacing to ~10
times the Jeans length at the arm peak. Bound clouds that form from spur
fragmentation have masses ~(1-3)x10^7 Msun each, a factor ~3-8 times larger
than in razor-thin models with the same gas surface density and stellar spiral
arm strength. We find that unmagnetized or weakly magnetized 2D models are
unstable to the wiggle instability (WI) previously identified by Wada & Koda
(2004)...

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## Large scale magnetic fields in viscous resistive accretion disks. I. Ejection from weakly magnetized disks

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 23/03/2010

Relevância na Pesquisa

46.29%

Cold steady-state disk wind theory from near Keplerian accretion disks
requires a large scale magnetic field at near equipartition strength. However
the minimum magnetization has never been tested. We investigate the time
evolution of an accretion disk threaded by a weak vertical magnetic field. The
strength of the field is such that the disk magnetization falls off rapidly
with radius. Four 2.5D numerical simulations of viscous resistive accretion
disk are performed using the magnetohydrodynamic code PLUTO. In these
simulations, a mean field approach is used and turbulence is assumed to give
rise to anomalous transport coefficients (alpha prescription). The large scale
magnetic field introduces only a small perturbation to the disk structure, with
accretion driven by the dominant viscous torque. A super fast magnetosonic jet
is observed to be launched from the innermost regions and remains stationary
over more than 953 Keplerian orbits. The self-confined jet is launched from a
finite radial zone in the disk which remains constant over time. Ejection is
made possible because the magnetization reaches unity at the disk surface, due
to the steep density decrease. However, no ejection is reported when the
midplane magnetization becomes too small. The asymptotic jet velocity remains
nevertheless too low to explain observed jets due to the negligible power
carried away by the jet. Astrophysical disks with superheated surface layers
could drive analogous outflows even if their midplane magnetization is low.
Sufficient angular momentum would be extracted by the turbulent viscosity to
allow the accretion process to continue. The magnetized outflows would be no
more than byproducts...

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## Does Magnetic Field-Rotation Misalignment Solve the Magnetic Braking Catastrophe in Protostellar Disk Formation?

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 25/01/2013

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26.31%

Stars form in dense cores of molecular clouds that are observed to be
significantly magnetized. In the simplest case of a laminar (non-turbulent)
core with the magnetic field aligned with the rotation axis, both analytic
considerations and numerical simulations have shown that the formation of a
large, $10^2\au$-scale, rotationally supported protostellar disk is suppressed
by magnetic braking in the ideal MHD limit for a realistic level of core
magnetization. This theoretical difficulty in forming protostellar disks is
termed "magnetic braking catastrophe". A possible resolution to this problem,
proposed by \citeauthor{HennebelleCiardi2009} and \citeauthor{Joos+2012}, is
that misalignment between the magnetic field and rotation axis may weaken the
magnetic braking enough to enable disk formation. We evaluate this possibility
quantitatively through numerical simulations. We confirm the basic result of
\citeauthor{Joos+2012} that the misalignment is indeed conducive to disk
formation. In relatively weakly magnetized cores with dimensionless
mass-to-flux ratio $\gtrsim 5$, it enabled the formation of rotationally
supported disks that would otherwise be suppressed if the magnetic field and
rotation axis are aligned. For more strongly magnetized cores...

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## Magnetohydrodynamic Simulations of Accretion Disks around a Weakly Magnetized Neutron Star in Strong Gravity

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

46.18%

We carried out two dimensional high-resolution magnetohydrodynamic (MHD)
simulations of an accretion disk around a weakly magnetized neutron star.
General relativistic effects are taken into account by using pseudo-Newtonian
potential of Pacz\'{y}nski, B., P. J. Witta (1980). When magnetic loops connect
the neutron star and the accretion disk, the twist injection from the disk or
from the rotating neutron star triggers expansion of the loops. Since the
expanding magnetic loops prevent inflow toward the magnetic poles of the
neutron star, disk matter accumulates on the boundary between the magnetosphere
and the disk. Magnetic reconnection taking place in the loops creates a channel
along which the disk matter can accrete and unloads the magnetosphere. This
process produces quasi-periodic variation of the accretion flow in the
innermost region of the disk. We found two kinds of oscillations. One is the
magnetospheric oscillation regulated by magnetic reconnection. The other is the
radial disk oscillation. The typical frequency of the oscillations is 100 Hz to
2 kHz. Furthermore, we predict that QPO sources inevitably accompany X-ray
flares by magnetic reconnection and bipolar outflows of hot X-ray emitting
plasma similar to the optical jets in protostars.; Comment: 5 pages...

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## Local Dynamical Instabilities in Magnetized, Radiation Pressure Supported Accretion Disks

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 03/11/2000

Relevância na Pesquisa

36.26%

We present a general linear dispersion relation which describes the coupled
behavior of magnetorotational, photon bubble, and convective instabilities in
weakly magnetized, differentially rotating accretion disks. We presume the
accretion disks to be geometrically thin and supported vertically by radiation
pressure. We fully incorporate the effects of a nonzero radiative diffusion
length on the linear modes. In an equilibrium with purely vertical magnetic
field, the vertical magnetorotational modes are completely unaffected by
compressibility, stratification, and radiative diffusion. However, in the
presence of azimuthal fields, which are expected in differentially rotating
flows, the growth rate of all magnetorotational modes can be reduced
substantially below the orbital frequency. This occurs if diffusion destroys
radiation sound waves on the length scale of the instability, and the magnetic
energy density of the azimuthal component exceeds the non-radiative thermal
energy density. While sluggish in this case, the magnetorotational instability
still persists and will still tap the free energy of the differential rotation.
Photon bubble instabilities are generically present in radiation pressure
dominated flows where diffusion is present. We show that their growth rates are
limited to a maximum value which is reached at short wavelengths where the
modes may be viewed as unstable slow magnetosonic waves. We also find that
vertical radiation pressure destabilizes upward propagating fast waves...

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## Precession of Magnetically Driven Warped Disks and Low-Frequency QPOs in Low-Mass X-Ray Binaries

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

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An accretion disk around a rotating magnetized star is subjected to magnetic
torques which induce disk warping and precession. These torques arise from
interactions between the stellar field and the induced surface currents on the
disk. Applying these new effects to weakly magnetized ($B\sim 10^7-10^9$G)
neutron stars in low-mass X-ray binaries, we study the global hydrodynamical
warping/precession modes of the disk under the combined influences of
relativistic frame dragging, classical precession due to the oblateness of the
neutron star, and the magnetic torques. Under quite general conditions, the
magnetic warping torque can overcome the ``Bardeen-Petterson'' viscous damping
and makes the modes grow. The modes are confined to the inner region of the
disk, and have frequencies close to the sum of the Lense-Thirring frequency,
the classical precession frequency, and the magnetically driven precession
frequency evaluated at the inner disk radius $r_{in}$. As $\dot M$ increases,
the mode frequency is reduced relative to the total precession frequency at
$r_{in}$ since the mode is less concentrated around $r_{in}$ due to the
increasing viscous stress associated with the large $\dot M$. Because of this,
and because the magnetically driven precession is retrograde and depends
strongly on $\dot M$...

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## Evolution of self-gravitating magnetized disks. II- Interaction between MHD turbulence and gravitational instabilities

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 16/09/2004

Relevância na Pesquisa

56.26%

We present 3D magnetohydrodynamic (MHD) numerical simulations of the
evolution of self--gravitating and weakly magnetized disks with an adiabatic
equation of state. Such disks are subject to the development of both the
magnetorotational and gravitational instabilities, which transport angular
momentum outward. As in previous studies, our hydrodynamical simulations show
the growth of strong m=2 spiral structure. This spiral disturbance drives
matter toward the central object and disappears when the Toomre parameter Q has
increased well above unity. When a weak magnetic field is present as well, the
magnetorotational instability grows and leads to turbulence. In that case, the
strength of the gravitational stress tensor is lowered by a factor of about~2
compared to the hydrodynamical run and oscillates periodically, reaching very
small values at its minimum. We attribute this behavior to the presence of a
second spiral mode with higher pattern speed than the one which dominates in
the hydrodynamical simulations. It is apparently excited by the high frequency
motions associated with MHD turbulence. The nonlinear coupling between these
two spiral modes gives rise to a stress tensor that oscillates with a frequency
which is a combination of the frequencies of each of the modes. This
interaction between MHD turbulence and gravitational instabilities therefore
results in a smaller mass accretion rate onto the central object.; Comment: 31 pages...

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## Collisions between Dark Matter Confined High Velocity Clouds and Magnetized Galactic Disks: The Smith Cloud

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 18/11/2015

Relevância na Pesquisa

36.08%

The Galaxy's population of High Velocity Clouds (HVCs) may include a
subpopulation that is confined by dark matter minihalos and falling toward the
Galactic disk. We present the first magnetohydrodynamic simulational study of
dark matter-dominated HVCs colliding with a weakly magnetized galactic disk.
Our HVCs have baryonic masses of $5 \times 10^6\,$M$_{\odot}$ and dark matter
minihalo masses of 0, $3 \times 10^8$, or $1 \times 10^9\,$M$_{\odot}$. They
are modeled on the Smith Cloud, which is said to have collided with the disk 70
Myr ago. We find that, in all cases, the cloud's collision with the galactic
disk creates a hole in the disk, completely disperses the cloud, and forms a
bubble-shaped structure on the far side of the disk. In contrast, when present,
the dark matter minihalo continues unimpeded along its trajectory. Later, as
the minihalo passes through the bubble structure and galactic halo, it accretes
up to $6.0 \times 10^5\,$M$_{\odot}$ in baryonic material, depending on the
strengths of the magnetic field and minihalo gravity. These simulations suggest
that if the Smith Cloud is associated with a dark matter minihalo and collided
with the Galactic disk, the minihalo has accreted the observed gas. However, if
the Smith Cloud is dark matter-free...

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## Vertical Structure of Magnetized Accretion Disks around Young Stars

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 03/12/2015

Relevância na Pesquisa

46.68%

We model the vertical structure of magnetized accretion disks subject to
viscous and resistive heating, and irradiation by the central star. We apply
our formalism to the radial structure of magnetized accretion disks threaded by
a poloidal magnetic field dragged during the process of star formation
developed by Shu and coworkers. We consider disks around low mass protostars, T
Tauri, and FU Orionis stars. We consider two levels of disk magnetization,
$\lambda_{sys} = 4$ (strongly magnetized disks), and $\lambda_{sys} = 12$
(weakly magnetized disks). The rotation rates of strongly magnetized disks have
large deviations from Keplerian rotation. In these models, resistive heating
dominates the thermal structure for the FU Ori disk. The T Tauri disk is very
thin and cold because it is strongly compressed by magnetic pressure; it may be
too thin compared with observations. Instead, in the weakly magnetized disks,
rotation velocities are close to Keplerian, and resistive heating is always
less than 7\% of the viscous heating. In these models, the T Tauri disk has a
larger aspect ratio, consistent with that inferred from observations. All the
disks have spatially extended hot atmospheres where the irradiation flux is
absorbed, although most of the mass ($\sim 90-95$ \%) is in the disk midplane.
With the advent of ALMA one expects direct measurements of magnetic fields and
their morphology at disk scales. It will then be possible to determine the
mass-to-flux ratio of magnetized accretion disks around young stars...

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## Radially Extended, Stratified, Local Models of Isothermal Disks

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 16/12/2010

Relevância na Pesquisa

36.26%

We consider local, stratified, numerical models of isothermal accretion
disks. The novel feature of our treatment is that radial extent L_x and
azimuthal extent L_y satisfy H << L_x, L_y << R, where H is the scale height
and R is the local radius. This enables us to probe mesoscale structure in
stratified thin disks. We evolve the model at several resolutions, sizes, and
initial magnetic field strengths. Consistent with earlier work, we find that
the saturated, turbulent state consists of a weakly magnetized disk midplane
coupled to a strongly magnetized corona, with a transition at |z| ~ 2H. The
saturated \alpha ~ 0.01 - 0.02. A two-point correlation function analysis
reveals that the central 4H of the disk is dominated by small scale turbulence
that is statistically similar to unstratified disk models, while the coronal
magnetic fields are correlated on scales ~ 10 H. Nevertheless angular momentum
transport through the corona is small. A study of magnetic field loops in the
corona reveals few open field lines and predominantly toroidal loops with a
characteristic distance between footpoints that is ~ H. Finally we find
quasi-periodic oscillations with characteristic timescale ~ 30 \Omega^{-1} in
the magnetic field energy density. These oscillations are correlated with
oscillations in the mean azimuthal field; we present a phenomenological...

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## The Stability of Magnetized Rotating Plasmas with Superthermal Fields

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

36.15%

During the last decade it has become evident that the magnetorotational
instability is at the heart of the enhanced angular momentum transport in
weakly magnetized accretion disks around neutron stars and black holes. In this
paper, we investigate the local linear stability of differentially rotating,
magnetized flows and the evolution of the magnetorotational instability beyond
the weak-field limit. We show that, when superthermal toroidal fields are
considered, the effects of both compressibility and magnetic tension forces,
which are related to the curvature of toroidal field lines, should be taken
fully into account. We demonstrate that the presence of a strong toroidal
component in the magnetic field plays a non-trivial role. When strong fields
are considered, the strength of the toroidal magnetic field not only modifies
the growth rates of the unstable modes but also determines which modes are
subject to instabilities. We find that, for rotating configurations with
Keplerian laws, the magnetorotational instability is stabilized at low
wavenumbers for toroidal Alfven speeds exceeding the geometric mean of the
sound speed and the rotational speed. We discuss the significance of our
findings for the stability of cold, magnetically dominated...

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## Two timescale dispersal of magnetized protoplanetary disks

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 24/10/2013

Relevância na Pesquisa

36.29%

Protoplanetary disks are likely to be threaded by a weak net flux of vertical
magnetic field that is a remnant of the much larger fluxes present in molecular
cloud cores. If this flux is approximately conserved its dynamical importance
will increase as mass is accreted, initially by stimulating magnetorotational
disk turbulence and subsequently by enabling wind angular momentum loss. We use
fits to numerical simulations of ambipolar dominated disk turbulence to
construct simplified one dimensional evolution models for weakly magnetized
protoplanetary disks. We show that the late onset of significant angular
momentum loss in a wind can give rise to "two timescale" disk evolution in
which a long phase of viscous evolution precedes rapid dispersal as the wind
becomes dominant. The wide dispersion in disk lifetimes could therefore be due
to varying initial levels of net flux. Magnetohydrodynamic (MHD) wind triggered
dispersal differs from photoevaporative dispersal in predicting mass loss from
small (less that 1 AU) scales, where thermal winds are suppressed. Our specific
models are based on a limited set of simulations that remain uncertain, but
qualitatively similar evolution appears likely if mass is lost from disks more
quickly than flux...

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