476 research outputs found
New composite models of partially ionized protoplanetary disks
We study an accretion disk in which three different regions may coexist: MHD
turbulent regions, dead zones and gravitationally unstable regions. Although
the dead zones are stable, there is some transport due to the Reynolds stress
associated with waves emitted from the turbulent layers. We model the transport
in each of the different regions by its own parameter, this being 10
to times smaller in dead zones than in active layers. In
gravitationally unstable regions, is determined by the fact that the
disk self-adjusts to a state of marginal stability. We construct steady-state
models of such disks. We find that for uniform mass flow, the disk has to be
more massive, hotter and thicker at the radii where there is a dead zone. In
disks in which the dead zone is very massive, gravitational instabilities are
present. Whether such models are realistic or not depends on whether
hydrodynamical fluctuations driven by the turbulent layers can penetrate all
the way inside the dead zone. This may be more easily achieved when the ratio
of the mass of the active layer to that of the dead zone is relatively large,
which in our models corresponds to in the dead zone being about 10% of
in the active layers. If the disk is at some stage of its evolution
not in steady-state, then the surface density will evolve toward the
steady-state solution. However, if in the dead zone is much smaller
than in the active zone, the timescale for the parts of the disk beyond a few
AU to reach steady-state may become longer than the disk lifetime. Steady-state
disks with dead zones are a more favorable environment for planet formation
than standard disks, since the dead zone is typically 10 times more massive
than a corresponding turbulent zone at the same location.Comment: 13 pages, 5 figures, accepted for publication in Ap
Magnetothermal instabilities in magnetized anisotropic plasmas
Using the transport equations for an ideal anisotropic collisionless plasma
derived from the Vlasov equation by the 16-moment method, we analyse the
influence of pressure anisotropy exhibited by collisionless magnetized plasmas
on the magnetothermal (MTI) and heat-flux-driven buoyancy (HBI) instabilities.
We calculate the dispersion relation and the growth rates for these
instabilities in the presence of a background heat flux and for configurations
with static pressure anisotropy, finding that when the frequency at which heat
conduction acts is much larger than any other frequency in the system (i.e.
weak magnetic field) the pressure anisotropy has no effect on the MTI/HBI,
provided the degree of anisotropy is small. In contrast, when this ordering of
timescales does not apply the instability criteria depend on pressure
anisotropy. Specifically, the growth time of the instabilities in the
anisotropic case can be almost one order of magnitude smaller than its
isotropic counterpart. We conclude that in plasmas where pressure anisotropy is
present the MTI/HBI are modified. However, in environments with low magnetic
fields and small anisotropy such as the ICM the results obtained from the
16-moment equations under the approximations considered are similar to those
obtained from ideal MHD.Comment: v3: 16 pages, 2 figures, fixed typos, added references and a final
note on related wor
Angular Momentum Transfer in Star-Discs Encounters: The Case of Low-Mass Discs
A prerequisite for the formation of stars and planetary systems is that
angular momentum is transported in some way from the inner regions of the
accretion disc. Tidal effects may play an important part in this angular
momentum transport. Here the angular momentum transfer in an star-disc
encounter is investigated numerically for a variety of encounter parameters in
the case of low mass discs. Although good agreement is found with analytical
results for the entire disc, the loss {\it inside} the disc can be up to an
order of magnitude higher than previously assumed. The differences in angular
momentum transport by secondaries on a hyperbolic, parabolic and elliptical
path are shown, and it is found that a succession of distant encounters might
be equally, if not more, successful in removing angular momentum than single
close encounter.Comment: 11pages, 8 figures, 1 tabl
Hybrid viscosity and the magnetoviscous instability in hot, collisionless accretion disks
We aim to illustrate the role of hot protons in enhancing the
magnetorotational instability (MRI) via the ``hybrid'' viscosity, which is due
to the redirection of protons interacting with static magnetic field
perturbations, and to establish that it is the only relevant mechanism in this
situation. It has recently been shown by Balbus \cite{PBM1} and Islam & Balbus
\cite{PBM11} using a fluid approach that viscous momentum transport is key to
the development of the MRI in accretion disks for a wide range of parameters.
However, their results do not apply in hot, advection-dominated disks, which
are collisionless. We develop a fluid picture using the hybrid viscosity
mechanism, that applies in the collisionless limit. We demonstrate that viscous
effects arising from this mechanism can significantly enhance the growth of the
MRI as long as the plasma \beta \gapprox 80. Our results facilitate for the
first time a direct comparison between the MHD and quasi-kinetic treatments of
the magnetoviscous instability in hot, collisionless disks.Comment: To appear in the proceedings of the first Kodai-Trieste workshop on
Plasma Astrophysics (Aug 27-Sept 07 2007), Springer Astrophysics and Space
Science Proceedings serie
Accretion of low angular momentum material onto black holes: 2D magnetohydrodynamical case
We report on the second phase of our study of slightly rotating accretion
flows onto black holes. We consider magnetohydrodynamical (MHD) accretion flows
with a spherically symmetric density distribution at the outer boundary, but
with spherical symmetry broken by the introduction of a small,
latitude-dependent angular momentum and a weak radial magnetic field. We study
accretion flows by means of numerical 2D, axisymmetric, MHD simulations with
and without resistive heating. Our main result is that the properties of the
accretion flow depend mostly on an equatorial accretion torus which is made of
the material that has too much angular momentum to be accreted directly. The
torus accretes, however, because of the transport of angular momentum due to
the magnetorotational instability (MRI). Initially, accretion is dominated by
the polar funnel, as in the hydrodynamic inviscid case, where material has zero
or very low angular momentum. At the later phase of the evolution, the torus
thickens towards the poles and develops a corona or an outflow or both.
Consequently, the mass accretion through the funnel is stopped. The accretion
of rotating gas through the torus is significantly reduced compared to the
accretion of non-rotating gas (i.e., the Bondi rate). It is also much smaller
than the accretion rate in the inviscid, weakly rotating case.Our results do
not change if we switch on or off resistive heating. Overall our simulations
are very similar to those presented by Stone, Pringle, Hawley and Balbus
despite different initial and outer boundary conditions. Thus, we confirm that
MRI is very robust and controls the nature of radiatively inefficient accretion
flows.Comment: submitted in Ap
Simulation of the Magnetothermal Instability
In many magnetized, dilute astrophysical plasmas, thermal conduction occurs
almost exclusively parallel to magnetic field lines. In this case, the usual
stability criterion for convective stability, the Schwarzschild criterion,
which depends on entropy gradients, is modified. In the magnetized long mean
free path regime, instability occurs for small wavenumbers when (dP/dz)(dln
T/dz) > 0, which we refer to as the Balbus criterion. We refer to the
convective-type instability that results as the magnetothermal instability
(MTI). We use the equations of MHD with anisotropic electron heat conduction to
numerically simulate the linear growth and nonlinear saturation of the MTI in
plane-parallel atmospheres that are unstable according to the Balbus criterion.
The linear growth rates measured from the simulations are in excellent
agreement with the weak field dispersion relation. The addition of isotropic
conduction, e.g. radiation, or strong magnetic fields can damp the growth of
the MTI and affect the nonlinear regime. The instability saturates when the
atmosphere becomes isothermal as the source of free energy is exhausted. By
maintaining a fixed temperature difference between the top and bottom
boundaries of the simulation domain, sustained convective turbulence can be
driven. MTI-stable layers introduced by isotropic conduction are used to
prevent the formation of unresolved, thermal boundary layers. We find that the
largest component of the time-averaged heat flux is due to advective motions as
opposed to the actual thermal conduction itself. Finally, we explore the
implications of this instability for a variety of astrophysical systems, such
as neutron stars, the hot intracluster medium of galaxy clusters, and the
structure of radiatively inefficient accretion flows.Comment: Accepted for publication in Astrophysics and Space Science as
proceedings of the 6th High Energy Density Laboratory Astrophysics (HEDLA)
Conferenc
Protodiscs around Hot Magnetic Rotator Stars
We develop equations and obtain solutions for the structure and evolution of
a protodisc region that is initially formed with no radial motion and
super-Keplerian rotation speed when wind material from a hot rotating star is
channelled towards its equatorial plane by a dipole-type magnetic field. Its
temperature is around K because of shock heating and the inflow of wind
material causes its equatorial density to increase with time. The centrifugal
force and thermal pressure increase relative to the magnetic force and material
escapes at its outer edge. The protodisc region of a uniformly rotating star
has almost uniform rotation and will shrink radially unless some instability
intervenes. In a star with angular velocity increasing along its surface
towards the equator, the angular velocity of the protodisc region decreases
radially outwards and magnetorotational instability (MRI) can occur within a
few hours or days. Viscosity resulting from MRI will readjust the angular
velocity distribution of the protodisc material and may assist in the formation
of a quasi-steady disc. Thus, the centrifugal breakout found in numerical
simulations for uniformly rotating stars does not imply that quasi-steady discs
with slow outflow cannot form around magnetic rotator stars with solar-type
differential rotation.Comment: Accepted for publication in MNRAS. 16 pages, 1 figure, 7 table
The Magnetohydrodynamics of Convection-Dominated Accretion Flows
Radiatively inefficient accretion flows onto black holes are unstable due to
both an outwardly decreasing entropy (`convection') and an outwardly decreasing
rotation rate (the `magnetorotational instability'; MRI). Using a linear
magnetohydrodynamic stability analysis, we show that long-wavelength modes are
primarily destabilized by the entropy gradient and that such `convective' modes
transport angular momentum inwards. Moreover, the stability criteria for the
convective modes are the standard Hoiland criteria of hydrodynamics. By
contrast, shorter wavelength modes are primarily destabilized by magnetic
tension and differential rotation. These `MRI' modes transport angular momentum
outwards. The convection-dominated accretion flow (CDAF) model, which has been
proposed for radiatively inefficient accretion onto a black hole, posits that
inward angular momentum transport and outward energy transport by
long-wavelength convective fluctuations are crucial for determining the
structure of the accretion flow. Our analysis suggests that the CDAF model is
applicable to a magnetohydrodynamic accretion flow provided the magnetic field
saturates at a sufficiently sub-equipartition value (plasma beta >> 1), so that
long-wavelength convective fluctuations can fit inside the accretion disk.
Numerical magnetohydrodynamic simulations are required to determine whether
such a sub-equipartition field is in fact obtained.Comment: 17 pages including 3 figures. Accepted for publication in ApJ. New
appendix and figure were added; some changes of the text were made in
response to the referee
The thermal-viscous disk instability model in the AGN context
Accretion disks in AGN should be subject to the same type of instability as
in cataclysmic variables (CVs) or in low-mass X-ray binaries (LMXBs), which
leads to dwarf nova and soft X-ray transient outbursts. It has been suggested
that this thermal/viscous instability can account for the long term variability
of AGNs. We test this assertion by presenting a systematic study of the
application of the disk instability model (DIM) to AGNs. We are using the
adaptative grid numerical code we have developed in the context of CVs,
enabling us to fully resolve the radial structure of the disk. We show that,
because in AGN disks the Mach numbers are very large, the heating and cooling
fronts are so narrow that they cannot be resolved by the numerical codes that
have been used until now. In addition, these fronts propagate on time scales
much shorter than the viscous time. As a result, a sequence of heating and
cooling fronts propagate back and forth in the disk, leading only to small
variations of the accretion rate onto the black hole, with short quiescent
states occurring for very low mass transfer rates only. Truncation of the inner
part of the disk by e.g. an ADAF does not alter this result, but enables longer
quiescent states. Finally we discuss the effects of irradiation by the central
X-ray source, and show that, even for extremely high irradiation efficiencies,
outbursts are not a natural outcome of the model.Comment: Astronomy & Astrophysics - in pres
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