487 research outputs found
Local interstellar gasdynamical stability in spiral arm flow
The stability of two-dimensional interstellar gas flow passing through a spiral potential has been investigated. The background flow is assumed to move in a tightly wound potential, which may be regarded as external or self-generated. The unperturbed flow, which may be time dependent, is self-gravitating and satisfies the Roberts equations of motion. A polytropic, single-fluid assumption has been used. Magnetic effects are not considered. The motivation behind this work is to try to understand how much of the diversity of spiral arm morphology can be understood by large scale gas dynamical processes alone. To this end, it is suggested that spurring and feathering, and forming molecular cloud complexes may be closely related in the sense of having dynamically similar origins
Differential Rotation and Turbulence in Extended HI Disks
When present, extended disks of neutral hydrogen around spiral galaxies show
a remarkably uniform velocity dispersion of approx 6 km/s. Since stellar winds
and supernovae are largely absent in such regions, neither the magnitude nor
the constancy of this number can be accounted for in the classical picture in
which interstellar turbulence is driven by stellar energy sources. Here we
suggest that magnetic fields with strengths of a few microgauss in these
extended disks allow energy to be extracted from galactic differential rotation
through MHD driven turbulence. The magnitude and constancy of the observed
velocity dispersion may be understood if its value is Alfvenic. Moreover, by
providing a simple explanation for a lower bound to the gaseous velocity
fluctuations, MHD processes may account for the sharp outer edge to star
formation in galaxy disks.Comment: 9 pages, no figures, to appear in ApJ, LaTeX uses aas2pp4.sty,
eliminated duplicate paragrap
Local Axisymmetric Diffusive Stability of Weakly-Magnetized, Differentially-Rotating, Stratified Fluids
We study the local stability of stratified, differentially-rotating fluids to
axisymmetric perturbations in the presence of a weak magnetic field and of
finite resistivity, viscosity and heat conductivity. This is a generalization
of the Goldreich-Schubert-Fricke (GSF) double-diffusive analysis to the
magnetized and resistive, triple-diffusive case. Our fifth-order dispersion
relation admits a novel branch which describes a magnetized version of
multi-diffusive modes. We derive necessary conditions for axisymmetric
stability in the inviscid and perfect-conductor (double-diffusive) limits. In
each case, rotation must be constant on cylinders and angular velocity must not
decrease with distance from the rotation axis for stability, irrespective of
the relative strength of viscous, resistive and heat diffusion. Therefore, in
both double-diffusive limits, solid body rotation marginally satisfies our
stability criteria. The role of weak magnetic fields is essential to reach
these conclusions. The triple-diffusive situation is more complex, and its
stability criteria are not easily stated. Numerical analysis of our general
dispersion relation confirms our analytic double-diffusive criteria, but also
shows that an unstable double-diffusive situation can be significantly
stabilized by the addition of a third, ostensibly weaker, diffusion process. We
describe a numerical application to the Sun's upper radiative zone and
establish that it would be subject to unstable multi-diffusive modes if
moderate or strong radial gradients of angular velocity were present.Comment: 29 pages, 1 table, accepted for publication in Ap
The Non-Linear Dependence of Flux on Black Hole Mass and Accretion Rate in Core Dominated Jets
We derive the non-linear relation between the core flux F_{nu} of accretion
powered jets at a given frequency and the mass M of the central compact object.
For scale invariant jet models, the mathematical structure of the equations
describing the synchrotron emission from jets enables us to cancel out the
model dependent complications of jet dynamics, retaining only a simple, model
independent algebraic relation between F_{nu} and M. This approach allows us to
derive the F_{nu}-M relation for any accretion disk scenario that provides a
set of input boundary conditions for the magnetic field and the relativistic
particle pressure in the jet, such as standard and advection dominated
accretion flow (ADAF) disk solutions. Surprisingly, the mass dependence of
F_{nu} is very similar in different accretion scenarios. For typical
flat-spectrum core dominated radio jets and standard accretion scenarios we
find F_{nu}~M^{17/12}. The 7-9 orders of magnitude difference in black hole
mass between microquasars and AGN jets imply that AGN jets must be about 3-4
orders of magnitude more radio loud than microquasars, i.e., the ratio of radio
to bolometric luminosity is much smaller in microquasars than in AGN jets.
Because of the generality of these results, measurements of this F_{nu}-M
dependence are a powerful probe of jet and accretion physics. We show how our
analysis can be extended to derive a similar scaling relation between the
accretion rate mdot and F_{nu} for different accretion disk models. For
radiatively inefficient accretion modes we find that the flat spectrum emission
follows F_{nu}~(mdot*M)^{17/12}.Comment: Added key words and acknowledgements, minor editorial corrections. 6
pages, to appear in MNRAS 343, L59-L6
On the Minimum Energy Configuration of a Rotating Barotropic Fluid: A Response to Narayan & Pringle astro-ph\/0208161
The authors respond to the commentary listed in the title
On Nonshearing Magnetic Configurations in Differentially Rotating Disks
A new class of disk MHD equilibrium solutions is described, which is valid within the standard local (``shearing sheet'') approximation scheme. These solutions have the following remarkable property: velocity streamlines and magnetic lines of force rotate rigidly, even in the presence of differential rotation. This situation comes about because the Lorentz forces acting upon modified epicycles compel fluid elements to follow magnetic lines of force. Field line (and streamline) configurations may be elliptical or hyperbolic, prograde or retrograde. These structures have previously known hydrodynamical analogs: the ``planet'' solutions described by Goodman, Narayan, & Goldreich. The primary focus of this investigation is configurations in the disk plane. A related family of solutions lying in a vertical plane is briefly discussed; other families of solutions may exist. Whether these MHD structures are stable is not yet known, but could readily be determined by three-dimensional simulations. If stable or quasi-stable, these simple structures may find important applications in both accretion and galactic disks
Simulations of MHD Instabilities in Intracluster Medium Including Anisotropic Thermal Conduction
We perform a suite of simulations of cooling cores in clusters of galaxies in
order to investigate the effect of the recently discovered heat flux buoyancy
instability (HBI) on the evolution of cores. Our models follow the
3-dimensional magnetohydrodynamics (MHD) of cooling cluster cores and capture
the effects of anisotropic heat conduction along the lines of magnetic field,
but do not account for the cosmological setting of clusters or the presence of
AGN. Our model clusters can be divided into three groups according to their
final thermodynamical state: catastrophically collapsing cores, isothermal
cores, and an intermediate group whose final state is determined by the initial
configuration of magnetic field. Modeled cores that are reminiscent of real
cluster cores show evolution towards thermal collapse on a time scale which is
prolonged by a factor of ~2-10 compared with the zero-conduction cases. The
principal effect of the HBI is to re-orient field lines to be perpendicular to
the temperature gradient. Once the field has been wrapped up onto spherical
surfaces surrounding the core, the core is insulated from further conductive
heating (with the effective thermal conduction suppressed to less than 1/100th
of the Spitzer value) and proceeds to collapse. We speculate that, in real
clusters, the central AGN and possibly mergers play the role of "stirrers,"
periodically disrupting the azimuthal field structure and allowing thermal
conduction to sporadically heat the core.Comment: 16 pages, 3 tables, 17 figures, accepted to ApJ with minor revisions,
to appear in Volume 704, Oct 20, 2009 issu
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
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
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
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