520 research outputs found
Angular Momentum Transport in Particle and Fluid Disks
We examine the angular momentum transport properties of disks composed of
macroscopic particles whose velocity dispersions are externally enhanced
(``stirred''). Our simple Boltzmann equation model serves as an analogy for
unmagnetized fluid disks in which turbulence may be driven by thermal
convection. We show that interparticle collisions in particle disks play the
same role as fluctuating pressure forces and viscous dissipation in turbulent
disks: both transfer energy in random motions associated with one direction to
those associated with another, and convert kinetic energy into heat. The
direction of angular momentum transport in stirred particle and fluid disks is
determined by the direction of external stirring and by the properties of the
collision term in the Boltzmann equation (or its analogue in the fluid
problem). In particular, our model problem yields inward transport for
vertically or radially stirred disks, provided collisions are suitably
inelastic; the transport is outwards in the elastic limit. Numerical
simulations of hydrodynamic turbulence driven by thermal convection find inward
transport; this requires that fluctuating pressure forces do little to no work,
and is analogous to an externally stirred particle disk in which collisions are
highly inelastic.Comment: 15 pages; final version accepted by ApJ; minor changes, some
clarificatio
Convection-Dominated Accretion Flows
Non-radiating, advection-dominated, accretion flows are convectively
unstable. We calculate the two-dimensional (r-theta) structure of such flows
assuming that (1) convection transports angular momentum inwards, opposite to
normal viscosity and (2) viscous transport by other mechanisms (e.g., magnetic
fields) is weak (alpha << 1). Under such conditions convection dominates the
dynamics of the accretion flow and leads to a steady state structure that is
marginally stable to convection. We show that the marginally stable flow has a
constant temperature and rotational velocity on spherical shells, a net flux of
energy from small to large radii, zero net accretion rate, and a radial density
profile proportional to r^{-1/2}, flatter than the r^{-3/2} profile
characteristic of spherical accretion flows. This solution accurately describes
the full two-dimensional structure of recent axisymmetric numerical simulations
of advection-dominated accretion flows.Comment: final version accepted by ApJ; discussion expanded, references adde
Turbulence and Mixing in the Intracluster Medium
The intracluster medium (ICM) is stably stratified in the hydrodynamic sense
with the entropy increasing outwards. However, thermal conduction along
magnetic field lines fundamentally changes the stability of the ICM, leading to
the "heat-flux buoyancy instability" when and the "magnetothermal
instability" when . The ICM is thus buoyantly unstable regardless of
the signs of and . On the other hand, these
temperature-gradient-driven instabilities saturate by reorienting the magnetic
field (perpendicular to when and parallel to when ), without generating sustained convection. We show that
after an anisotropically conducting plasma reaches this nonlinearly stable
magnetic configuration, it experiences a buoyant restoring force that resists
further distortions of the magnetic field. This restoring force is analogous to
the buoyant restoring force experienced by a stably stratified adiabatic
plasma. We argue that in order for a driving mechanism (e.g, galaxy motions or
cosmic-ray buoyancy) to overcome this restoring force and generate turbulence
in the ICM, the strength of the driving must exceed a threshold, corresponding
to turbulent velocities . For weaker driving, the ICM
remains in its nonlinearly stable magnetic configuration, and turbulent mixing
is effectively absent. We discuss the implications of these findings for the
turbulent diffusion of metals and heat in the ICM.Comment: 8 pages, 2 figs., submitted to the conference proceedings of "The
Monster's Fiery Breath;" a follow up of arXiv:0901.4786 focusing on the
general mixing properties of the IC
Collisionless Isotropization of the Solar-Wind Protons by Compressive Fluctuations and Plasma Instabilities
Compressive fluctuations are a minor yet significant component of
astrophysical plasma turbulence. In the solar wind, long-wavelength compressive
slow-mode fluctuations lead to changes in and in , where and are the perpendicular and parallel
temperatures of the protons, is the magnetic field strength, and
is the proton density. If the amplitude of the compressive
fluctuations is large enough, crosses one or more instability
thresholds for anisotropy-driven microinstabilities. The enhanced field
fluctuations from these microinstabilities scatter the protons so as to reduce
the anisotropy of the pressure tensor. We propose that this scattering drives
the average value of away from the marginal stability boundary
until the fluctuating value of stops crossing the boundary. We
model this "fluctuating-anisotropy effect" using linear Vlasov--Maxwell theory
to describe the large-scale compressive fluctuations. We argue that this effect
can explain why, in the nearly collisionless solar wind, the average value of
is close to unity.Comment: 11 pages, published in Ap
Relativistic Jets and Long-Duration Gamma-ray Bursts from the Birth of Magnetars
We present time-dependent axisymmetric magnetohydrodynamic simulations of the
interaction of a relativistic magnetized wind produced by a proto-magnetar with
a surrounding stellar envelope, in the first seconds after core
collapse. We inject a super-magnetosonic wind with ergs
s into a cavity created by an outgoing supernova shock. A strong
toroidal magnetic field builds up in the bubble of plasma and magnetic field
that is at first inertially confined by the progenitor star. This drives a jet
out along the polar axis of the star, even though the star and the magnetar
wind are each spherically symmetric. The jet has the properties needed to
produce a long-duration gamma-ray burst (GRB). At s after core bounce,
the jet has escaped the host star and the Lorentz factor of the material in the
jet at large radii cm is similar to that in the magnetar wind
near the source. Most of the spindown power of the central magnetar escapes via
the relativistic jet. There are fluctuations in the Lorentz factor and energy
flux in the jet on second timescale. These may contribute to
variability in GRB emission (e.g., via internal shocks).Comment: 5 pages, 3 figures, accepted in MNRAS letter, presented at the
conference "Astrophysics of Compact Objects", 1-7 July, Huangshan, Chin
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