23,650 research outputs found
Galaxy Motions, Turbulence and Conduction in Clusters of Galaxies
Unopposed radiative cooling in clusters of galaxies results in excessive mass
deposition rates. However, the cool cores of galaxy clusters are continuously
heated by thermal conduction and turbulent heat diffusion due to minor mergers
or the galaxies orbiting the cluster center. These processes can either reduce
the energy requirements for AGN heating of cool cores, or they can prevent
overcooling altogether. We perform 3D MHD simulations including field-aligned
thermal conduction and self-gravitating particles to model this in detail.
Turbulence is not confined to the wakes of galaxies but is instead
volume-filling, due to the excitation of large-scale g-modes. We systematically
probe the parameter space of galaxy masses and numbers. For a wide range of
observationally motivated galaxy parameters, the magnetic field is randomized
by stirring motions, restoring the conductive heat flow to the core. The
cooling catastrophe either does not occur or it is sufficiently delayed to
allow the cluster to experience a major merger that could reset conditions in
the intracluster medium. Whilst dissipation of turbulent motions is negligible
as a heat source, turbulent heat diffusion is extremely important; it
predominates in the cluster center. However, thermal conduction becomes
important at larger radii, and simulations without thermal conduction suffer a
cooling catastrophe. Conduction is important both as a heat source and to
reduce stabilizing buoyancy forces, enabling more efficient diffusion.
Turbulence enables conduction, and conduction enables turbulence. In these
simulations, the gas vorticity---which is a good indicator of trapped
g-modes--increases with time. The vorticity growth is approximately mirrored by
the growth of the magnetic field, which is amplified by turbulence.Comment: Submitted to MNRA
The evolution of interstellar clouds in a streaming hot plasma including heat conduction
To examine the evolution of giant molecular clouds in the stream of a hot
plasma we performed two-dimensional hydrodynamical simulations that take full
account of self-gravity, heating and cooling effects and heat conduction by
electrons. We use the thermal conductivity of a fully ionized hydrogen plasma
proposed by Spitzer and a saturated heat flux according to Cowie & McKee in
regions where the mean free path of the electrons is large compared to the
temperature scaleheight. Significant structural and evolutionary differences
occur between simulations with and without heat conduction. Dense clouds in
pure dynamical models experience dynamical destruction by Kelvin-Helmholtz (KH)
instability. In static models heat conduction leads to evaporation of such
clouds. Heat conduction acting on clouds in a gas stream smooths out steep
temperature and density gradients at the edge of the cloud because the
conduction timescale is shorter than the cooling timescale. This diminishes the
velocity gradient between the streaming plasma and the cloud, so that the
timescale for the onset of KH instabilities increases, and the surface of the
cloud becomes less susceptible to KH instabilities. The stabilisation effect of
heat conduction against KH instability is more pronounced for smaller and less
massive clouds. As in the static case more realistic cloud conditions allow
heat conduction to transfer hot material onto the cloud's surface and to mix
the accreted gas deeper into the cloud.Comment: 19 pages, 12 figures, accepted in Astronomy and Astrophysic
A Quantitative Model of Energy Release and Heating by Time-dependent, Localized Reconnection in a Flare with a Thermal Loop-top X-ray Source
We present a quantitative model of the magnetic energy stored and then
released through magnetic reconnection for a flare on 26 Feb 2004. This flare,
well observed by RHESSI and TRACE, shows evidence of non-thermal electrons only
for a brief, early phase. Throughout the main period of energy release there is
a super-hot (T>30 MK) plasma emitting thermal bremsstrahlung atop the flare
loops. Our model describes the heating and compression of such a source by
localized, transient magnetic reconnection. It is a three-dimensional
generalization of the Petschek model whereby Alfven-speed retraction following
reconnection drives supersonic inflows parallel to the field lines, which form
shocks heating, compressing, and confining a loop-top plasma plug. The
confining inflows provide longer life than a freely-expanding or
conductively-cooling plasma of similar size and temperature. Superposition of
successive transient episodes of localized reconnection across a current sheet
produces an apparently persistent, localized source of high-temperature
emission. The temperature of the source decreases smoothly on a time scale
consistent with observations, far longer than the cooling time of a single
plug. Built from a disordered collection of small plugs, the source need not
have the coherent jet-like structure predicted by steady-state reconnection
models. This new model predicts temperatures and emission measure consistent
with the observations of 26 Feb 2004. Furthermore, the total energy released by
the flare is found to be roughly consistent with that predicted by the model.
Only a small fraction of the energy released appears in the super-hot source at
any one time, but roughly a quarter of the flare energy is thermalized by the
reconnection shocks over the course of the flare. All energy is presumed to
ultimately appear in the lower-temperature T<20 MK, post-flare loops
Is the Plasma Within Bubbles and Superbubbles Hot or Cold?
I review what is known about the temperature of the plasma within stellar
wind bubbles and superbubbles. Classical theory suggests that it should be hot,
with characteristic temperatures of order a million degrees. This temperature
should be set by the balance between heating by the internal termination shocks
of the central stellar winds and supernovae, which expand at thousands of km/s,
and cooling by conductive evaporation of cold gas off the shell walls. However,
if the hot interior gas becomes dense enough due to evaporation or ablation off
of interior clouds, it will cool in less than a dynamical time, leading to a
cold interior. The observational evidence appears mixed. On the one hand, X-ray
emission has been observed from both stellar wind bubbles and superbubbles. On
the other hand, no stellar wind bubble or superbubble has yet been observed
emitting at the rate predicted by the classical theory: they are either too
faint or too bright, by up to an order of magnitude. Alternate explanations
have been proposed for the observed emission, including off-center supernova
remnants hitting the shell walls of superbubbles, and residual emission from
highly-ionized gas out of coronal equilibrium. Furthermore, the structures of
post-main sequence stellar wind bubbles, expanding into what are presumably old
stellar wind bubbles, appear in at least some cases to show that the bubble
interior is cold, not hot. (The classical example of this is NGC 6888.) What is
the actual state of bubble and superbubble interiors?Comment: 7 pages, 1 figures, to be published in Astrophysical Plasmas: Codes,
Models and Observations, RMxAA Conf Ser, 2000. Requires rmaa.cl
Quasisteady Configurations of Conductive Intracluster Media
The radial distributions of temperature, density, and gas entropy among
cool-core clusters tend to be quite similar, suggesting that they have entered
a quasi-steady state. If that state is regulated by a combination of thermal
conduction and feedback from a central AGN, then the characteristics of those
radial profiles ought to contain information about the spatial distribution of
AGN heat input and the relative importance of thermal conduction. This paper
addresses those topics by deriving steady-state solutions for clusters in which
radiative cooling, electron thermal conduction, and thermal feedback fueled by
accretion are all present, with the aim of interpreting the configurations of
cool-core clusters in terms of steady-state models. It finds that the core
configurations of many cool-core clusters have entropy levels just below those
of conductively balanced solutions in which magnetic fields have suppressed
electron thermal conduction to ~1/3 of the full Spitzer value, suggesting that
AGN feedback is triggered when conduction can no longer compensate for
radiative cooling. And even when feedback is necessary to heat the central ~30
kpc, conduction may still be the most important heating mechanism within a
cluster's central ~100 kpc.Comment: ApJ in press, 13 pages, 5 figure
Cycling Joule Thomson refrigerator
A symmetrical adsorption pump/compressor system having a pair of mirror image legs and a Joule Thomson expander, or valve, interposed between the legs thereof for providing a, efficient refrigeration cycle is described. The system further includes a plurality of gas operational heat switches adapted selectively to transfer heat from a thermal load and to transfer or discharge heat through a heat projector, such as a radiator or the like. The heat switches comprise heat pressurizable chambers adapted for alternate pressurization in response to adsorption and desorption of a pressurizing gas confined therein
Ultra-fast self-assembly and stabilization of reactive nanoparticles in reduced graphene oxide films.
Nanoparticles hosted in conductive matrices are ubiquitous in electrochemical energy storage, catalysis and energetic devices. However, agglomeration and surface oxidation remain as two major challenges towards their ultimate utility, especially for highly reactive materials. Here we report uniformly distributed nanoparticles with diameters around 10 nm can be self-assembled within a reduced graphene oxide matrix in 10 ms. Microsized particles in reduced graphene oxide are Joule heated to high temperature (∼1,700 K) and rapidly quenched to preserve the resultant nano-architecture. A possible formation mechanism is that microsized particles melt under high temperature, are separated by defects in reduced graphene oxide and self-assemble into nanoparticles on cooling. The ultra-fast manufacturing approach can be applied to a wide range of materials, including aluminium, silicon, tin and so on. One unique application of this technique is the stabilization of aluminium nanoparticles in reduced graphene oxide film, which we demonstrate to have excellent performance as a switchable energetic material
The HBI in a quasi-global model of the intracluster medium
In this paper we investigate how convective instabilities influence heat
conduction in the intracluster medium (ICM) of cool-core galaxy clusters. The
ICM is a high-beta, weakly collisional plasma in which the transport of
momentum and heat is aligned with the magnetic field. The anisotropy of heat
conduction, in particular, gives rise to instabilities that can access energy
stored in a temperature gradient of either sign. We focus on the heat-flux
buoyancy-driven instability (HBI), which feeds on the outwardly increasing
temperature profile of cluster cool cores. Our aim is to elucidate how the
global structure of a cluster impacts on the growth and morphology of the
linear HBI modes when in the presence of Braginskii viscosity, and ultimately
on the ability of the HBI to thermally insulate cores. We employ an idealised
quasi-global model, the plane-parallel atmosphere, which captures the essential
physics -- e.g. the global radial profile of the cluster -- while letting the
problem remain analytically tractable. Our main result is that the dominant HBI
modes are localised to the the innermost (~<20%) regions of cool cores. It is
then probable that, in the nonlinear regime, appreciable field-line insulation
will be similarly localised. Thus, while radio-mode feedback appears necessary
in the central few tens of kpc, heat conduction may be capable of offsetting
radiative losses throughout most of a cool core over a significant fraction of
the Hubble time. Finally, our linear solutions provide a convenient numerical
test for the nonlinear codes that tackle the saturation of such convective
instabilities in the presence of anisotropic transport.Comment: MNRAS, in press; minor modifications from v
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