40,415 research outputs found
Gravitational collapse of magnetized clouds II. The role of Ohmic dissipation
We formulate the problem of magnetic field dissipation during the accretion
phase of low-mass star formation, and we carry out the first step of an
iterative solution procedure by assuming that the gas is in free-fall along
radial field lines. This so-called ``kinematic approximation'' ignores the back
reaction of the Lorentz force on the accretion flow. In quasi steady-state, and
assuming the resistivity coefficient to be spatially uniform, the problem is
analytically soluble in terms of Legendre's polynomials and confluent
hypergeometric functions. The dissipation of the magnetic field occurs inside a
region of radius inversely proportional to the mass of the central star (the
``Ohm radius''), where the magnetic field becomes asymptotically straight and
uniform. In our solution, the magnetic flux problem of star formation is
avoided because the magnetic flux dragged in the accreting protostar is always
zero. Our results imply that the effective resistivity of the infalling gas
must be higher by several orders of magnitude than the microscopic electric
resistivity, to avoid conflict with measurements of paleomagnetism in
meteorites and with the observed luminosity of regions of low-mass star
formation.Comment: 20 pages, 4 figures, The Astrophysical Journal, in pres
In-Beam Background Suppression Shield
The long (3ms) proton pulse of the European Spallation Source (ESS) gives
rise to unique and potentially high backgrounds for the instrument suite. In
such a source an instrument capabilities will be limited by it's Signal to
Noise (S/N) ratio. The instruments with a direct view of the moderator, which
do not use a bender to help mitigate the fast neutron background, are the most
challenging. For these beam lines we propose the innovative shielding of
placing blocks of material directly into the guide system, which allow a
minimum attenuation of the cold and thermal fluxes relative to the background
suppression. This shielding configuration has been worked into a beam line
model using Geant4. We study particularly the advantages of single crystal
sapphire and silicon blocks .Comment: 12 pages, 8 figures, proceeding of NDS 2015, 4th International
Workshop on Neutron Delivery Systems, 28 -30 September 2015, ILL Grenoble,
Franc
Soliton motion in a parametrically ac-driven damped Toda lattice
We demonstrate that a staggered parametric ac driving term can support stable
progressive motion of a soliton in a Toda lattice with friction, while an
unstaggered drivng force cannot. A physical context of the model is that of a
chain of anharmonically coupled particles adsorbed on a solid surface of a
finite size. The ac driving force models a standing acoustic wave excited on
the surface. Simulations demonstrate that the state left behind the moving
soliton, with the particles shifted from their equilibrium positions, gradually
relaxes back to the equilibrium state that existed before the passage of the
soliton. Perturbation theory predicts that the ac-driven soliton exists if the
amplitude of the drive exceeds a certain threshold. The analytical prediction
for the threshold is in reasonable agreement with that found numerically.
Collisions between two counter propagating solitons were also simulated,
demonstrating that the collisions are, essentially fully elastic
Breathing oscillations of a trapped impurity in a Bose gas
Motivated by a recent experiment [J. Catani et al., arXiv:1106.0828v1
preprint, 2011], we study breathing oscillations in the width of a harmonically
trapped impurity interacting with a separately trapped Bose gas. We provide an
intuitive physical picture of such dynamics at zero temperature, using a
time-dependent variational approach. In the Gross-Pitaevskii regime we obtain
breathing oscillations whose amplitudes are suppressed by self trapping, due to
interactions with the Bose gas. Introducing phonons in the Bose gas leads to
the damping of breathing oscillations and non-Markovian dynamics of the width
of the impurity, the degree of which can be engineered through controllable
parameters. Our results reproduce the main features of the impurity dynamics
observed by Catani et al. despite experimental thermal effects, and are
supported by simulations of the system in the Gross-Pitaevskii regime.
Moreover, we predict novel effects at lower temperatures due to self-trapping
and the inhomogeneity of the trapped Bose gas.Comment: 7 pages, 3 figure
Notes on Matter in Horava-Lifshitz Gravity
We investigate the dynamics of a scalar field governed by the Lifshitz-type
action which should appear naturally in Horava-Lifshitz gravity. The wave of
the scalar field may propagate with any speed without an upper bound. To
preserve the causality, the action cannot have a generic form. Due to the
superluminal propagation, a formation of a singularity may cause the breakdown
of the predictability of the theory. To check whether such a catastrophe could
occur in Horava-Lifshitz gravity, we investigate the dynamics of a dust. It
turns out that the dust does not collapse completely to form a singularity in a
generic situation, but expands again after it attains a maximum energy density.Comment: 14 pages, references adde
Relationship between five-dimensional black holes and de Sitter spaces
We study a close relationship between the topological anti-de Sitter
(TAdS)-black holes and topological de Sitter (TdS) spaces including the
Schwarzschild-de Sitter (SdS) black hole in five-dimensions. We show that all
thermal properties of the TdS spaces can be found from those of the TAdS black
holes by replacing by . Also we find that all thermal information for
the cosmological horizon of the SdS black hole is obtained from either the
hyperbolic-AdS black hole or the Schwarzschild-TdS space by substituting
with . For this purpose we calculate thermal quantities of bulk,
(Euclidean) conformal field theory (ECFT) and moving domain wall by using the
A(dS)/(E)CFT correspondences. Further we compute logarithmic corrections to the
Bekenstein-Hawking entropy, Cardy-Verlinde formula and Friedmann equation due
to thermal fluctuations. It implies that the cosmological horizon of the TdS
spaces is nothing but the event horizon of the TAdS black holes and the dS/ECFT
correspondence is valid for the TdS spaces in conjunction with the AdS/CFT
correspondence for the TAdS black holes.Comment: 17 page
Cloud for Gaming
Cloud for Gaming refers to the use of cloud computing technologies to build
large-scale gaming infrastructures, with the goal of improving scalability and
responsiveness, improve the user's experience and enable new business models.Comment: Encyclopedia of Computer Graphics and Games. Newton Lee (Editor).
Springer International Publishing, 2015, ISBN 978-3-319-08234-
Gauss-Bonnet Black Holes in AdS Spaces
We study thermodynamic properties and phase structures of topological black
holes in Einstein theory with a Gauss-Bonnet term and a negative cosmological
constant. The event horizon of these topological black holes can be a
hypersurface with positive, zero or negative constant curvature. When the
horizon is a zero curvature hypersurface, the thermodynamic properties of black
holes are completely the same as those of black holes without the Gauss-Bonnet
term, although the two black hole solutions are quite different. When the
horizon is a negative constant curvature hypersurface, the thermodynamic
properties of the Gauss-Bonnet black holes are qualitatively similar to those
of black holes without the Gauss-Bonnet term. When the event horizon is a
hypersurface with positive constant curvature, we find that the thermodynamic
properties and phase structures of black holes drastically depend on the
spacetime dimension and the coefficient of the Gauss-Bonnet term: when
, the properties of black hole are also qualitatively similar to the
case without the Gauss-Bonnet term, but when , a new phase of locally
stable small black hole occurs under a critical value of the Gauss-Bonnet
coefficient, and beyond the critical value, the black holes are always
thermodynamically stable. However, the locally stable small black hole is not
globally preferred, instead a thermal anti-de Sitter space is globally
preferred. We find that there is a minimal horizon radius, below which the
Hawking-Page phase transition will not occur since for these black holes the
thermal anti de Sitter space is always globally preferred.Comment: Revtex, 17 pages with 9 eps figures, v2: section II removed and
references added, the version to appear in PR
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