6,329 research outputs found
On the relevance of numerical simulations to booming sand
We have performed a simulation study of 3D cohesionless granular flows down
an inclined chute. We find that the oscillations observed in [L.E. Silbert,
Phys. Rev. Lett., 94, 098002 (2005)] near the angle of repose are harmonic
vibrations of the lowest normal mode. Their frequencies depend on the contact
stiffness as well as on the depth of the flow. Could these oscillations account
for the phenomena of "booming sand"? We estimate an effective contact stiffness
from the Hertz law, but this leads to frequencies several times higher than
observed. However, the Hertz law also predicts interpenetrations of a few
nanometers, indicating that the oscillations frequencies are governed by the
surface stiffness, which can be much lower than the bulk one. This is in
agreement with previous studies ascribing the ability to sing to the presence
of a soft coating on the grain surface.Comment: accepted for publication in Physical Review E http://pre.aps.org;
Physical Review E (2012) to be publishe
Active galactic nucleus feedback in clusters of galaxies
Observations made during the last ten years with the Chandra X-ray
Observatory have shed much light on the cooling gas in the centers of clusters
of galaxies and the role of active galactic nucleus (AGN) heating. Cooling of
the hot intracluster medium in cluster centers can feed the supermassive black
holes found in the nuclei of the dominant cluster galaxies leading to AGN
outbursts which can reheat the gas, suppressing cooling and large amounts of
star formation. AGN heating can come in the form of shocks, buoyantly rising
bubbles that have been inflated by radio lobes, and the dissipation of sound
waves.Comment: Refereed review article published in Chandra's First Decade of
Discovery Special Feature edition of the Proceedings of the National Academy
of Science
The Insignificance of Global Reheating in the Abell 1068 Cluster: X-Ray Analysis
We report on a Chandra observation of the massive, medium redshift (z=0.1386)
cooling flow cluster Abell 1068. We detect a clear temperature gradient in the
X-ray emitting gas from kT ~ 5 keV in the outer part of the cluster down to
roughly 2 keV in the core, and a striking increase in the metallicity of the
gas toward the cluster center. The total spectrum from the cluster can be fit
by a cooling flow model with a total mass deposition rate of 150 solar
masses/yr. Within the core (r < 30 kpc), the mass depositon rate of 40 solar
masses/yr is comparable to estimates for the star formation rate from optical
data. We find an apparent correlation between the cD galaxy's optical isophotes
and enhanced metallicity isocontours in the central ~100 kpc of the cluster. We
show that the approximate doubling of the metallicity associated with the cD
can be plausibly explained by supernova explosions associated with the cD's
ambient stellar population and the recent starburst. Finally, we calculate the
amount of heating due to thermal conduction and show that this process is
unlikely to offset cooling in Abell 1068.Comment: Accepted for publication in ApJ, 26 pages, 12 b+w figures, 3 color
figure
Heteronuclear ionizing collisions between laser-cooled metastable helium atoms
We have investigated cold ionizing heteronuclear collisions in dilute
mixtures of metastable (2 3S1) 3He and 4He atoms, extending our previous work
on the analogous homonuclear collisions [R. J. W. Stas et al., PRA 73, 032713
(2006)]. A simple theoretical model of such collisions enables us to calculate
the heteronuclear ionization rate coefficient, for our quasi-unpolarized gas,
in the absence of resonant light (T = 1.2 mK): K34(th) = 2.4*10^-10 cm^3/s.
This calculation is supported by a measurement of K34 using magneto-optically
trapped mixtures containing about 1*10^8 atoms of each species, K34(exp) =
2.5(8)*10^-10 cm^3/s. Theory and experiment show good agreement.Comment: 8 pages, 6 figure
A Degenerate Bose-Fermi Mixture of Metastable Atoms
We report the observation of simultaneous quantum degeneracy in a dilute
gaseous Bose-Fermi mixture of metastable atoms. Sympathetic cooling of helium-3
(fermion) by helium-4 (boson), both in the lowest triplet state, allows us to
produce ensembles containing more than 10^6 atoms of each isotope at
temperatures below 1 micro-Kelvin, and achieve a fermionic degeneracy parameter
of T/Tf=0.45. Due to their high internal energy, the detection of individual
metastable atoms with sub-nanosecond time resolution is possible, permitting
the study of bosonic and fermionic quantum gases with unprecedented precision.
This may lead to metastable helium becoming the mainstay of quantum atom
optics.Comment: 4 pages, 3 figures submitted to PR
A Large Atom Number Metastable Helium Bose-Einstein Condensate
We have produced a Bose-Einstein condensate of metastable helium (4He*)
containing over 1.5x10^7 atoms, which is a factor of 25 higher than previously
achieved. The improved starting conditions for evaporative cooling are obtained
by applying one-dimensional Doppler cooling inside a magnetic trap. The same
technique is successfully used to cool the spin-polarized fermionic isotope
(3He*), for which thermalizing collisions are highly suppressed. Our detection
techniques include absorption imaging, time-of-flight measurements on a
microchannel plate detector and ion counting to monitor the formation and decay
of the condensate.Comment: 4 pages, 3 figures (changed content
Inelastic collapse of a randomly forced particle
We consider a randomly forced particle moving in a finite region, which
rebounds inelastically with coefficient of restitution r on collision with the
boundaries. We show that there is a transition at a critical value of r,
r_c\equiv e^{-\pi/\sqrt{3}}, above which the dynamics is ergodic but beneath
which the particle undergoes inelastic collapse, coming to rest after an
infinite number of collisions in a finite time. The value of r_c is argued to
be independent of the size of the region or the presence of a viscous damping
term in the equation of motion.Comment: 4 pages, REVTEX, 2 EPS figures, uses multicol.sty and epsf.st
Propagating front in an excited granular layer
A partial monolayer of ~ 20000 uniform spherical steel beads, vibrated
vertically on a flat plate, shows remarkable ordering transitions and
cooperative behavior just below 1g maximum acceleration. We study the stability
of a quiescent disordered or ``amorphous'' state formed when the acceleration
is switched off in the excited ``gaseous'' state. The transition from the
amorphous state back to the gaseous state upon increasing the plate's
acceleration is generally subcritical: An external perturbation applied to one
bead initiates a propagating front that produces a rapid transition. We measure
the front velocity as a function of the applied acceleration. This phenomenon
is explained by a model based on a single vibrated particle with multiple
attractors that is perturbed by collisions. A simulation shows that a
sufficiently high rate of interparticle collisions can prevent trapping in the
attractor corresponding to the nonmoving ground state.Comment: 16 pages, 9 figures, revised version, to appear in Phys. Rev. E, May
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