2,293 research outputs found
Superfluid turbulence and pulsar glitch statistics
Experimental evidence is reviewed for the existence of superfluid turbulence
in a differentially rotating, spherical shell at high Reynolds numbers
(\Rey\gsim 10^3), such as the outer core of a neutron star. It is shown that
torque variability increases with \Rey, suggesting that glitch activity in
radio pulsars may be a function of \Rey as well. The \Rey distribution of
the 67 glitching radio pulsars with characteristic ages {\rm
yr} is constructed from radio timing data and cooling curves and compared with
the \Rey distribution of all 348 known pulsars with {\rm
yr}. The two distributions are different, with a Kolmogorov-Smirnov probability
. The conclusion holds for (modified) Urca and
nonstandard cooling, and for Newtonian and superfluid viscosities
Interferometric Observations of the Nuclear Region of Arp220 at Submillimeter Wavelengths
We report the first submillimeter interferometric observations of an
ultraluminous infrared galaxy. We observed Arp220 in the CO J=3-2 line and
342GHz continuum with the single baseline CSO-JCMT interferometer consisting of
the Caltech Submillimeter Observatory (CSO) and the James Clerk Maxwell
Telescope (JCMT). Models were fit to the measured visibilities to constrain the
structure of the source. The morphologies of the CO J=3-2 line and 342GHz
continuum emission are similar to those seen in published maps at 230 and
110GHz. We clearly detect a binary source separated by about 1 arcsec in the
east-west direction in the 342GHz continuum. The CO J=3-2 visibility
amplitudes, however, indicate a more complicated structure, with evidence for a
compact binary at some velocities and rather more extended structure at others.
Less than 30% of the total CO J=3-2 emission is detected by the interferometer,
which implies the presence of significant quantities of extended gas. We also
obtained single-dish CO J=2-1, CO J=3-2 and HCN J=4-3 spectra. The HCN J=4-3
spectrum, unlike the CO spectra, is dominated by a single redshifted peak. The
HCN J=4-3/CO J=3-2, HCN J=4-3/HCN J=1-0 and CO J=3-2/2-1 line ratios are larger
in the redshifted (eastern) source, which suggests that the two sources may
have different physical conditions. This result might be explained by the
presence of an intense starburst that has begun to deplete or disperse the
densest gas in the western source, while the eastern source harbors undispersed
high density gas.Comment: 17 pages, 9 figures, 4 Tables. accepted by Ap
Adaptive Lévy processes and area-restricted search in human foraging
A considerable amount of research has claimed that animals’ foraging behaviors display movement lengths with power-law distributed tails, characteristic of Lévy flights and Lévy walks. Though these claims have recently come into question, the proposal that many animals forage using Lévy processes nonetheless remains. A Lévy process does not consider when or where resources are encountered, and samples movement lengths independently of past experience. However, Lévy processes too have come into question based on the observation that in patchy resource environments resource-sensitive foraging strategies, like area-restricted search, perform better than Lévy flights yet can still generate heavy-tailed distributions of movement lengths. To investigate these questions further, we tracked humans as they searched for hidden resources in an open-field virtual environment, with either patchy or dispersed resource distributions. Supporting previous research, for both conditions logarithmic binning methods were consistent with Lévy flights and rank-frequency methods–comparing alternative distributions using maximum likelihood methods–showed the strongest support for bounded power-law distributions (truncated Lévy flights). However, goodness-of-fit tests found that even bounded power-law distributions only accurately characterized movement behavior for 4 (out of 32) participants. Moreover, paths in the patchy environment (but not the dispersed environment) showed a transition to intensive search following resource encounters, characteristic of area-restricted search. Transferring paths between environments revealed that paths generated in the patchy environment were adapted to that environment. Our results suggest that though power-law distributions do not accurately reflect human search, Lévy processes may still describe movement in dispersed environments, but not in patchy environments–where search was area-restricted. Furthermore, our results indicate that search strategies cannot be inferred without knowing how organisms respond to resources–as both patched and dispersed conditions led to similar Lévy-like movement distributions
Numerical simulation of small perturbation on an accretion disk due to the collision of a star with the disk near the black hole
In this paper, perturbations of an accretion disk by a star orbiting around a
black hole are studied. We report on a numerical experiment, which has been
carried out by using a parallel-machine code originally developed by D\"{o}nmez
(2004). An initially steady state accretion disk near a non-rotating
(Schwarzschild) black hole interacts with a "star", modeled as an initially
circular region of increased density. Part of the disk is affected by the
interaction. In some cases, a gap develops and shock wave propagates through
the disk. We follow the evolution for order of one dynamical period and we show
how the non-axisymetric density perturbation further evolves and moves
downwards where the material of the disk and the star become eventually
accreted onto the central body.
When the star perturbs the steady state accretion disk, the disk around the
black hole is destroyed by the effect of perturbation. The perturbed accretion
disk creates a shock wave during the evolution and it loses angular momentum
when the gas hits on the shock waves. Colliding gas with the shock wave is the
one of the basic mechanism of emitting the rays in the accretion disk. The
series of supernovae occurring in the inner disk could entirely destroy the
disk in that region which leaves a more massive black hole behind, at the
center of galaxies.Comment: 20pages, 8 figures, accepted for publication in Astrophysics and
Space Scienc
A galactic-scale origin for stellar clustering
We recently presented a model for the cluster formation efficiency (CFE),
i.e. the fraction of star formation occurring in bound stellar clusters. It
utilizes the idea that the formation of stars and stellar clusters occurs
across a continuous spectrum of ISM densities. Bound stellar clusters naturally
arise from the high-density end of this density spectrum. Due to short
free-fall times, these high-density regions can achieve high star formation
efficiencies (SFEs) and can be unaffected by gas expulsion. Lower-density
regions remain gas-rich and substructured, and are unbound upon gas expulsion.
The model enables the CFE to be calculated using galactic-scale observables. I
present a brief summary of the model physics, assumptions and caveats, and show
that it agrees well with observations. Fortran and IDL routines for calculating
the CFE are publicly available at http://www.mpa-garching.mpg.de/cfe.Comment: 4 pages, 1 figure; to appear in The Labyrinth of Star Formation,
(eds.) D. Stamatellos, S. Goodwin, and D. Ward-Thompson, Springer, in pres
Nanomechanical investigation of soft biological cell adhesion using atomic force microscopy
Mechanical coupling between living cells is a complex process that is important for a variety of biological processes. In this study the effects of specific biochemical treatment on cell-to-cell adhesion and single cell mechanics were systematically investigated using atomic force microscopy (AFM) single cell force spectroscopy. Functionalised AFM tipless cantilevers were used for attaching single suspended cells that were brought in contact with substrate cells. Cell-to-cell adhesion parameters, such as maximum unbinding force (F max) and work or energy of detachment (W D), were extracted from the retraction force–displacement (F–d) curves. AFM indentation experiments were performed by indenting single cells with a spherical microbead attached to the cantilever. Hertzian contact model was applied to determine the elastic modulus (E) of single cells. Following treatment of the cells with neutralising antibody for epithelial (E)-cadherin, F max was increased by 25%, whereas W D decreased by 11% in response to a 43% increase in E. The results suggest that although the adhesion force between cells was increased after treatment, the energy of adhesion was decreased due to the reduced displacement separation as manifested by the loss of elastic deformation. Conclusively, changes in single cell mechanics are important underlying factors contributing to cell-to-cell adhesion and hence cytomechanical characterization is critical for cell adhesion measurements
Blue Stragglers in Galactic Open Clusters and the Integrated Spectral Energy Distributions
Synthetic integrated spectral properties of the old Galactic open clusters
are studies in this work, where twenty-seven Galactic open clusters of ages >=
1Gyr are selected as the working sample. Based on the photometric observations
of these open clusters, synthetic integrated spectrum has been made for the
stellar population of each cluster. The effects of blue straggler stars (BSSs)
on the conventional simple stellar population (SSP) model are analyzed on an
individual cluster base. It is shown that the BSSs, whose holding positions in
the color-magnitude diagrams (CMDs) cannot be predicted by the current
single-star evolution theory, present significant modifications to the
integrated properties of theoretical SSP model. The synthesized integrated
spectral energy distributions (ISEDs) of our sample clusters are dramatically
different from the SSPs based on isochrone only. The BSSs corrected ISEDs of
stellar populations show systematic enhancements towards shorter wavelength in
the spectra. When measured with wide-band colors in unresolvable conditions,
the age of a stellar population can be seriously under-estimated by the
conventional SSP model. Therefore, considering the common existence of BSS
component in real stellar populations, a considerable amount of alternations on
the conventional ISEDs should be expected when applying the technique of
evolutionary population synthesis (EPS) to more complicated stellar systems.Comment: 45 pages, 21 figures Accepted for publication in ApJ (Feburary 1,
2005 issue
High glucose up-regulates ENaC and SGK1 expression in HCD-cells
Background/Aim: Diabetic nephropathy is associated with progressive renal damage, leading to impaired function and end-stage renal failure. Secondary hypertension stems from a deranged ability of cells within the kidney to resolve and appropriately regulate sodium resorption in response to hyperglycaemia. However, the mechanisms by which glucose alters sodium re-uptake have not been fully characterised.
Methods: Here we present RT-PCR, western blot and immunocytochemistry data confirming mRNA and protein expression of the serum and glucocorticoid inducible kinase (SGK1) and the a conducting subunit of the epithelial sodium channel (ENaC) in a model in vitro system of the human cortical collecting duct (HCD). We examined changes in expression of these elements in response to glucose challenge, designed to mimic hyperglycaemia associated with type 2 diabetes mellitus. Changes in Na+ concentration were assessed using single-cell microfluorimetry.
Results: Incubation with glucose, the Ca2+-ionophore ionomycin and the cytokine TGF-beta 1 were all found to evoke significant and time-dependent increases in both SGK1 and alpha ENaC protein expression. These molecular changes were correlated to an increase in Na+-uptake at the single-cell level.
Conclusion: Together these data offer a potential explanation for glucose-evoked Na+-resorption and a potential contributory role of SGK1 and ENaCs in development of secondary hypertension, commonly linked to diabetic nephropathy
Simulation for the oblique impact of a lattice system
The oblique collision between an elastic disk and an elastic wall is
numerically studied.
We investigate the dependency of the tangential coefficient of restitution on
the incident angle of impact.
From the results of simulation, our model reproduces experimental results and
can be explained by a phenomenological theory of the oblique impact.Comment: 30 pages, 9 figures, submitted to J. Phys. Soc. Japa
Diffuse Gamma-ray Emission from the Galactic Center - A Multiple Energy Injection Model
We suggest that the energy source of the observed diffuse gamma-ray emission
from the direction of the Galactic center is the Galactic black hole Sgr A*,
which becomes active when a star is captured at a rate of
yr^{-1}. Subsequently the star is tidally disrupted and its matter is accreted
into the black hole. During the active phase relativistic protons with a
characteristic energy erg per capture are ejected. Over
90% of these relativistic protons disappear due to proton-proton collisions on
a timescale years in the small central bulge region with
radius pc within Sgr A*, where the density is cm^{-3}. The
gamma-ray intensity, which results from the decay of neutral pions produced by
proton-proton collisions, decreases according to , where t is
the time after last stellar capture. Less than 5% of relativistic protons
escaped from the central bulge region can survive and maintain their energy for
>10^7 years due to much lower gas density outside, where the gas density can
drop to cm. They can diffuse to a pc region before
disappearing due to proton-proton collisions. The observed diffuse GeV
gamma-rays resulting from the decay of neutral pions produced via collision
between these escaped protons and the gas in this region is expected to be
insensitive to time in the multi-injection model with the characteristic
injection rate of 10^{-5} yr^{-1}. Our model calculated GeV and 511 keV
gamma-ray intensities are consistent with the observed results of EGRET and
INTEGRAL, however, our calculated inflight annihilation rate cannot produce
sufficient intensity to explain the COMPTEL data.Comment: 8 pages, 3 figures, accepted by A&
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