438 research outputs found
Solar activity during the Holocene: the Hallstatt cycle and its consequence for grand minima and maxim
Cosmogenic isotopes provide the only quantitative proxy for analyzing the
long-term solar variability over a centennial timescale. While essential
progress has been achieved in both measurements and modeling of the cosmogenic
proxy, uncertainties still remain in the determination of the geomagnetic
dipole moment evolution. Here we improve the reconstruction of solar activity
over the past nine millennia using a multi-proxy approach. We used records of
the 14C and 10Be cosmogenic isotopes, current numerical models of the isotope
production and transport in Earth's atmosphere, and available geomagnetic field
reconstructions, including a new reconstruction relying on an updated
archeo-/paleointensity database. The obtained series were analyzed using the
singular spectrum analysis (SSA) method to study the millennial-scale trends. A
new reconstruction of the geomagnetic dipole field moment, GMAG.9k, is built
for the last nine millennia. New reconstructions of solar activity covering the
last nine millennia, quantified in sunspot numbers, are presented and analyzed.
A conservative list of grand minima and maxima is provided. The primary
components of the reconstructed solar activity, as determined using the SSA
method, are different for the series based on 14C and 10Be. These primary
components can only be ascribed to long-term changes in the terrestrial system
and not to the Sun. They have been removed from the reconstructed series. In
contrast, the secondary SSA components of the reconstructed solar activity are
found to be dominated by a common ~2400-yr quasi-periodicity, the so-called
Hallstatt cycle, in both the 14C and 10Be based series. This Hallstatt cycle
thus appears to be related to solar activity. Finally, we show that the grand
minima and maxima occurred intermittently over the studied period, with
clustering near highs and lows of the Hallstatt cycle, respectively.Comment: In press in Astronomy & Astrophysics, doi:
10.1051/0004-6361/20152729
Probing active forces via a fluctuation-dissipation relation: Application to living cells
We derive a new fluctuation-dissipation relation for non-equilibrium systems
with long-term memory. We show how this relation allows one to access new
experimental information regarding active forces in living cells that cannot
otherwise be accessed. For a silica bead attached to the wall of a living cell,
we identify a crossover time between thermally controlled fluctuations and
those produced by the active forces. We show that the probe position is
eventually slaved to the underlying random drive produced by the so-called
active forces.Comment: 5 page
Charge distribution in two-dimensional electrostatics
We examine the stability of ringlike configurations of N charges on a plane
interacting through the potential . We interpret the equilibrium distributions in terms of a shell
model and compare predictions of the model with the results of numerical
simulations for systems with up to 100 particles.Comment: LaTe
Classification in sparse, high dimensional environments applied to distributed systems failure prediction
Network failures are still one of the main causes of distributed systems’ lack of reliability. To overcome this problem we present an improvement over a failure prediction system, based on Elastic Net Logistic Regression and the application of rare events prediction techniques, able to work with sparse, high dimensional datasets. Specifically, we prove its stability, fine tune its hyperparameter and improve its industrial utility by showing that, with a slight change in dataset creation, it can also predict the location of a failure, a key asset when trying to take a proactive approach to failure management
Rapidly rotating second-generation progenitors for the blue hook stars of {\omega} Cen
Horizontal Branch stars belong to an advanced stage in the evolution of the
oldest stellar galactic population, occurring either as field halo stars or
grouped in globular clusters. The discovery of multiple populations in these
clusters, that were previously believed to have single populations gave rise to
the currently accepted theory that the hottest horizontal branch members (the
blue hook stars, which had late helium-core flash ignition, followed by deep
mixing) are the progeny of a helium-rich "second generation" of stars. It is
not known why such a supposedly rare event (a late flash followed by mixing) is
so common that the blue hook of {\omega} Cen contains \sim 30% of horizontal
branch stars 10 , or why the blue hook luminosity range in this massive cluster
cannot be reproduced by models. Here we report that the presence of helium core
masses up to \sim 0.04 solar masses larger than the core mass resulting from
evolution is required to solve the luminosity range problem. We model this by
taking into account the dispersion in rotation rates achieved by the
progenitors, whose premain sequence accretion disc suffered an early disruption
in the dense environment of the cluster's central regions where
second-generation stars form. Rotation may also account for frequent
late-flash-mixing events in massive globular clusters.Comment: 44 pages, 8 figures, 2 tables in Nature, online june 22, 201
Non-universal equilibrium crystal shape results from sticky steps
The anisotropic surface free energy, Andreev surface free energy, and
equilibrium crystal shape (ECS) z=z(x,y) are calculated numerically using a
transfer matrix approach with the density matrix renormalization group (DMRG)
method. The adopted surface model is a restricted solid-on-solid (RSOS) model
with "sticky" steps, i.e., steps with a point-contact type attraction between
them (p-RSOS model). By analyzing the results, we obtain a first-order shape
transition on the ECS profile around the (111) facet; and on the curved surface
near the (001) facet edge, we obtain shape exponents having values different
from those of the universal Gruber-Mullins-Pokrovsky-Talapov (GMPT) class. In
order to elucidate the origin of the non-universal shape exponents, we
calculate the slope dependence of the mean step height of "step droplets"
(bound states of steps) using the Monte Carlo method, where p=(dz/dx,
dz/dy)$, and represents the thermal averag |p| dependence of , we
derive a |p|-expanded expression for the non-universal surface free energy
f_{eff}(p), which contains quadratic terms with respect to |p|. The first-order
shape transition and the non-universal shape exponents obtained by the DMRG
calculations are reproduced thermodynamically from the non-universal surface
free energy f_{eff}(p).Comment: 31 pages, 21 figure
Computing the Roughening Transition of Ising and Solid-On-Solid Models by BCSOS Model Matching
We study the roughening transition of the dual of the 2D XY model, of the
Discrete Gaussian model, of the Absolute Value Solid-On-Solid model and of the
interface in an Ising model on a 3D simple cubic lattice. The investigation
relies on a renormalization group finite size scaling method that was proposed
and successfully tested a few years ago. The basic idea is to match the
renormalization group flow of the interface observables with that of the
exactly solvable BCSOS model. Our estimates for the critical couplings are
, and for
the XY-model, the Discrete Gaussian model and the Absolute Value Solid-On-Solid
model, respectively. For the inverse roughening temperature of the Ising
interface we find . To the best of our knowledge,
these are the most precise estimates for these parameters published so far.Comment: 25 pages, LaTeX file, no figure
MICROMEGAS chambers for hadronic calorimetry at a future linear collider
Prototypes of MICROMEGAS chambers, using bulk technology and analog readout,
with 1x1cm2 readout segmentation have been built and tested. Measurements in
Ar/iC4H10 (95/5) and Ar/CO2 (80/20) are reported. The dependency of the
prototypes gas gain versus pressure, gas temperature and amplification gap
thickness variations has been measured with an 55Fe source and a method for
temperature and pressure correction of data is presented. A stack of four
chambers has been tested in 200GeV/c and 7GeV/c muon and pion beams
respectively. Measurements of response uniformity, detection efficiency and hit
multiplicity are reported. A bulk MICROMEGAS prototype with embedded digital
readout electronics has been assembled and tested. The chamber layout and first
results are presented
Power laws in microrheology experiments on living cells: comparative analysis and modelling
We compare and synthesize the results of two microrheological experiments on
the cytoskeleton of single cells. In the first one, the creep function J(t) of
a cell stretched between two glass plates is measured after applying a constant
force step. In the second one, a micrometric bead specifically bound to
transmembrane receptors is driven by an oscillating optical trap, and the
viscoelastic coefficient is retrieved. Both and
exhibit power law behavior: and , with the same exponent
. This power law behavior is very robust ; is
distributed over a narrow range, and shows almost no dependance on the cell
type, on the nature of the protein complex which transmits the mechanical
stress, nor on the typical length scale of the experiment. On the contrary, the
prefactors and appear very sensitive to these parameters. Whereas
the exponents are normally distributed over the cell population, the
prefactors and follow a log-normal repartition. These results are
compared with other data published in the litterature. We propose a global
interpretation, based on a semi-phenomenological model, which involves a broad
distribution of relaxation times in the system. The model predicts the power
law behavior and the statistical repartition of the mechanical parameters, as
experimentally observed for the cells. Moreover, it leads to an estimate of the
largest response time in the cytoskeletal network: s.Comment: 47 pages, 14 figures // v2: PDF file is now Acrobat Reader 4 (and up)
compatible // v3: Minor typos corrected - The presentation of the model have
been substantially rewritten (p. 17-18), in order to give more details -
Enhanced description of protocols // v4: Minor corrections in the text : the
immersion angles are estimated and not measured // v5: Minor typos corrected.
Two references were clarifie
Enhanced stability of the square lattice of a classical bilayer Wigner crystal
The stability and melting transition of a single layer and a bilayer crystal
consisting of charged particles interacting through a Coulomb or a screened
Coulomb potential is studied using the Monte-Carlo technique. A new melting
criterion is formulated which we show to be universal for bilayer as well as
for single layer crystals in the case of (screened) Coulomb, Lennard--Jones and
1/r^{12} repulsive inter-particle interactions. The melting temperature for the
five different lattice structures of the bilayer Wigner crystal is obtained,
and a phase diagram is constructed as a function of the interlayer distance. We
found the surprising result that the square lattice has a substantial larger
melting temperature as compared to the other lattice structures. This is a
consequence of the specific topology of the defects which are created with
increasing temperature and which have a larger energy as compared to the
defects in e.g. a hexagonal lattice.Comment: Accepted for publication in Physical Review
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