6,885 research outputs found
How genealogies are affected by the speed of evolution
In a series of recent works it has been shown that a class of simple models
of evolving populations under selection leads to genealogical trees whose
statistics are given by the Bolthausen-Sznitman coalescent rather than by the
well known Kingman coalescent in the case of neutral evolution. Here we show
that when conditioning the genealogies on the speed of evolution, one finds a
one parameter family of tree statistics which interpolates between the
Bolthausen-Sznitman and Kingman's coalescents. This interpolation can be
calculated explicitly for one specific version of the model, the exponential
model. Numerical simulations of another version of the model and a
phenomenological theory indicate that this one-parameter family of tree
statistics could be universal. We compare this tree structure with those
appearing in other contexts, in particular in the mean field theory of spin
glasses
The electron's dance
A joint Fermilab/SLAC publicationParis' Trocadéro science exhibition allows science enthusiasts to see--and even control--a real electron accelerator
A Systematic Search for Corotating Interaction Regions in Apparently Single Galactic Wolf-Rayet Stars. II. A Global View of the Wind Variability
This study is the second part of a survey searching for large-scale
spectroscopic variability in apparently single Wolf-Rayet (WR) stars. In a
previous paper (Paper I), we described and characterized the spectroscopic
variability level of 25 WR stars observable from the northern hemisphere and
found 3 new candidates presenting large-scale wind variability, potentially
originating from large-scale structures named Co-rotating Interaction Regions
(CIRs). In this second paper, we discuss an additional 39 stars observable from
the southern hemisphere. For each star in our sample, we obtained 4-5
high-resolution spectra with a signal-to-noise ratio of ~100 and determined its
variability level using the approach described in Paper I. In total, 10 new
stars are found to show large-scale spectral variability of which 7 present
CIR-type changes (WR 8, WR 44, WR 55, WR 58, WR 61, WR 63, WR 100). Of the
remaining stars, 20 were found to show small-amplitude changes and 9 were found
to show no spectral variability as far as can be concluded from the data in
hand. Also, we discuss the spectroscopic variability level of all single
galactic WR stars that are brighter than v~12.5, and some WR stars with 12.5 <
v <= 13.5; i.e. all the stars presented in our two papers and 4 more stars for
which spectra have already been published in the literature. We find that 23/68
stars (33.8 %) present large-scale variability, but only 12/54 stars (~22.1 %)
are potentially of CIR-type. Also, we find 31/68 stars (45.6 %) that only show
small-scale variability, most likely due to clumping in the wind. Finally, no
spectral variability is detected based on the data in hand for 14/68 (20.6 %)
stars. Interestingly, the variability with the highest amplitude also have the
widest mean velocity dispersion.Comment: 14 pages, 24 figures, 2 tables, Accepted in Ap
Changes on the nanostructure of cementitius calcium silicate hydrates (C-S-H) induced by aqueous carbonation
The nanostructure of the main binding phase of the hydrated cements, the calcium silicate hydrates (C-S-H), and their structural changes due to aqueous carbonation have been characterized using TEM, nitrogen physisorption, and SAXS. Synthetic C-S-H has been used for this purpose. Two different morphologies were identified, similar to the high density and low density C-S-H types. When submitting the sample to a CO 2 flux, the low density phase was completely carbonated. The carbonation by-products, calcium carbonate, and silica gel were also identified and characterized. The precipitation of the silica gel increased the specific surface area from 95 to 132 m 2/g, and its structure, formed by particles of ~5 nm typical radius, was observed by small angle X-ray scattering. In addition, the resistance of the high density C-S-H to carbonation is reported, and the passivating effect of the precipitated calcium carbonate is also discussed. Finally, the results have been compared with carbonation features observed in Portland cement carbonated experimentally at downhole conditions.
A Compositional Deadlock Detector for Android Java
We develop a static deadlock analysis for commercial Android Java applications, of sizes in the tens of millions of
LoC, under active development at Facebook. The analysis runs
primarily at code-review time, on only the modified code and
its dependents; we aim at reporting to developers in under 15
minutes.
To detect deadlocks in this setting, we first model the real
language as an abstract language with balanced re-entrant locks,
nondeterministic iteration and branching, and non-recursive
procedure calls. We show that the existence of a deadlock in this
abstract language is equivalent to a certain condition over the
sets of critical pairs of each program thread; these record, for all
possible executions of the thread, which locks are currently held
at the point when a fresh lock is acquired. Since the critical pairs
of any program thread is finite and computable, the deadlock
detection problem for our language is decidable, and in NP.
We then leverage these results to develop an open-source
implementation of our analysis adapted to deal with real Java
code. The core of the implementation is an algorithm which
computes critical pairs in a compositional, abstract interpretation
style, running in quasi-exponential time. Our analyser is built in
the INFER verification framework and has been in industrial
deployment for over two years; it has seen over two hundred
fixed deadlock reports with a report fix rate of ∼54%
The VLT-FLAMES Tarantula Survey. VII. A low velocity dispersion for the young massive cluster R136
Detailed studies of resolved young massive star clusters are necessary to
determine their dynamical state and evaluate the importance of gas expulsion
and early cluster evolution. In an effort to gain insight into the dynamical
state of the young massive cluster R136 and obtain the first measurement of its
velocity dispersion, we analyse multi-epoch spectroscopic data of the inner
regions of 30 Doradus in the Large Magellanic Cloud (LMC) obtained as part of
the VLT-FLAMES Tarantula Survey. Following a quantitative assessment of the
variability, we use the radial velocities of non-variable sources to place an
upper limit of 6 km/s on the line-of-sight velocity dispersion of stars within
a projected distance of 5 pc from the centre of the cluster. After accounting
for the contributions of undetected binaries and measurement errors through
Monte Carlo simulations, we conclude that the true velocity dispersion is
likely between 4 and 5 km/s given a range of standard assumptions about the
binary distribution. This result is consistent with what is expected if the
cluster is in virial equilibrium, suggesting that gas expulsion has not altered
its dynamics. We find that the velocity dispersion would be ~25 km/s if
binaries were not identified and rejected, confirming the importance of the
multi-epoch strategy and the risk of interpreting velocity dispersion
measurements of unresolved extragalactic young massive clusters.Comment: 18 pages, 7 figures, accepted by A&
Effect of Water Activity on Reaction Kinetics and Intergranular Transport: Insights from the Ca(OH) 2 + MgCO 3 → CaCO 3 + Mg(OH) 2 Reaction at 1·8 GPa
The kinetics of the irreversible reaction Ca(OH)2 + MgCO3 → CaCO3 + Mg(OH)2 were investigated at high pressures and temperatures relevant to metamorphic petrology, using both in situ synchrotron X-ray diffraction and post-mortem analysis of reaction rim growth on recovered samples. Reaction kinetics are found to strongly depend on water content; comparable bulk-reaction kinetics are obtained under water-saturated (excess water, c. 10 wt %) and under intermediate (0·1–1 wt % water) conditions when temperature is increased by c. 300 K. In contrast, similar reaction kinetics were observed at ∼673 K and 823 K between intermediate and dry experiments, respectively, where dry refers to a set of experiments with water activity below 1·0 (no free water), as buffered by the CaO–Ca(OH)2 assemblage. Given the activation energies at play, this gap—corresponding to the loss of no more than 1 wt % of water by the assemblage—leads to a difference of several orders of magnitude in reaction kinetics at a given temperature. Further analysis, at the microscopic scale, of the intermediate and dry condition samples, shows that intergranular transport of calcium controls the reaction progress. Grain boundary diffusivities could be retrieved from the classic treatment of reaction rim growth rate. In turn, once modeled, this rate was used to fit the bulk kinetic data derived from X-ray powder diffraction, offering an alternative means to derive calcium diffusivity data. Based on a comparison with effective grain boundary data for Ca and Mg from the literature, it is inferred that both dry and intermediate datasets are consistent with a water-saturated intergranular medium with different levels of connectivity. The very high diffusivity of Ca in the CaCO3 + Mg(OH)2 rims, in comparison with that of Mg in enstatite rims found by earlier workers, emphasizes the prominent role of the interactions between diffusing species and mineral surfaces in diffusion kinetics. Furthermore, we show that the addition of water is likely to change the relative diffusivity of Mg and Ca in carbonate aggregates. From a qualitative point of view, we confirm, in a carbonate-bearing system, that small water content variations within the 0–1 wt % range have tremendous effects on both intergranular transport mechanisms and kinetics. We also propose that the water content dependent diffusivity of major species (Mg, Ca) in low-porosity metamorphic rocks is strongly dependent on the interaction between diffusing species and mineral surfaces. This parameter, which will vary from one rock-type to another, needs also to considered when extrapolating (P, T, t, xH2O) laboratory diffusion data to metamorphic processes
Discrete Feynman-Kac formulas for branching random walks
Branching random walks are key to the description of several physical and
biological systems, such as neutron multiplication, genetics and population
dynamics. For a broad class of such processes, in this Letter we derive the
discrete Feynman-Kac equations for the probability and the moments of the
number of visits of the walker to a given region in the phase space.
Feynman-Kac formulas for the residence times of Markovian processes are
recovered in the diffusion limit.Comment: 4 pages, 3 figure
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