20,488 research outputs found
Eurasian watermilfoil biomass associated with insect herbivores in New York
A study of aquatic plant biomass within Cayuga Lake, New
York spans twelve years from 1987-1998. The exotic Eurasian
watermilfoil
(
Myriophyllum spicatum
L.) decreased in the
northwest end of the lake from 55% of the total biomass in
1987 to 0.4% in 1998 and within the southwest end from
50% in 1987 to 11% in 1998. Concurrent with the watermilfoil
decline was the resurgence of native species of submersed
macrophytes. During this time we recorded for the
first time in Cayuga Lake two herbivorous insect species: the
aquatic moth
Acentria ephemerella
, first observed in 1991, and
the aquatic weevil
Euhrychiopsis lecontei
, first found in 1996
.
Densities of
Acentria
in southwest Cayuga Lake averaged 1.04
individuals per apical meristem of Eurasian watermilfoil for
the three-year period 1996-1998. These same meristems had
Euhrychiopsis
densities on average of only 0.02 individuals per
apical meristem over the same three-year period. A comparison
of herbivore densities and lake sizes from five lakes in
1997 shows that
Acentria
densities correlate positively with
lake surface area and mean depth, while
Euhrychiopsis
densities
correlate negatively with lake surface area and mean
depth. In these five lakes,
Acentria
densities correlate negatively
with percent composition and dry mass of watermilfoil.
However,
Euhrychiopsis
densities correlate positively with percent
composition and dry mass of watermilfoil. Finally,
Acentria
densities correlate negatively with
Euhrychiopsis
densities
suggesting interspecific competition
Hard sphere crystallization gets rarer with increasing dimension
We recently found that crystallization of monodisperse hard spheres from the
bulk fluid faces a much higher free energy barrier in four than in three
dimensions at equivalent supersaturation, due to the increased geometrical
frustration between the simplex-based fluid order and the crystal [J.A. van
Meel, D. Frenkel, and P. Charbonneau, Phys. Rev. E 79, 030201(R) (2009)]. Here,
we analyze the microscopic contributions to the fluid-crystal interfacial free
energy to understand how the barrier to crystallization changes with dimension.
We find the barrier to grow with dimension and we identify the role of
polydispersity in preventing crystal formation. The increased fluid stability
allows us to study the jamming behavior in four, five, and six dimensions and
compare our observations with two recent theories [C. Song, P. Wang, and H. A.
Makse, Nature 453, 629 (2008); G. Parisi and F. Zamponi, Rev. Mod. Phys, in
press (2009)].Comment: 15 pages, 5 figure
Chameleons with Field Dependent Couplings
Certain scalar-tensor theories exhibit the so-called chameleon mechanism,
whereby observational signatures of scalar fields are hidden by a combination
of self-interactions and interactions with ambient matter. Not all
scalar-tensor theories exhibit such a chameleon mechanism, which has been
originally found in models with inverse power run-away potentials and field
independent couplings to matter. In this paper we investigate field-theories
with field-dependent couplings and a power-law potential for the scalar field.
We show that the theory indeed is a chameleon field theory. We find the
thin-shell solution for a spherical body and investigate the consequences for
E\"ot-Wash experiments, fifth-force searches and Casimir force experiments.
Requiring that the scalar-field evades gravitational tests, we find that the
coupling is sensitive to a mass-scale which is of order of the Hubble scale
today.Comment: 17 pages, 20 figure
The relationship between anti-mullerian hormone in women receiving fertility assessments and age at menopause in subfertile women: evidence from large population studies
<p>Context: Anti-MĂĽllerian hormone (AMH) concentration reflects ovarian aging and is argued to be a useful predictor of age at menopause (AMP). It is hypothesized that AMH falling below a critical threshold corresponds to follicle depletion, which results in menopause. With this threshold, theoretical predictions of AMP can be made. Comparisons of such predictions with observed AMP from population studies support the role for AMH as a forecaster of menopause.</p>
<p>Objective: The objective of the study was to investigate whether previous relationships between AMH and AMP are valid using a much larger data set.</p>
<p>Setting: AMH was measured in 27 563 women attending fertility clinics.</p>
<p>Study Design: From these data a model of age-related AMH change was constructed using a robust regression analysis. Data on AMP from subfertile women were obtained from the population-based Prospect-European Prospective Investigation into Cancer and Nutrition (Prospect-EPIC) cohort (n = 2249). By constructing a probability distribution of age at which AMH falls below a critical threshold and fitting this to Prospect-EPIC menopausal age data using maximum likelihood, such a threshold was estimated.</p>
<p>Main Outcome: The main outcome was conformity between observed and predicted AMP.</p>
<p>Results: To get a distribution of AMH-predicted AMP that fit the Prospect-EPIC data, we found the critical AMH threshold should vary among women in such a way that women with low age-specific AMH would have lower thresholds, whereas women with high age-specific AMH would have higher thresholds (mean 0.075 ng/mL; interquartile range 0.038–0.15 ng/mL). Such a varying AMH threshold for menopause is a novel and biologically plausible finding. AMH became undetectable (<0.2 ng/mL) approximately 5 years before the occurrence of menopause, in line with a previous report.</p>
<p>Conclusions: The conformity of the observed and predicted distributions of AMP supports the hypothesis that declining population averages of AMH are associated with menopause, making AMH an excellent candidate biomarker for AMP prediction. Further research will help establish the accuracy of AMH levels to predict AMP within individuals.</p>
Relativistic Partial Wave Analysis Using the Velocity Basis of the Poincare Group
The velocity basis of the Poincare group is used in the direct product space
of two irreducible unitary representations of the Poincare group. The velocity
basis with total angular momentum j will be used for the definition of
relativistic Gamow vectors.Comment: 14 pages; revte
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Internal charge behaviour of nanocomposites
The incorporation of 23 nm titanium dioxide nanoparticles into an epoxy matrix to form a nanocomposite structure is described. It is shown that the use of nanometric particles results in a substantial change in the behaviour of the composite, which can be traced to the mitigation of internal charge when a comparison is made with conventional TiO2 fillers. A variety of diagnostic techniques (including dielectric spectroscopy, electroluminescence, thermally stimulated current, photoluminescence) have been used to augment pulsed electro-acoustic space charge measurement to provide a basis for understanding the underlying physics of the phenomenon. It would appear that, when the size of the inclusions becomes small enough, they act co-operatively with the host structure and cease to exhibit interfacial properties leading to Maxwell-Wagner polarization. It is postulated that the particles are surrounded by high charge concentrations in the Gouy-Chapman-Stern layer. Since nanoparticles have very high specific areas, these regions allow limited charge percolation through nano-filled dielectrics. The practical consequences of this have also been explored in terms of the electric strength exhibited. It would appear that there was a window in which real advantages accrue from the nano-formulated material. An optimum loading of about 10% (by weight) is indicated
The Peculiar Multi-Wavelength Evolution Of V1535 Sco
We present multi-wavelength observations of the unusual nova V1535 Sco
throughout its outburst in 2015. Early radio observations were consistent with
synchrotron emission, and early X-ray observations revealed the presence of
high-energy (>1 keV) photons. These indicated that strong shocks were present
during the first ~2 weeks of the nova's evolution. The radio spectral energy
distribution was consistent with thermal emission from week 2 to week 6.
Starting in week 7, the radio emission again showed evidence of synchrotron
emission and there was an increase in X-ray emission, indicating a second shock
event. The optical spectra show evidence for at least two separate outflows,
with the faster outflow possibly having a bipolar morphology. The optical and
near infrared light curves and the X-ray measurements of the hydrogen column
density indicated that the companion star is likely a K giant.Comment: 20 pages, 13 figures, under review at ApJ, updated to match the most
recent version submitted to the refere
Dynamical Arrest in Attractive Colloids: The Effect of Long-Range Repulsion
We study gelation in suspensions of model colloidal particles with
short-ranged attractive and long-ranged repulsive interactions by means of
three-dimensional fluorescence confocal microscopy. At low packing fractions,
particles form stable equilibrium clusters. Upon increasing the packing
fraction the clusters grow in size and become increasingly anisotropic until
finally associating into a fully connected network at gelation. We find a
surprising order in the gel structure. Analysis of spatial and orientational
correlations reveals that the gel is composed of dense chains of particles
constructed from face-sharing tetrahedral clusters. Our findings imply that
dynamical arrest occurs via cluster growth and association.Comment: Final version: Phys. Rev. Lett. 94, 208301 (2005
The Superconductor-Insulator Transition in a Tunable Dissipative Environment
We study the influence of a tunable dissipative environment on the dynamics
of Josephson junction arrays near the superconductor-insulator transition. The
experimental realization of the environment is a two dimensional electron gas
coupled capacitively to the array. This setup allows for the well-controlled
tuning of the dissipation by changing the resistance of the two dimensional
electron gas. The capacitive coupling cuts off the dissipation at low
frequencies. We determine the phase diagram and calculate the temperature and
dissipation dependence of the array conductivity. We find good agreement with
recent experimental results.Comment: 4 pages, 4 .eps figures, revte
Geometrical Frustration: A Study of 4d Hard Spheres
The smallest maximum kissing-number Voronoi polyhedron of 3d spheres is the
icosahedron and the tetrahedron is the smallest volume that can show up in
Delaunay tessalation. No periodic lattice is consistent with either and hence
these dense packings are geometrically frustrated. Because icosahedra can be
assembled from almost perfect tetrahedra, the terms "icosahedral" and
"polytetrahedral" packing are often used interchangeably, which leaves the true
origin of geometric frustration unclear. Here we report a computational study
of freezing of 4d hard spheres, where the densest Voronoi cluster is compatible
with the symmetry of the densest crystal, while polytetrahedral order is not.
We observe that, under otherwise comparable conditions, crystal nucleation in
4d is less facile than in 3d. This suggest that it is the geometrical
frustration of polytetrahedral structures that inhibits crystallization.Comment: 4 pages, 3 figures; revised interpretatio
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