3,258 research outputs found
Resource amount and discontinuity influence flight and reproduction in \u3ci\u3eHippodamia convergens\u3c/i\u3e (Coleoptera: Coccinellidae)
Industrial-scale agriculture creates a mosaic of large monocultures in the landscape, where seasonal cropping cycles generate discontinuous resource availability for insect predators both spatially and temporally. In this environment, selection will favor predator movement and reproductive behaviors that optimize the location and effective utilization of resource (prey) pulses that are both patchy and ephemeral in nature. Using a model system to study predator movement and reproduction, we tested how discontinuous periods of food resource access that mimic fluctuating resource populations (aphids) would influence flight behavior and reproduction of a highly mobile predator, Hippodamia convergens (convergent lady beetle), and possibly modify energetic trade-offs between these behaviors. Adult beetles were provided either short (3 h) or long (6 h) food pulses daily (continuous availability) or short (6 h) or long (12 h) food pulses every other day (discontinuous availability). We measured preoviposition period, fecundity, and fertility during an 18-day oviposition period, and female tethered flight activity (3 h) before and after the oviposition period. We found that discontinuous food access delayed the onset of oviposition in the high food quantity treatment; fewer females laid eggs overall, and 18-day fecundity was lower compared with continuous provision of the same food quantity. A longer preoviposition period was associated with fewer reproductive days and lower fitness. Flight distance and fecundity were negatively correlated, suggesting that energetic expenditure in flight can deplete energetic reserves otherwise used for subsequent reproduction. The negative effects of discontinuous resource access at fine temporal scales reveal how gaps in resource availability could influence lady beetle population dynamics and their ecosystem services within the agricultural landscape
Models of Metal Poor Stars with Gravitational Settling and Radiative Accelerations: I. Evolution and Abundance Anomalies
Evolutionary models have been calculated for Pop II stars of 0.5 to
1.0 from the pre-main-sequence to the lower part of the giant branch.
Rosseland opacities and radiative accelerations were calculated taking into
account the concentration variations of 28 chemical species, including all
species contributing to Rosseland opacities in the OPAL tables. The effects of
radiative accelerations, thermal diffusion and gravitational settling are
included. While models were calculated both for Z=0.00017 and 0.0017, we
concentrate on models with Z=0.00017 in this paper. These are the first Pop II
models calculated taking radiative acceleration into account. It is shown that,
at least in a 0.8 star, it is a better approximation not to let Fe
diffuse than to calculate its gravitational settling without including the
effects of . In the absence of any turbulence outside of
convection zones, the effects of atomic diffusion are large mainly for stars
more massive than 0.7. Overabundances are expected in some stars with
\teff \ge 6000K. Most chemical species heavier than CNO are affected. At 12
Gyr, overabundance factors may reach 10 in some cases (e.g. for Al or Ni) while
others are limited to 3 (e.g. for Fe). The calculated surface abundances are
compared to recent observations of abundances in globular clusters as well as
to observations of Li in halo stars. It is shown that, as in the case of Pop I
stars, additional turbulence appears to be present.Comment: 40 pages, 17 color figures, to appear in The Astrophysical Journal,
April 2002 (paper with original high resolution figures can be found at
http://www.cerca.umontreal.ca/~richer/Fichiersps/popII_1.ps
Nitrogen Use Efficiency of Timothy Populations
The objective of this study was to determine the variability in N use efficiency among field-grown timothy (Phelum pratense L.) populations. Shoot biomass and N uptake were measured at the end of the spring growth cycle on six timothy populations fertilized with three N rates at two sites in Eastern Canada. The variability in shoot biomass among populations was similar under limiting and nonlimiting N conditions. The ranking of the populations, however, differed under limiting and non-limiting N conditions, and also between the two sites under limiting N conditions. The differences in shoot biomass among populations under highly N deficient conditions were more related to N conversion efficiency than N uptake efficiency. These preliminary results indicate significant interactions between the N nutrition status and timothy populations, and the importance of N conversion efficiency under highly N deficient conditions
The Interaction Of Multiple Convection Zones In A-type Stars
A-type stars have a complex internal structure with the possibility of
multiple convection zones. If not sufficiently separated, such zones will
interact through the convectively stable regions that lie between them. It is
therefore of interest to ask whether the typical conditions that exist within
such stars are such that these convections zones can ever be considered as
disjoint.
In this paper we present results from numerical simulations that help in
understanding how increasing the distance between the convectively unstable
regions are likely to interact through the stable region that separates them.
This has profound implications for mixing and transport within these stars.Comment: 9 pages, 15 figures, Preprint accepted for publication in MNRA
Curing kinetics and thermomechanical properties of latent epoxy/carbon fiber composites
In this work, resins based on diglycidyl ether of bisphenol A (DGEBA) epoxy and a latent hardener, dicyandiamide (DICY), as well as carbon fiber (CF) composites based on them, were prepared with three commercial accelerators: a methylene bis (phenyl dimethyl urea), a cycloaliphatic substituted urea, and a modified polyamine. The curing kinetics of the three DGEBA/DICY/accelerator systems were investigated by chemorheology and differential scanning calorimetry (DSC), in isothermal and over temperature change conditions. Differences in the reaction onset temperature, and in the glass transition temperature (Tg) were
highlighted. For curing of thick resin samples, a slow curing cycle at the lowest possible temperature was used, followed by high temperature (160 – 180 °C) post-curing. Indeed, fast curing at higher temperatures caused the formation of hot spots and led to local burning of the samples. The obtained thermomechanical properties, assessed by ultimate tensile testing and dynamic mechanical analysis (DMA) in single cantilever configuration, were all in the expected range for epoxy resins, with tensile moduli close to 3 GPa and Tg > 140 °C. The longterm stability of these resins at room temperature was verified by DSC. Composite samples were prepared by hand lay-up by manually impregnating four layers of 5-harness satin CF textile, and curing in vacuum bag. Impregnation quality and void content were assessed by optical microscopy. The flexural properties of the post-cured composites were assessed by three-point bending test at room temperature and showed no relevant differences, all composites having bending moduli of 45 - 50 GPa. Finally, composites cured with a faster high
temperature curing cycle (20 min at 140 °C) were prepared with the DGEBA/DICY/ methylene bis (phenyl dimethyl urea) system, obtaining similar properties as with the slower curing cycle, showing that the prepreg system allowed more flexibility in terms of curing cycle than the bulk resin samples
Transverse emittance measurement in 2D and 4D performed on a Low Energy Beam Transport line: benchmarking and data analysis
2D and 4D transverse phase-space of a low-energy ion-beam is measured with
two of the most common emittance scanners. The article covers the description
of the installation, the setup, the settings, the experiment and the benchmark
of the two emittance meters. We compare the results from three series of
measurements and present the advantages and drawbacks of the two systems.
Coupling between phase-space planes, correlations and mitigation of deleterious
effects are discussed. The influence of background noise and aberrations of
trace-space figures on emittance measurements and RMS calculations is
highlighted, especially for low density beams and halos. A new data analysis
method using noise reduction, filtering, and reconstruction of the emittance
figure is described. Finally, some basic concepts of phase-space theory and
application to beam transport are recalled
New angles on D-branes
A low-energy background field solution is presented which describes several
D-membranes oriented at angles with respect to one another. The mass and charge
densities for this configuration are computed and found to saturate the BPS
bound, implying the preservation of one-quarter of the supersymmetries.
T-duality is exploited to construct new solutions with nontrivial angles from
the basic one.Comment: Latex, 12 pages, still no figures, references update
Gravitational settling in pulsating subdwarf B stars and their progenitors
Diffusion of atoms can be important during quiescent phases of stellar
evolution. Particularly in the very thin inert envelopes of subdwarf B stars,
diffusive movements will considerably change the envelope structure and the
surface abundances on a short timescale. Also, the subdwarfs will inherit the
effects of diffusion in their direct progenitors, namely giants near the tip of
the red giant branch. This will influence the global evolution and the
pulsational properties of subdwarf B stars. We investigate the impact of
gravitational settling, thermal diffusion and concentration diffusion on the
evolution and pulsations of subdwarf B stars. Our diffusive stellar models are
compared with models evolved without diffusion. We constructed subdwarf B
models with a mass of 0.465 Msun from a 1 and 3 Msun ZAMS progenitor. The low
mass star ignited helium in an energetic flash, while the intermediate mass
star started helium fusion gently. For each progenitor type we computed series
with and without atomic diffusion. Atomic diffusion in red giants causes the
helium core mass at the onset of helium ignition to be larger. We find an
increase of 0.0015 Msun for the 1 Msun model and 0.0036 Msun for the 3 Msun
model. The effects on the red giant surface abundances are small after the
first dredge up. The evolutionary tracks of the diffusive subdwarf B models are
shifted to lower surface gravities and effective temperatures due to outward
diffusion of hydrogen. This affects both the frequencies of the excited modes
and the overall frequency spectrum. Especially the structure and pulsations of
the post-non-degenerate sdB star are drastically altered, proving that atomic
diffusion cannot be ignored in these stars.Comment: 10 pages, 6 figures, accepted for publication in A&
Tailoring the interfacial interactions in ferroelectric fluorinated polymer/ceramic nanocomposites
In this work composites of PVDF-TrFE containing 60 vol% untreated and surface modified BaTiO3 were produced by solvent casting with two procedures. Their morphology and structure were characterized by scanning electron microscope, X-ray diffraction and differential scanning calorimetry. The effect of the processing conditions and of the surface modification of BaTiO3 on the viscoelastic, dielectric and piezoelectric properties was investigated. The surface modification of BaTiO3 allowed obtaining composite films with low porosity and good filler dispersion, and hence higher storage modulus and lower loss tangent, in a wider processing window. Furthermore it reduced the dielectric losses at low frequency and modified the decay kinetic of the d33 piezoelectric coefficient with respect to composites made with untreated particles
Imaging Oxygen Distribution in Marine Sediments. The Importance of Bioturbation and Sediment Heterogeneity
The influence of sediment oxygen heterogeneity, due to bioturbation, on diffusive oxygen flux was investigated. Laboratory experiments were carried out with 3 macrobenthic species presenting different bioturbation behaviour patterns:the polychaetes Nereis diversicolor and Nereis virens, both constructing ventilated galleries in the sediment column, and the gastropod Cyclope neritea, a burrowing species which does not build any structure. Oxygen two-dimensional distribution in sediments was quantified by means of the optical planar optode technique. Diffusive oxygen fluxes (mean and integrated) and a variability index were calculated on the captured oxygen images. All species increased sediment oxygen heterogeneity compared to the controls without animals. This was particularly noticeable with the polychaetes because of the construction of more or less complex burrows. Integrated diffusive oxygen flux increased with oxygen heterogeneity due to the production of interface available for solute exchanges between overlying water and sediments. This work shows that sediment heterogeneity is an important feature of the control of oxygen exchanges at the sediment–water interface
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