5,863 research outputs found
Locked oscillator phase modulator, appendix d final report
Design parameters for linear phase modulation of locked oscillato
A modified Oster-Murray-Harris mechanical model of morphogenesis
There are two main modeling paradigms for biological pattern formation in developmental biology: chemical prepattern models and cell aggregation models. This paper focuses on an example of a cell aggregation model, the mechanical model developed by Oster, Murray, and Harris [Development, 78 (1983), pp. 83--125]. We revisit the Oster--Murray--Harris model and find that, due to the infinitesimal displacement assumption made in the original version of this model, there is a restriction on the types of boundary conditions that can be prescribed. We derive a modified form of the model which relaxes the infinitesimal displacement assumption. We analyze the dynamics of this model using linear and multiscale nonlinear analysis and show that it has the same linear behavior as the original Oster--Murray--Harris model. Nonlinear analysis, however, predicts that the modified model will allow for a wider range of parameters where the solution evolves to a bounded steady state. The results from both analyses are verified through numerical simulations of the full nonlinear model in one and two dimensions. The increased range of boundary conditions that are well-posed, as well as a wider range of parameters that yield bounded steady states, renders the modified model more applicable to, and more robust for, comparisons with experiments
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&
Reply to ‘Comment on ‘The impact of vitamin D pathway genetic variation and circulating 25-hydroxyvitamin D on cancer outcome: systematic review and meta-analysis’’
High-resolution sub-ice-shelf seafloor records of 20th-century ungrounding and retreat of Pine Island Glacier, West Antarctica.
Pine Island Glacier Ice Shelf (PIGIS) has been thinning rapidly over recent decades, resulting in a progressive drawdown of the inland ice and an upstream migration of the grounding line. The resultant ice loss from Pine Island Glacier (PIG) and its neighboring ice streams presently contributes an estimated ∼10% to global sea level rise, motivating efforts to constrain better the rate of future ice retreat. One route toward gaining a better understanding of the processes required to underpin physically based projections is provided by examining assemblages of landforms and sediment exposed over recent decades by the ongoing ungrounding of PIG. Here we present high-resolution bathymetry and sub-bottom-profiler data acquired by autonomous underwater vehicle (AUV) surveys beneath PIGIS in 2009 and 2014, respectively. We identify landforms and sediments associated with grounded ice flow, proglacial and subglacial sediment transport, overprinting of lightly grounded ice-shelf keels, and stepwise grounding line retreat. The location of a submarine ridge (Jenkins Ridge) coincides with a transition from exposed crystalline bedrock to abundant sediment cover potentially linked to a thick sedimentary basin extending upstream of the modern grounding line. The capability of acquiring high-resolution data from AUV platforms enables observations of landforms and understanding of processes on a scale that is not possible in standard offshore geophysical surveys
The Impact of Vitamin D Pathway Genetic Variation and Circulating 25-Hydroxyvitamin D on Cancer Outcome: Systematic Review and Meta-Analysis
The Hot and Energetic Universe: Astrophysics of feedback in local AGN
Understanding the astrophysics of feedback in active galactic nuclei (AGN) is
key to understanding the growth and co-evolution of supermassive black holes
and galaxies. AGN-driven winds/outflows are potentially the most effective way
of transporting energy and momentum from the nuclear scales to the host galaxy,
quenching star formation by sweeping away the gas reservoir. Key questions in
this field are: 1) how do accretion disks around black holes launch
winds/outflows, and how much energy do these carry? 2) How are the energy and
metals accelerated in winds/outflows transferred and deposited into the
circumgalactic medium? X-ray observations are a unique way to address these
questions because they probe the phase of the outflows which carries most of
the kinetic energy. We show how a high throughput, high spectral resolution
instrument like the X-ray Integral Field Unit (X-IFU) on Athena+ will allow us
to address these questions by determining the physical parameters (ionization
state, density, temperature, abundances, velocities, geometry, etc.) of the
outflows on a dynamical time-scale, in a broad sample of nearby bright AGN. The
X-IFU will also allow direct spectral imaging of the impact of these winds on
the host galaxy for local AGN, forming a template for understanding AGN at
higher redshifts where wind shocks cannot be resolved.Comment: Supporting paper for the science theme "The Hot and Energetic
Universe" to be implemented by the Athena+ X-ray observatory
(http://www.the-athena-x-ray-observatory.eu). 10 pages, 6 figure
The discontinuous nature of chromospheric activity evolution
Chromospheric activity has been thought to decay smoothly with time and,
hence, to be a viable age indicator. Measurements in solar type stars in open
clusters seem to point to a different conclusion: chromospheric activity
undergoes a fast transition from Hyades level to that of the Sun after about 1
Gyr of main--sequence lifetime and any decaying trend before or after this
transition must be much less significant than the short term variations.Comment: 6 pages, 1 figure, to be published in Astrophysics and Space Scienc
The Growth of Eimeria tenella: Characterization and Application of Quantitative Methods to Assess Sporozoite Invasion and Endogenous Development in Cell Culture
In vitro development of the complete life cycle of Eimeria species has been achieved in primary cultures of avian epithelial cells with low efficiency. The use of immortalized cell lines simplifies procedures but only allows partial development through one round of parasite invasion and intracellular replication. We have assessed the suitability of Madin-Darby Bovine Kidney (MDBK) cells to support qualitative and quantitative studies on sporozoite invasion and intracellular development of Eimeria tenella. Analysis of parasite ultrastructure by transmission electron microscopy and serial block face—scanning electron microscopy proved the suitability of the system to generate good quality schizonts and first-generation merozoites. Parasite protein expression profiles elucidated by mass spectrometry corroborated previous findings occurring during the development of the parasite such as the presence of alternative types of surface antigen at different stages and increased abundance of proteins from secretory organelles during invasion and endogenous development. Quantitative PCR (qPCR) allowed the tracking of development by detecting DNA division, whereas reverse transcription qPCR of sporozoite- and merozoite-specific genes could detect early changes before cell division and after merozoite formation, respectively. These results correlated with the analysis of development using ImageJ semi-automated image analysis of fluorescent parasites, demonstrating the suitability and reproducibility of the MDBK culture system. This systems also allowed the evaluation of the effects on invasion and development when sporozoites were pre-incubated with anticoccidial drugs, showing similar effects to those reported before. We have described through this study a series of methods and assays for the further application of this in vitro culture model to more complex studies of Eimeria including basic research on parasite cell biology and host-parasite interactions and for screening anticoccidial drugs
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