6,457 research outputs found
Nucleation of small silicon carbide dust clusters in AGB stars
Silicon carbide (SiC) grains are a major dust component in carbon-rich AGB
stars. The formation pathways of these grains are, however, not fully
understood.\ We calculate ground states and energetically low-lying structures
of (SiC), clusters by means of simulated annealing (SA) and Monte
Carlo simulations of seed structures and subsequent quantum-mechanical
calculations on the density functional level of theory. We derive the infrared
(IR) spectra of these clusters and compare the IR signatures to observational
and laboratory data.\ According to energetic considerations, we evaluate the
viability of SiC cluster growth at several densities and temperatures,
characterising various locations and evolutionary states in circumstellar
envelopes.\ We discover new, energetically low-lying structures for
SiC, SiC, SiC and SiC, and
new ground states for SiC and SiC. The clusters
with carbon-segregated substructures tend to be more stable by 4-9 eV than
their bulk-like isomers with alternating Si-C bonds. However, we find ground
states with cage ("bucky"-like) geometries for SiC and
SiC and low-lying, stable cage structures for n 12. The
latter findings indicate thus a regime of clusters sizes that differs from
small clusters as well as from large-scale crystals. Thus, and owing to their
stability and geometry, the latter clusters may mark a transition from a
quantum-confined cluster regime to crystalline, solid bulk-material.
The calculated vibrational IR spectra of the ground-state SiC clusters shows
significant emission. They include the 10-13 m wavelength range and the
11.3 m feature inferred from laboratory measurements and observations,
respectively, though the overall intensities are rather low.Comment: 16 pages, 25 figures, 3 tables, accepted for publication in Ap
Challenging the paradigm of singularity excision in gravitational collapse
A paradigm deeply rooted in modern numerical relativity calculations
prescribes the removal of those regions of the computational domain where a
physical singularity may develop. We here challenge this paradigm by performing
three-dimensional simulations of the collapse of uniformly rotating stars to
black holes without excision. We show that this choice, combined with suitable
gauge conditions and the use of minute numerical dissipation, improves
dramatically the long-term stability of the evolutions. In turn, this allows
for the calculation of the waveforms well beyond what previously possible,
providing information on the black-hole ringing and setting a new mark on the
present knowledge of the gravitational-wave emission from the stellar collapse
to a rotating black hole.Comment: 4 pages, 4 figures, accepted for publication on Phys. Rev. Let
Redox control of multidrug resistance and Its possible modulation by antioxidants
Clinical efficacy of anticancer chemotherapies is dramatically hampered by multidrug resistance (MDR) dependent on inherited traits, acquired defence against toxins, and adaptive mechanisms mounting in tumours. There is overwhelming evidence that molecular events leading to MDR are regulated by redox mechanisms. For example, chemotherapeutics which overrun the first obstacle of redox-regulated cellular uptake channels (MDR1, MDR2, and MDR3) induce a concerted action of phase I/II metabolic enzymes with a temporal redox-regulated axis. This results in rapid metabolic transformation and elimination of a toxin. This metabolic axis is tightly interconnected with the inducible Nrf2-linked pathway, a key switch-on mechanism for upregulation of endogenous antioxidant enzymes and detoxifying systems. As a result, chemotherapeutics and cytotoxic by-products of their metabolism (ROS, hydroperoxides, and aldehydes) are inactivated and MDR occurs. On the other hand, tumour cells are capable of mounting an adaptive antioxidant response against ROS produced by chemotherapeutics and host immune cells. The multiple redox-dependent mechanisms involved in MDR prompted suggesting redox-active drugs (antioxidants and prooxidants) or inhibitors of inducible antioxidant defence as a novel approach to diminish MDR. Pitfalls and progress in this direction are discussed
Quasi-periodic Oscillations in the X-ray Light Curves from Relativistic Tori
We use a relativistic ray-tracing code to analyze the X-ray emission from a
pressure-supported oscillating relativistic torus around a black hole. We show
that a strong correlation exists between the {\it intrinsic} frequencies of the
torus normal modes and the {\it extrinsic} frequencies seen in the observed
light curve power spectrum. This correlation demonstrates the feasibility of
the oscillating-torus model to explain the multiple peaks seen in black hole
high-frequency quasi-periodic oscillations. Using an optically thin,
monochromatic emission model, we also determine how a relativistically
broadened emission line and the amplitude of the X-ray modulations are
dependent on the observer's inclination angle and on the torus oscillation
amplitudes. Observations of these features can provide important information
about the torus as well as the black hole.Comment: 4 pages, 3 figures, submitted to ApJ
Exact and heuristic allocation of multi-kernel applications to multi-FPGA platforms
FPGA-based accelerators demonstrated high energy efficiency compared to GPUs and CPUs. However, single FPGA designs may not achieve sufficient task parallelism. In this work, we optimize the mapping of high-performance multi-kernel applications, like Convolutional Neural Networks, to multi-FPGA platforms. First, we formulate the system level optimization problem, choosing within a huge design space the parallelism and number of compute units for each kernel in the pipeline. Then we solve it using a combination of Geometric Programming, producing the optimum performance solution given resource and DRAM bandwidth constraints, and a heuristic allocator of the compute units on the FPGA cluster.Peer ReviewedPostprint (author's final draft
Light-addressable liquid crystal polymer dispersed liquid crystal
Scattering-free liquid crystal polymer-dispersed liquid crystal polymer (LCPDLC)
films are fabricated by combining a room temperature polymerizable liquid crystal (LC)
monomer with a mesogenic photosensitive LC. The morphological and photosensitive
properties of the system are analysed with polarized optical microscopy and high resolution
scanning and transmission electron microscopy. A two-phase morphology comprised of
oriented fibril-like polymeric structures interwoven with nanoscale domains of phase
separated LC exists. The nanoscale of the structures enables an absence of scattering which
allows imaging through the LCPDLC sample without optical distortion. The use of a
mesogenic monomer enables much smaller phase separated domains as compared to nonmesogenic systems. All-optical experiments show that the transmitted intensity, measured
through parallel polarizers, can be modulated by the low power density radiation (31
mW/cm2) of a suitable wavelength (532 nm). The reversible and repeatable transmission
change is due to the photoinduced trans-cis photoisomerization process. The birefringence
variation (0.01) obtained by optically pumping the LCPDLC films allow their use as an alloptical phase modulato
The Spread of Opinions and Proportional Voting
Election results are determined by numerous social factors that affect the
formation of opinion of the voters, including the network of interactions
between them and the dynamics of opinion influence. In this work we study the
result of proportional elections using an opinion dynamics model similar to
simple opinion spreading over a complex network. Erdos-Renyi, Barabasi-Albert,
regular lattices and randomly augmented lattices are considered as models of
the underlying social networks. The model reproduces the power law behavior of
number of candidates with a given number of votes found in real elections with
the correct slope, a cutoff for larger number of votes and a plateau for small
number of votes. It is found that the small world property of the underlying
network is fundamental for the emergence of the power law regime.Comment: 10 pages, 7 figure
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