1,721 research outputs found
Conditional symmetries and Riemann invariants for inhomogeneous hydrodynamic-type systems
A new approach to the solution of quasilinear nonelliptic first-order systems
of inhomogeneous PDEs in many dimensions is presented. It is based on a version
of the conditional symmetry and Riemann invariant methods. We discuss in detail
the necessary and sufficient conditions for the existence of rank-2 and rank-3
solutions expressible in terms of Riemann invariants. We perform the analysis
using the Cayley-Hamilton theorem for a certain algebraic system associated
with the initial system. The problem of finding such solutions has been reduced
to expanding a set of trace conditions on wave vectors and their profiles which
are expressible in terms of Riemann invariants. A couple of theorems useful for
the construction of such solutions are given. These theoretical considerations
are illustrated by the example of inhomogeneous equations of fluid dynamics
which describe motion of an ideal fluid subjected to gravitational and Coriolis
forces. Several new rank-2 solutions are obtained. Some physical interpretation
of these results is given.Comment: 19 page
Common-Path Interference and Zener Tunneling in Bilayer Graphene p-n Junctions
Interference and tunneling are two signature quantum effects that are often
perceived as the yin and yang of quantum mechanics: particle simultaneously
propagating along several distinct classical paths versus particle penetrating
through a classically inaccessible region via a single least-action path. Here
we demonstrate that the Dirac quasiparticles in graphene provide a dramatic
departure from this paradigm. We show that Zener tunneling in gapped bilayer
graphene (BLG), which governs transport through p-n heterojunctions, exhibits
common-path interference that takes place under the tunnel barrier. Due to a
symmetry peculiar to the BLG bandstructure, interfering tunneling paths form
`conjugate pairs', giving rise to high-contrast oscillations in transmission as
a function of the gate-tunable bandgap and other control parameters of the
junction. The common-path interference is solely due to forward-propagating
waves; in contrast to Fabry-Perot-type interference in resonant tunneling
structures it does not rely on multiple backscattering. The oscillations
manifest themselves in the junction I-V characteristic as N-shaped branches
with negative differential conductivity, enabling new high-speed active-circuit
devices with architectures which are not available in electronic semiconductor
devices.Comment: 7 pgs, 5 fg
Simple Estimation of X- Trion Binding Energy in Semiconductor Quantum Wells
A simple illustrative wave function with only three variational parameters is
suggested to calculate the binding energy of negatively charged excitons (X-)
as a function of quantum well width. The results of calculations are in
agreement with experimental data for GaAs, CdTe and ZnSe quantum wells, which
differ considerably in exciton and trion binding energy. The normalized X-
binding energy is found to be nearly independent of electron-to-hole mass ratio
for any quantum well heterostructure with conventional parameters. Its
dependence on quantum well width follows an universal curve. The curve is
described by a simple phenomenological equation.Comment: 8 pages, 3 Postscript figure
The Spitzer c2d survey of large, nearby, interstellar clouds. X. The Chamaeleon II pre-main-sequence population as observed with IRAC and MIPS
We discuss the results from the combined IRAC and MIPS c2d Spitzer Legacy survey observations and complementary optical and NIR data of the Chamaeleon II (Cha II) dark cloud. We perform a census of the young population in an area of similar to 1.75 deg^(2) and study the spatial distribution and properties of the cloud members and candidate pre-main-sequence (PMS) objects and their circumstellar matter. Our census is complete down to the substellar regime (M approximate to 0.03 M☉). From the analysis of the volume density of the PMS objects and candidates we find two groups of objects with volume densities higher than 25 M☉ pc^(-3) and 5-10 members each. A multiplicity fraction of about 13% +/- 3% is observed for objects with separations 0.8" < θ < 6.0" (142-1065 AU). No evidence for variability between the two epochs of the c2d IRAC data set, Δt ~ 6 hr, is detected. We estimate a star formation efficiency of 1%-4%, consistent with the estimates for Taurus and Lupus, but lower than for Cha I. This might mean that different star formation activities in the Chamaeleon clouds reflect a different history of star formation. We also find that Cha II is turning some 6-7 M☉ into stars every Myr, which is low in comparison with the star formation rate in other c2d clouds. The disk fraction of 70%-80% that we estimate in Cha II is much higher than in other star-forming regions and indicates that the population in this cloud is dominated by objects with active accretion. Finally, the Cha II outflows are discussed; a new Herbig-Haro outflow, HH 939, driven by the classical T Tauri star Sz 50, has been discovered
Directionality in protein fold prediction
<p>Abstract</p> <p>Background</p> <p>Ever since the ground-breaking work of Anfinsen et al. in which a denatured protein was found to refold to its native state, it has been frequently stated by the protein fold prediction community that all the information required for protein folding lies in the amino acid sequence. Recent in vitro experiments and in silico computational studies, however, have shown that cotranslation may affect the folding pathway of some proteins, especially those of ancient folds. In this paper aspects of cotranslational folding have been incorporated into a protein structure prediction algorithm by adapting the Rosetta program to fold proteins as the nascent chain elongates. This makes it possible to conduct a pairwise comparison of folding accuracy, by comparing folds created sequentially from each end of the protein.</p> <p>Results</p> <p>A single main result emerged: in 94% of proteins analyzed, following the sense of translation, from N-terminus to C-terminus, produced better predictions than following the reverse sense of translation, from the C-terminus to N-terminus. Two secondary results emerged. First, this superiority of N-terminus to C-terminus folding was more marked for proteins showing stronger evidence of cotranslation and second, an algorithm following the sense of translation produced predictions comparable to, and occasionally better than, Rosetta.</p> <p>Conclusions</p> <p>There is a directionality effect in protein fold prediction. At present, prediction methods appear to be too noisy to take advantage of this effect; as techniques refine, it may be possible to draw benefit from a sequential approach to protein fold prediction.</p
The Mass Distributions of Starless and Protostellar Cores in Gould Belt Clouds
Using data from the SCUBA Legacy Catalogue (850 um) and Spitzer Space
Telescope (3.6 - 70 um), we explore dense cores in the Ophiuchus, Taurus,
Perseus, Serpens, and Orion molecular clouds. We develop a new method to
discriminate submillimeter cores found by SCUBA as starless or protostellar,
using point source photometry from Spitzer wide field surveys. First, we
identify infrared sources with red colors associated with embedded young
stellar objects (YSOs). Second, we compare the positions of these
YSO-candidates to our submillimeter cores. With these identifications, we
construct new, self-consistent starless and protostellar core mass functions
(CMFs) for the five clouds. We find best fit slopes to the high-mass end of the
CMFs of -1.26 +/- 0.20, -1.22 +/- 0.06, -0.95 +/- 0.20, and -1.67 +/- 0.72 for
Ophiuchus, Taurus, Perseus, and Orion, respectively. Broadly, these slopes are
each consistent with the -1.35 power-law slope of the Salpeter IMF at higher
masses, but suggest some differences. We examine a variety of trends between
these CMF shapes and their parent cloud properties, potentially finding a
correlation between the high-mass slope and core temperature. We also find a
trend between core mass and effective size, but we are very limited by
sensitivity. We make similar comparisons between core mass and size with visual
extinction (for A_V >= 3) and find no obvious trends. We also predict the
numbers and mass distributions of cores that future surveys with SCUBA-2 may
detect in each of these clouds.Comment: 56 pages, 18 figures, fixed typo in Eq 1, results in paper remain
unchange
Dust emissivity in the Submm/Mm: SCUBA and SIMBA observations of Barnard 68
We have observed the dark cloud Barnard 68 with SCUBA at 850 um and with
SIMBA at 1.2 mm. The submillimetre and millimetre dust emission correlate well
with the extinction map of Alves, Lada and Lada (2001).The A_V/850um
correlation is clearly not linear and suggests lower temperatures for the dust
in the inner core of the cloud. Assuming a model for the temperature gradient,
we derive the cloud-averaged dust emissivities (normalised to the V-Band
extinction efficiency) at 850 um and 1.2 mm. We find k_850um/k_V = 4.0 +/- 1.0
x 10^-5 and k_1.2mm/k_V = 9.0 +/- 3.0 x 10^-6. These values are compared with
other determinations in this wavelength regime and with expectations for models
of diffuse dust and grain growth in dense clouds.Comment: 8 pages, 4 figures, A&A accepted (Letter), referee forma
Epigenetic reprogramming of cancer cells under embryonic microenvironment
The idea of epigenetic reprogramming of cancer cells by an embryonic microenvironment
possesses potential interest from the prospect of both basic science and potential therapeutic strategies.
Chick embryo extract (CEE) has been used for the successful expansion of many specific stem cells
and has demonstrated the ability to facilitate DNA demethylation. The current study was conducted to
compare the status of DNA methylation in highly metastatic and less metastatic osteosarcoma cells and
to investigate whether CEE may affect the epigenetic regulation of tumor suppressor genes and thus
change the metastatic phenotypes of highly metastatic osteosarcoma cells
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