623 research outputs found
Decentralised Learning MACs for Collision-free Access in WLANs
By combining the features of CSMA and TDMA, fully decentralised WLAN MAC
schemes have recently been proposed that converge to collision-free schedules.
In this paper we describe a MAC with optimal long-run throughput that is almost
decentralised. We then design two \changed{schemes} that are practically
realisable, decentralised approximations of this optimal scheme and operate
with different amounts of sensing information. We achieve this by (1)
introducing learning algorithms that can substantially speed up convergence to
collision free operation; (2) developing a decentralised schedule length
adaptation scheme that provides long-run fair (uniform) access to the medium
while maintaining collision-free access for arbitrary numbers of stations
Vision, challenges and opportunities for a Plant Cell Atlas
With growing populations and pressing environmental problems, future economies will be increasingly plant-based. Now is the time to reimagine plant science as a critical component of fundamental science, agriculture, environmental stewardship, energy, technology and healthcare. This effort requires a conceptual and technological framework to identify and map all cell types, and to comprehensively annotate the localization and organization of molecules at cellular and tissue levels. This framework, called the Plant Cell Atlas (PCA), will be critical for understanding and engineering plant development, physiology and environmental responses. A workshop was convened to discuss the purpose and utility of such an initiative, resulting in a roadmap that acknowledges the current knowledge gaps and technical challenges, and underscores how the PCA initiative can help to overcome them
Physical properties of Heusler-like Fe2VAl
A comprehensive characterization of the compound Fe2VAl was carried out. Samples grown by arc melting or the Bridgman method have Al and Fe deficiencies of up to 5 at. %. Czochralski-grown samples were Fe rich and Al deficient. X-ray diffraction implies appreciable antisite disorder in all of our samples. Fourier-transform infrared (FTIR) spectroscopy measurements showed that the carrier density and scattering time had little sample-to-sample variation or temperature dependence for near-stoichiometric samples. FTIR and dc resistivity suggest that the transport properties of Fe2VAl are influenced by both localized and delocalized carriers, with the former primarily responsible for the negative temperature coefficient of resistivity. Magnetization measurements reveal that near-stoichiometric samples have superparamagnetic clusters with at least two sizes of moments. We conclude that in Fe2VAl, antisite disorder causes significant modification to the semimetallic band structure proposed theoretically. With antisite disorder considered, we are now able to explain most of the physical properties of Fe2VAl. None of our data suggest heavy-fermion behavior in our samples
Vlasov-Maxwell, self-consistent electromagnetic wave emission simulations in the solar corona
1.5D Vlasov-Maxwell simulations are employed to model electromagnetic
emission generation in a fully self-consistent plasma kinetic model for the
first time in the solar physics context. The simulations mimic the plasma
emission mechanism and Larmor drift instability in a plasma thread that
connects the Sun to Earth with the spatial scales compressed appropriately. The
effects of spatial density gradients on the generation of electromagnetic
radiation are investigated. It is shown that 1.5D inhomogeneous plasma with a
uniform background magnetic field directed transverse to the density gradient
is aperiodically unstable to Larmor-drift instability. The latter results in a
novel effect of generation of electromagnetic emission at plasma frequency.
When density gradient is removed (i.e. when plasma becomes stable to
Larmor-drift instability) and a density, super-thermal, hot beam is
injected along the domain, in the direction perpendicular to the magnetic
field, plasma emission mechanism generates non-escaping Langmuir type
oscillations which in turn generate escaping electromagnetic radiation. It is
found that in the spatial location where the beam is injected, the standing
waves, oscillating at the plasma frequency, are excited. These can be used to
interpret the horizontal strips observed in some dynamical spectra. Quasilinear
theory predictions: (i) the electron free streaming and (ii) the beam long
relaxation time, in accord with the analytic expressions, are corroborated via
direct, fully-kinetic simulation. Finally, the interplay of Larmor-drift
instability and plasma emission mechanism is studied by considering
electron beam in the Larmor-drift unstable (inhomogeneous) plasma.
http://www.maths.qmul.ac.uk/~tsiklauri/movie1.mpg *
http://www.maths.qmul.ac.uk/~tsiklauri/movie2.mpg *
http://www.maths.qmul.ac.uk/~tsiklauri/movie3.mpgComment: Solar Physics (in press, the final, accepted version
The Look-back Time Evolution of Far-Ultraviolet Flux from the Brightest Cluster Elliptical Galaxies at z < 0.2
We present the GALEX UV photometry of the elliptical galaxies in Abell
clusters at moderate redshifts (z < 0.2) for the study of the look-back time
evolution of the UV upturn phenomenon. The brightest elliptical galaxies (M_r <
-22) in 12 remote clusters are compared with the nearby giant elliptical
galaxies of comparable optical luminosity in the Fornax and Virgo clusters. The
sample galaxies presented here appear to be quiescent without signs of massive
star formation or strong nuclear activity, and show smooth, extended profiles
in their UV images indicating that the far-UV (FUV) light is mostly produced by
hot stars in the underlying old stellar population. Compared to their
counterparts in nearby clusters, the FUV flux of cluster giant elliptical
galaxies at moderate redshifts fades rapidly with ~ 2 Gyrs of look-back time,
and the observed pace in FUV - V color evolution agrees reasonably well with
the prediction from the population synthesis models where the dominant FUV
source is hot horizontal-branch stars and their progeny. A similar amount of
color spread (~ 1 mag) in FUV - V exists among the brightest cluster elliptical
galaxies at z ~ 0.1, as observed among the nearby giant elliptical galaxies of
comparable optical luminosity.Comment: Accepted for publication in the Special GALEX ApJ Supplement,
December 200
Photoswitchable diacylglycerols enable optical control of protein kinase C.
Increased levels of the second messenger lipid diacylglycerol (DAG) induce downstream signaling events including the translocation of C1-domain-containing proteins toward the plasma membrane. Here, we introduce three light-sensitive DAGs, termed PhoDAGs, which feature a photoswitchable acyl chain. The PhoDAGs are inactive in the dark and promote the translocation of proteins that feature C1 domains toward the plasma membrane upon a flash of UV-A light. This effect is quickly reversed after the termination of photostimulation or by irradiation with blue light, permitting the generation of oscillation patterns. Both protein kinase C and Munc13 can thus be put under optical control. PhoDAGs control vesicle release in excitable cells, such as mouse pancreatic islets and hippocampal neurons, and modulate synaptic transmission in Caenorhabditis elegans. As such, the PhoDAGs afford an unprecedented degree of spatiotemporal control and are broadly applicable tools to study DAG signaling
GALEX Ultraviolet Photometry of Globular Clusters in M31
We present ultraviolet photometry for globular clusters (GCs) in M31 from 15
square deg of imaging using the Galaxy Evolution Explorer (GALEX). We detect
200 and 94 GCs with certainty in the near-ultraviolet (NUV; 1750 - 2750
Angstroms) and far-ultraviolet (FUV; 1350 - 1750 Angstroms) bandpasses,
respectively. Our rate of detection is about 50% in the NUV and 23% in the FUV,
to an approximate limiting V magnitude of 19. Out of six clusters with
[Fe/H]>-1 seen in the NUV, none is detected in the FUV bandpass. Furthermore,
we find no candidate metal-rich clusters with significant FUV flux, because of
the contribution of blue horizontal-branch (HB) stars, such as NGC 6388 and NGC
6441, which are metal-rich Galactic GCs with hot HB stars. We show that our
GALEX photometry follows the general color trends established in previous UV
studies of GCs in M31 and the Galaxy. Comparing our data with Galactic GCs in
the UV and with population synthesis models, we suggest that the age range of
M31 and Galactic halo GCs are similar.Comment: This paper will be published as part of the Galaxy Evolution Explorer
(GALEX) Astrophysical Journal Letters Special Issue. Links to the full set of
papers will be available at http://www.galex.caltech.edu/PUBLICATIONS/ after
November 22, 200
GALEX UV Color Relations for Nearby Early-Type Galaxies
We use GALEX/optical photometry to construct color-color relationships for
early-type galaxies sorted by morphological type. We have matched objects in
the GALEX GR1 public release and the first IR1.1 internal release, with the RC3
early-type galaxies having a morphological type -5.5<T<-1.5 with mean error in
T<1.5, and mean error on (B-V)T<0.05. After visual inspection of each match, we
are left with 130 galaxies with a reliable GALEX pipeline photometry in the
far-UV and near-UV bands. This sample is divided into Ellipticals (-5.5<T<-3.5)
and Lenticulars (-3.5<T<-1.5). After correction for the Galactic extinction,
the color-color diagrams FUV-NUV vs. (B-V)_{Tc} are plotted for the two
subsamples. We find a tight anti-correlation between the FUV-NUV and (B-V)_{Tc}
colors for Ellipticals, the UV color getting bluer when the (B-V)_{Tc} get
redder. This relationship very likely is an extension of the color-metallicity
relationship into the GALEX NUV band. We suspect that the main source of the
correlation is metal line blanketing in the NUV band. The FUV-NUV vs B-V
correlation has larger scatter for lenticular galaxies; we speculate this
reflects the presence of low level star formation. If the latter objects (i.e.
those that are blue both in FUV-NUV and B-V) are interpreted as harboring
recent star formation activity, this would be the case for a few percent (~4%)
of Ellipticals and ~15% of Lenticulars; this would make about 10% of early-type
galaxies with residual star formation in our full sample of 130 early-type
galaxies. We also plot FUV-NUV vs. the Mg_2 index and central velocity
dispersion. We find a tight anti-correlation between FUV-NUV and the Mg_2
index(...).Comment: 25 pages, 5 figures, accepted for publication in ApJS (abstract
abridged), typos corrected in section 2.
Changes in control status of COPD over time and their consequences : A prospective international, study
ACKNOWLEDGMENTS The study was designed and coordinated by the Respiratory Effectiveness Group (REG; www.effectivenessevaluation.org; Cambridge, UK) and delivered by Optimum Patient Care (OPC; www.optimumpatientcare.org). Funding: The study was funded by an unrestricted grant from Novartis AG.Peer reviewedPostprin
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