251 research outputs found
Solution-Phase Synthesis of Heteroatom-Substituted Carbon Scaffolds for Hydrogen Storage
This paper reports a bottom-up solution-phase process for the preparation of pristine and heteroatom (boron, phosphorus, or nitrogen)-substituted carbon scaffolds that show good surface areas and enhanced hydrogen adsorption capacities and binding energies. The synthesis method involves heating chlorine-containing small organic molecules with metallic sodium at reflux in high-boiling solvents. For heteroatom incorporation, heteroatomic electrophiles are added to the reaction mixture. Under the reaction conditions, micrometer-sized graphitic sheets assembled by 3−5 nm-sized domains of graphene nanoflakes are formed, and when they are heteroatom-substituted, the heteroatoms are uniformly distributed. The substituted carbon scaffolds enriched with heteroatoms (boron ~7.3%, phosphorus ~8.1%, and nitrogen ~28.1%) had surface areas as high as 900 m^2 g^(−1) and enhanced reversible hydrogen physisorption capacities relative to pristine carbon scaffolds or common carbonaceous materials. In addition, the binding energies of the substituted carbon scaffolds, as measured by adsorption isotherms, were 8.6, 8.3, and 5.6 kJ mol^(−1) for the boron-, phosphorus-, and nitrogen-enriched carbon scaffolds, respectively
Modelling and Bayesian analysis of the Abakaliki smallpox data
The celebrated Abakaliki smallpox data have appeared numerous times in the epidemic modelling literature, but in almost all cases only a specific subset of the data is considered. The only previous analysis of the full data set relied on approximation methods to derive a likelihood and did not assess model adequacy. The data themselves continue to be of interest due to concerns about the possible re-emergence of smallpox as a bioterrorism weapon. We present the first full Bayesian statistical analysis using data-augmentation Markov chain Monte Carlo methods which avoid the need for likelihood approximations and which yield a wider range of results than previous analyses. We also carry out model assessment using simulation-based methods. Our findings suggest that the outbreak was largely driven by the interaction structure of the population, and that the introduction of control measures was not the sole reason for the end of the epidemic. We also obtain quantitative estimates of key quantities including reproduction numbers
A Bayesian nonparametric analysis of the 2003 outbreak of highly pathogenic avian influenza in the Netherlands
Infectious diseases on farms pose both public and animal health risks, so understanding how they spread between farms is crucial for developing disease control strategies to prevent future outbreaks. We develop novel Bayesian nonparametric methodology to fit spatial stochastic transmission models in which the infection rate between any two farms is a function that depends on the distance between them, but without assuming a specified parametric form. Making nonparametric inference in this context is challenging since the likelihood function of the observed data is intractable because the underlying transmission process is unobserved. We adopt a fully Bayesian approach by assigning a transformed Gaussian process prior distribution to the infection rate function, and then develop an efficient data augmentation Markov Chain Monte Carlo algorithm to perform Bayesian inference. We use the posterior predictive distribution to simulate the effect of different disease control methods and their economic impact. We analyse a large outbreak of avian influenza in the Netherlands and infer the between-farm infection rate, as well as the unknown infection status of farms which were pre-emptively culled. We use our results to analyse ring-culling strategies, and conclude that although effective, ring-culling has limited impact in high-density areas
Emergent Mesoscale Phenomena in Magnetized Accretion Disc Turbulence
We study how the structure and variability of magnetohydrodynamic (MHD)
turbulence in accretion discs converge with domain size. Our results are based
on a series of vertically stratified local simulations, computed using the
Athena code, that have fixed spatial resolution, but varying radial and
azimuthal extent (from \Delta R = 0.5H to 16H, where H is the vertical scale
height). We show that elementary local diagnostics of the turbulence, including
the Shakura-Sunyaev {\alpha} parameter, the ratio of Maxwell stress to magnetic
energy, and the ratio of magnetic to fluid stresses, converge to within the
precision of our measurements for spatial domains of radial size Lx \geq 2H. We
obtain {\alpha} = 0.02-0.03, consistent with recent results. Very small domains
(Lx = 0.5H) return anomalous results, independent of spatial resolution. The
convergence with domain size is only valid for a limited set of diagnostics:
larger spatial domains admit the emergence of dynamically important mesoscale
structures. In our largest simulations, the Maxwell stress shows a significant
large scale non-local component, while the density develops long-lived
axisymmetric perturbations (zonal flows) at the 20% level. Most strikingly, the
variability of the disc in fixed-sized patches decreases strongly as the
simulation volume increases. We find generally good agreement between our
largest local simulations and global simulations with comparable spatial
resolution. There is no direct evidence that the presence of curvature terms or
radial gradients in global calculations materially affect the turbulence,
except to perhaps introduce an outer radial scale for mesoscale structures. The
demonstrated importance of mean magnetic fields, seen in both large local and
global simulations implies that the growth and saturation of these fields is
likely of critical importance for the evolution of accretion discs. (abridged)Comment: 18 pages, 20 figures, accepted to MNRA
Turbulence in Global Simulations of Magnetized Thin Accretion Disks
We use a global magnetohydrodynamic simulation of a geometrically thin
accretion disk to investigate the locality and detailed structure of turbulence
driven by the magnetorotational instability (MRI). The model disk has an aspect
ratio , and is computed using a higher-order Godunov MHD
scheme with accurate fluxes. We focus the analysis on late times after the
system has lost direct memory of its initial magnetic flux state. The disk
enters a saturated turbulent state in which the fastest growing modes of the
MRI are well-resolved, with a relatively high efficiency of angular momentum
transport . The accretion stress
peaks at the disk midplane, above and below which exists a moderately
magnetized corona with patches of superthermal field. By analyzing the spatial
and temporal correlations of the turbulent fields, we find that the spatial
structure of the magnetic and kinetic energy is moderately well-localized (with
correlation lengths along the major axis of and respectively),
and generally consistent with that expected from homogenous incompressible
turbulence. The density field, conversely, exhibits both a longer correlation
length and a long correlation time, results which we ascribe to the importance
of spiral density waves within the flow. Consistent with prior results, we show
that the mean local stress displays a well-defined correlation with the local
vertical flux, and that this relation is apparently causal (in the sense of the
flux stimulating the stress) during portions of a global dynamo cycle. We argue
that the observed flux-stress relation supports dynamo models in which the
structure of coronal magnetic fields plays a central role in determining the
dynamics of thin-disk accretion.Comment: 24 pages and 25 figures. MNRAS in press. Version with high resolution
figures available from
http://jila.colorado.edu/~krb3u/Thin_Disk/thin_disk_turbulence.pd
Proximity as a Journalistic Keyword in the Digital Era : A study on the “closeness” of amateur news images
Proximity is an ambiguous journalistic notion for which there is no single definition. In this article, we re-evaluate the relevance and use of the concept in the digital news environment. Based on interviews with journalists in Finland and audience focus groups in Finland and the United Kingdom, we ask how new forms of visual amateur production incorporated into professional news journalism have transformed the concept. The concept of proximity has evolved from being a criterion of news selection into a central imperative of news production aiming to engage audiences. Through the prism of amateur news imagery, proximity appears as a spatio-temporal, emotional and strategic keyword.Peer reviewe
Galactic foreground contribution to the BEAST CMB Anisotropy Maps
We report limits on the Galactic foreground emission contribution to the
Background Emission Anisotropy Scanning Telescope (BEAST) Ka- and Q-band CMB
anisotropy maps. We estimate the contribution from the cross-correlations
between these maps and the foreground emission templates of an H map,
a de-striped version of the Haslam et al. 408 MHz map, and a combined 100
m IRAS/DIRBE map. Our analysis samples the BEAST
declination band into 24 one-hour (RA) wide sectors with pixels
each, where we calculate: (a) the linear correlation coefficient between the
anisotropy maps and the templates; (b) the coupling constants between the
specific intensity units of the templates and the antenna temperature at the
BEAST frequencies and (c) the individual foreground contributions to the BEAST
anisotropy maps. The peak sector contributions of the contaminants in the
Ka-band are of 56.5% free-free with a coupling constant of
K/R, and 67.4% dust with K/(MJy/sr). In the Q-band the
corresponding values are of 64.4% free-free with K/R and 67.5%
dust with K/(MJy/sr). Using a lower limit of 10% in the
relative uncertainty of the coupling constants, we can constrain the sector
contributions of each contaminant in both maps to % in 21 (free-free), 19
(dust) and 22 (synchrotron) sectors. At this level, all these sectors are found
outside of the b region. By performing the same
correlation analysis as a function of Galactic scale height, we conclude that
the region within should be removed from the BEAST maps for
CMB studies in order to keep individual Galactic contributions below %
of the map's rms.Comment: 17 pages PostScript file. Better resolution figures can be found in
the web page http://www.das.inpe.br/~alex/beast_foregrounds.html. Accepted
for publication in the ApJ Suppl. Serie
Distinct Mechanisms for Induction and Tolerance Regulate the Immediate Early Genes Encoding Interleukin 1β and Tumor Necrosis Factor α
Interleukin-1β and Tumor Necrosis Factor α play related, but distinct, roles in immunity and disease. Our study revealed major mechanistic distinctions in the Toll-like receptor (TLR) signaling-dependent induction for the rapidly expressed genes (IL1B and TNF) coding for these two cytokines. Prior to induction, TNF exhibited pre-bound TATA Binding Protein (TBP) and paused RNA Polymerase II (Pol II), hallmarks of poised immediate-early (IE) genes. In contrast, unstimulated IL1B displayed very low levels of both TBP and paused Pol II, requiring the lineage-specific Spi-1/PU.1 (Spi1) transcription factor as an anchor for induction-dependent interaction with two TLR-activated transcription factors, C/EBPβ and NF-κB. Activation and DNA binding of these two pre-expressed factors resulted in de novo recruitment of TBP and Pol II to IL1B in concert with a permissive state for elongation mediated by the recruitment of elongation factor P-TEFb. This Spi1-dependent mechanism for IL1B transcription, which is unique for a rapidly-induced/poised IE gene, was more dependent upon P-TEFb than was the case for the TNF gene. Furthermore, the dependence on phosphoinositide 3-kinase for P-TEFb recruitment to IL1B paralleled a greater sensitivity to the metabolic state of the cell and a lower sensitivity to the phenomenon of endotoxin tolerance than was evident for TNF. Such differences in induction mechanisms argue against the prevailing paradigm that all IE genes possess paused Pol II and may further delineate the specific roles played by each of these rapidly expressed immune modulators. © 2013 Adamik et al
Physical activity monitoring to assess disability progression in multiple sclerosis
Background: Clinical outcome measurement in multiple sclerosis (MS) usually requires a physical visit. Remote activity monitoring (RAM) using wearable technology provides a rational alternative, especially desirable when distance is involved or in a pandemic setting. Objective: To validate RAM in progressive MS using (1) traditional psychometric methods (2) brain atrophy. Methods: 56 people with progressive MS participated in a longitudinal study over 2.5 years. An arm-worn RAM device measured activity over six days, every six months, and incorporated triaxial accelerometry and transcutaneous physiological variable measurement. Five RAM variables were assessed: physical activity duration, step count, active energy expenditure, metabolic equivalents and a composite RAM score incorporating all four variables. Other assessments every six months included EDSS, MSFC, MSIS-29, Chalder Fatigue Scale and Beck’s Depression Inventory. Annualized brain atrophy was measured using SIENA. Results: RAM was tolerated well by people with MS; the device was worn 99.4% of the time. RAM had good convergent and divergent validity and was responsive, especially with respect to step count. Measurement of physical activity over one day was as responsive as six days. The composite RAM score positively correlated with brain volume loss. Conclusion: Remote activity monitoring is a valid and acceptable outcome measure in MS
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