2,499 research outputs found
Longitudinal Eigenvibration of Multilayer Colloidal Crystals and the Effect of Nanoscale Contact Bridges
Longitudinal contact-based vibrations of colloidal crystals with a controlled
layer thickness are studied. These crystals consist of 390 nm diameter
polystyrene spheres arranged into close packed, ordered lattices with a
thickness of one to twelve layers. Using laser ultrasonics, eigenmodes of the
crystals that have out-of-plane motion are excited. The particle-substrate and
effective interlayer contact stiffnesses in the colloidal crystals are
extracted using a discrete, coupled oscillator model. Extracted stiffnesses are
correlated with scanning electron microscope images of the contacts and atomic
force microscope characterization of the substrate surface topography after
removal of the spheres. Solid bridges of nanometric thickness are found to
drastically alter the stiffness of the contacts, and their presence is found to
be dependent on the self-assembly process. Measurements of the eigenmode
quality factors suggest that energy leakage into the substrate plays a role for
low frequency modes but is overcome by disorder- or material-induced losses at
higher frequencies. These findings help further the understanding of the
contact mechanics, and the effects of disorder in three-dimensional micro- and
nano-particulate systems, and open new avenues to engineer new types of micro-
and nanostructured materials with wave tailoring functionalities via control of
the adhesive contact properties
The Gravitational Lens Candidate FBQ 1633+3134
We present our ground-based optical imaging, spectral analysis, and high
resolution radio mapping of the gravitational lens candidate FBQ 1633+3134.
This z=1.52, B=17.7 quasar appears double on CCD images with an image
separation of 0.66 arcseconds and a flux ratio of ~3:1 across BVRI filters. A
single 0.27 mJy radio source is detected at 8.46 GHz, coincident to within an
arcsecond of both optical components, but no companion at radio wavelengths is
detected down to a flux level of 0.1 mJy (3 sigma). Spectral observations
reveal a rich metal-line absorption system consisting of a strong Mg II doublet
and associated Fe I and Fe II absorption features, all at an intervening
redshift of z=0.684, suggestive of a lensing galaxy. Point spread function
subtraction however shows no obvious signs of a third object between the two
quasar images, and places a detection limit of I > 23.0 if such an object
exists. Although the possibility that FBQ 1633+3134 is a binary quasar cannot
be ruled out, the evidence is consistent with it being a single quasar lensed
by a faint, metal-rich galaxy.Comment: 24 pages, 5 figures. Accepted by AJ. A calibration error affecting B
and V band apparent magnitudes has been corrected. The conclusions of the
paper are not change
Frequent disturbances and chronic pressures constrain stony coral recovery on Florida’s Coral Reef
Acute disturbances and chronic pressures have an important and increasing influence on the structure of coral reef communities. For the viability of benthic taxa such as stony corals, a balance between loss following disturbance and recovery is vital. Coral populations on reefs with lower exposure to chronic pressures are often presumed to have increased resilience, enabling them to recover quickly following disturbance, but decades of anthropogenic stress and degradation may undermine the systematic recovery and reassembly of benthic communities. This study explored spatiotemporal changes in benthic community structure over a 15 yr period at three distinct coral reef regions with a gradient of chronic pressures in Florida, USA, (southeast Florida, the Florida Keys and the Dry Tortugas). We specifically assessed the spatial scale, potential drivers of change and resilience in stony coral, octocoral, sponge and macroalgae cover. Spatiotemporal changes were assessed at four different scales: among regions, habitats, sub-regions, and habitat types within regions. Cover of stony corals remained very low or declined in every region from 2004 to 2018, with corresponding increases in macroalgae cover. Stony coral recovery was limited regardless of regional differences in chronic pressure. Octocorals exhibited greater resilience due to increased recovery following disturbance and generally had higher cover than stony corals on Florida’s Coral Reef, while sponge cover was very stable over the study period. Acute disturbances, which affected sites on average once every 3 yr, negatively impacted stony coral and/or octocoral cover in every region and habitat, contributing to the regionwide proliferation of macroalgae. This study determined that high disturbance frequency and chronic anthropogenic pressures on Florida’s Coral Reef have led to sustained declines in stony corals and corresponding proliferation of macroalgae. Stony corals were expected to recover during inter-disturbance periods, but in Florida, even in locations with lower chronic pressure, recovery is severely limited
Pathogen-specific immune fingerprints during acute infection: The diagnostic potential of human γδ T-cells
Pharmacokinetic behavior of intraperitoneal teicoplanin during treatment of peritonitis complicating continuous ambulatory peritoneal dialysis
The Process of Stellar Tidal Disruption by Supermassive Black Holes. The first pericenter passage
Tidal disruption events (TDEs) are among the brightest transients in the
optical, ultraviolet, and X-ray sky. These flares are set into motion when a
star is torn apart by the tidal field of a massive black hole, triggering a
chain of events which is -- so far -- incompletely understood. However, the
disruption process has been studied extensively for almost half a century, and
unlike the later stages of a TDE, our understanding of the disruption itself is
reasonably well converged. In this Chapter, we review both analytical and
numerical models for stellar tidal disruption. Starting with relatively simple,
order-of-magnitude physics, we review models of increasing sophistication, the
semi-analytic ``affine formalism,'' hydrodynamic simulations of the disruption
of polytropic stars, and the most recent hydrodynamic results concerning the
disruption of realistic stellar models. Our review surveys the immediate
aftermath of disruption in both typical and more unusual TDEs, exploring how
the fate of the tidal debris changes if one considers non-main sequence stars,
deeply penetrating tidal encounters, binary star systems, and sub-parabolic
orbits. The stellar tidal disruption process provides the initial conditions
needed to model the formation of accretion flows around quiescent massive black
holes, and in some cases may also lead to directly observable emission, for
example via shock breakout, gravitational waves or runaway nuclear fusion in
deeply plunging TDEs.Comment: Review chapter in book: 'The Tidal Disruption of Stars by Massive
Black Holes', Space Science Reviews, Springer. Comments welcom
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High-precision 41K/39K measurements by MC-ICP-MS indicate terrestrial variability of δ41K
Potassium is a major component in continental crust, the fourth-most abundant cation in seawater, and a key element in biological processes. Until recently, difficulties with existing analytical techniques hindered our ability to identify natural isotopic variability of potassium isotopes in terrestrial materials. However, measurement precision has greatly improved, and a range of K isotopic compositions has now been demonstrated in natural samples. In this study, we present a new technique for high-precision measurement of K isotopic ratios using high-resolution, cold
plasma multi-collector mass spectrometry. We apply this technique to demonstrate natural variability in the ratio of 41K to 39K in a diverse group of geological and biological samples, including silicate and evaporite minerals, seawater, and plant and animal tissues. The total range in 41K/39K ratios is ca. 2.6‰, with a long-term external reproducibility of 0.17‰ (2σ, N=108). Seawater and seawater-derived evaporite minerals are systematically enriched in 41K compared to silicate minerals by ca. 0.6‰, a result consistent with recent findings (1, 2). Although our average bulk-
silicate Earth value (-0.54‰) is indistinguishable from previously published values, we find systematic δ41K variability in some high-temperature sample suites, particularly those with evidence for the presence of fluids. The δ41K values of biological samples span a range of ca. 1.2‰ between terrestrial mammals, plants, and marine organisms. Implications of terrestrial K isotope variability for the atomic weight of K and K-based geochronology are discussed. Our results indicate that high-precision measurements of stable K isotopes, made using commercially available
mass spectrometers, can provide unique insights into the chemistry of potassium in geological and biological systems
Étude comparative des pratiques d’enseignement de la lecture en 4e primaire : des questions de didactique pointées par l’étude internationale PIRLS 2011
Cette étude vise à mettre en lumière des pratiques d‘enseignement de la lecture susceptibles
de rendre compte des disparités de performances observées dans différents systèmes éducatifs.
Les comparaisons portent sur les pratiques d’enseignement de la lecture déclarées par
les enseignant.e.s de huit systèmes éducatifs contrastés tant au plan de la langue enseignée
(français, anglais, allemand) qu’au plan des performances moyennes obtenues à l’épreuve
PIRLS 2011. Les résultats mettent en évidence des différences parfois importantes dans la fréquence
à laquelle sont mises en place certaines facettes de l’enseignement de la lecture et
plus spécifiquement de la compréhension. Ces pratiques témoignent de visions contrastées,
parfois éloignées de ce que l’on pourrait attendre d’un enseignement de la lecture experte.Peer reviewe
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