1,468 research outputs found
The fate of the homoctenids (Tentaculitoidea) during the Frasnian-Famennian mass extinction (Late Devonian)
The homoctenids (Tentaculitoidea) are small, conical-shelled marine animals which are amongst the most abundant and widespread of all Late Devonian fossils. They were a principal casualty of the Frasnian-Famennian (F-F, Late Devonian) mass extinction, and thus provide an insight into the extinction dynamics. Despite their abundance during the Late Devonian, they have been largely neglected by extinction studies. A number of Frasnian-Famennian boundary sections have been studied, in Poland, Germany, France, and the United States. These sections have yielded homoctenids, which allow precise recognition of the timing of the mass extinction. It is clear that the homoctenids almost disappear from the fossil record during the latest Frasnian “Upper Kellwasser Event”. The coincident extinction of this pelagic group, and the widespread development of intense marine anoxia within the water column, provides a causal link between anoxia and the F-F extinction. Most notable is the sudden demise of a group, which had been present in rock-forming densities, during this anoxic event. One new species, belonging to Homoctenus is described, but is not formally named here
Asiago eclipsing binaries program. II. V505 Per
The orbit and fundamental physical parameters of the double-lined eclipsing
binary V505 Per are derived by means of Echelle high resolution, high S/N
spectroscopy and B, V photometry. Effective temperatures, gravities, rotational
velocities and metallicities are obtained from atmospheric chi^2 analysis. An
E(B-V)<=0.01 mag reddening is derived from interstellar NaI and KI lines. The
distance to the system computed from orbital parameters (60.6 +/- 1 pc) is
identical to the newly re-reduced Hipparcos parallax (61.5 +/- 1.9 pc). The
masses of the two components (M(1) = 1.2693 +/- 0.0011 and M(2) = 1.2514 +/-
0.0012 Msun) place them in the transition region between convective and
radiative stellar cores of the HR diagram, with the more massive of the two
showing already the effect of evolution within the Main Sequence band (T(1) =
6512 +/- 21 K, T(2) = 6462 +/- 12 K, R(1) = 1.287 +/- 0.014, R(2) = 1.266 +/-
0.013 Rsun). This makes this system of particular relevance to theoretical
stellar models, as a test on the overshooting. We compare the firm
observational results for V505 Per component stars with the predictions of
various libraries of theoretical stellar models (BaSTI, Padova, Granada,
Yonsei-Yale, Victoria-Regina) as well as BaSTI models computed specifically for
the masses and chemical abundances of V505 Per. We found that the overshooting
at the masses of V505 Per component stars is already pretty low, but not null,
and described by efficiencies lambda(OV)=0.093 and 0.087 for the 1.27 and 1.25
Msun components, respectively. According to the computed BaSTI models, the age
of the system is about 0.9 Gyr and the element diffusion during this time has
reduced the surface metallicity from the initial [M/H]=-0.03 to the current
[M/H]=-0.13, in excellent agreement with observed [M/H]=-0.12 +/- 0.03.Comment: accepted in press by A&
XUV digital in-line holography using high-order harmonics
A step towards a successful implementation of timeresolved digital in-line
holography with extreme ultraviolet radiation is presented. Ultrashort XUV
pulses are produced as high-order harmonics of a femtosecond laser and a
Schwarzschild objective is used to focus harmonic radiation at 38 nm and to
produce a strongly divergent reference beam for holographic recording.
Experimental holograms of thin wires are recorded and the objects
reconstructed. Descriptions of the simulation and reconstruction theory and
algorithms are also given. Spatial resolution of few hundreds of nm is
potentially achievable, and micrometer resolution range is demonstrated.Comment: 8 pages, 8 figure
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Reduced Recombination and Capacitor-like Charge Buildup in an Organic Heterojunction
Organic photovoltaic (OPV) efficiencies continue to rise, raising their prospects for solar energy conversion. However, researchers have long considered how to suppress the loss of free carriers by recombination—poor diffusion and significant Coulombic attraction can cause electrons and holes to encounter each other at interfaces close to where they were photogenerated. Using femtosecond transient spectroscopies, we report the nanosecond grow-in of a large transient 20 Stark effect, caused by nanoscale electric fields of ~487 kV/cm between photogenerated free carriers in the device active layer. We find that particular morphologies of the active layer lead to an energetic cascade for charge carriers, suppressing pathways to recombination, which is ~2000 times less than predicted by Langevin theory. This in turn leads to the build-up of electric charge in donor and acceptor domains—away from the interface—resistant to bimolecular recombination. 25 Interestingly, this signal is only experimentally obvious in thick films, due to the different scaling of electro-absorption and photo-induced absorption signals in transient absorption spectroscopy. Rather than inhibiting device performance, we show that devices up to 600 nm thick maintain efficiencies of > 8 % because domains can afford much higher carrier densities. These observations suggest that with particular nanoscale morphologies, the bulk heterojunction can go beyond its established role in charge photogeneration, and can act as a capacitor, where adjacent free charges are held away from the interface and can be protected from bimolecular recombination
Self-organized Emergence of Navigability on Small-World Networks
This paper mainly investigates why small-world networks are navigable and how
to navigate small-world networks. We find that the navigability can naturally
emerge from self-organization in the absence of prior knowledge about
underlying reference frames of networks. Through a process of information
exchange and accumulation on networks, a hidden metric space for navigation on
networks is constructed. Navigation based on distances between vertices in the
hidden metric space can efficiently deliver messages on small-world networks,
in which long range connections play an important role. Numerical simulations
further suggest that high cluster coefficient and low diameter are both
necessary for navigability. These interesting results provide profound insights
into scalable routing on the Internet due to its distributed and localized
requirements.Comment: 3 figure
Matryoshka locally resonant sonic crystal
The results of numerical modelling of sonic crystals with resonant array
elements are reported. The investigated resonant elements include plain slotted
cylinders as well as various their combinations, in particular, Russian doll or
Matryoshka configurations. The acoustic band structure and transmission
characteristics of such systems have been computed with the use of finite
element methods. The general concept of a locally resonant sonic crystal is
proposed, which utilises acoustic resonances to form additional band gaps that
are decoupled from Bragg gaps. An existence of a separate attenuation mechanism
associated with the resonant elements, which increases performance in the lower
frequency regime has been identified. The results show a formation of broad
band gaps positioned significantly below the first Bragg frequency. For low
frequency broadband attenuation a most optimal configuration is the Matryoshka
sonic crystal, where each scattering unit is composed of multiple concentric
slotted cylinders. This system forms numerous gaps in the lower frequency
regime, below Bragg bands, whilst maintaining a reduced crystal size viable for
noise barrier technology. The finding opens new perspectives for construction
of sound barriers in the low frequency range usually inaccessible by
traditional means including conventional sonic crystals.Comment: 7 Pages, 12 Figure
GenSeed-HMM: A tool for progressive assembly using profile HMMs as seeds and its application in Alpavirinae viral discovery from metagenomic data
This work reports the development of GenSeed-HMM, a program that implements seed-driven progressive assembly, an approach to reconstruct specific sequences from unassembled data, starting from short nucleotide or protein seed sequences or profile Hidden Markov Models (HMM). The program can use any one of a number of sequence assemblers. Assembly is performed in multiple steps and relatively few reads are used in each cycle, consequently the program demands low computational resources. As a proof-of-concept and to demonstrate the power of HMM-driven progressive assemblies, GenSeed-HMM was applied to metagenomic datasets in the search for diverse ssDNA bacteriophages from the recently described Alpavirinae subfamily. Profile HMMs were built using Alpavirinae-specific regions from multiple sequence alignments using either the viral protein 1 (VP1) (major capsid protein) or VP4 (genome replication initiation protein). These profile HMMs were used by GenSeed-HMM (running Newbler assembler) as seeds to reconstruct viral genomes from sequencing datasets of human fecal samples. All contigs obtained were annotated and taxonomically classified using similarity searches and phylogenetic analyses. The most specific profile HMM seed enabled the reconstruction of 45 partial or complete Alpavirinae genomic sequences. A comparison with conventional (global) assembly of the same original dataset, using Newbler in a standalone execution, revealed that GenSeed-HMM outperformed global genomic assembly in several metrics employed. This approach is capable of detecting organisms that have not been used in the construction of the profile HMM, which opens up the possibility of diagnosing novel viruses, without previous specific information, constituting a de novo diagnosis. Additional applications include, but are not limited to, the specific assembly of extrachromosomal elements such as plastid and mitochondrial genomes from metagenomic data. Profile HMM seeds can also be used to reconstruct specific protein coding genes for gene diversity studies, and to determine all possible gene variants present in a metagenomic sample. Such surveys could be useful to detect the emergence of drug-resistance variants in sensitive environments such as hospitals and animal production facilities, where antibiotics are regularly used. Finally, GenSeed-HMM can be used as an adjunct for gap closure on assembly finishing projects, by using multiple contig ends as anchored seeds
Single-cell transcriptomics uncovers distinct molecular signatures of stem cells in chronic myeloid leukemia
Recent advances in single-cell transcriptomics are ideally placed to unravel intratumoral heterogeneity and selective resistance of cancer stem cell (SC) subpopulations to molecularly targeted cancer therapies. However, current single-cell RNA-sequencing approaches lack the sensitivity required to reliably detect somatic mutations. We developed a method that combines high-sensitivity mutation detection with whole-transcriptome analysis of the same single cell. We applied this technique to analyze more than 2,000 SCs from patients with chronic myeloid leukemia (CML) throughout the disease course, revealing heterogeneity of CML-SCs, including the identification of a subgroup of CML-SCs with a distinct molecular signature that selectively persisted during prolonged therapy. Analysis of nonleukemic SCs from patients with CML also provided new insights into cell-extrinsic disruption of hematopoiesis in CML associated with clinical outcome. Furthermore, we used this single-cell approach to identify a blast-crisis-specific SC population, which was also present in a subclone of CML-SCs during the chronic phase in a patient who subsequently developed blast crisis. This approach, which might be broadly applied to any malignancy, illustrates how single-cell analysis can identify subpopulations of therapy-resistant SCs that are not apparent through cell-population analysis
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