235 research outputs found
The Capabilities of the Microprobe Kontron Image Analysis System: Application to Mineral Beneficiation
Image analysis is a technique for determining quantities, grain size distributions, grain orientations, intergrowths, associations, liberations and degrees of alteration of minerals or phases in naturally occurring and processed materials. It is performed by analysing images of polished or thin sections of the material under study. To obtain accurate results the images must be faithful reproductions of the material, and each mineral or phase displayed in the images must be distinct enough to be automatically discriminated and identified without operator interaction. An image analysis system that meets these requirements was developed in the Process Mineralogy Section at CANMET by interfacing a microprobe, an energy dispersive X-ray analyser (EDXA), and an image analyser with communication in both directions between the units. The analytical procedure involves transferring a backscattered electron image (BSE) from the microprobe to the image analyser. The minerals or phases are discriminated and identified on the basis of their grey levels in the image. If the grey levels of two or more minerals or phases are too close for discrimination and identification, the minerals or phases are identified by scanning each grain with the electron beam of the microprobe under control of the image analyser, and analysing the grains with the EDXA. To perform the analysis a binary image is produced for each mineral displayed in the BSE image and prepared for analysis by using a variety of image analysis routines. The binary images are analysed and the data are classified, summarized and output in simple tables and graphs
Chromatin proteins and RNA are associated with DNA during all phases of mitosis.
Mitosis brings about major changes to chromosome and nuclear structure. We used recently developed proximity ligation assay-based techniques to investigate the association with DNA of chromatin-associated proteins and RNAs in Drosophila embryos during mitosis. All groups of tested proteins, histone-modifying and chromatin-remodeling proteins and methylated histones remained in close proximity to DNA during all phases of mitosis. We also found that RNA transcripts are associated with DNA during all stages of mitosis. Reduction of H3K27me3 levels or elimination of RNAs had no effect on the association of the components of PcG and TrxG complexes to DNA. Using a combination of proximity ligation assay-based techniques and super-resolution microscopy, we found that the number of protein-DNA and RNA-DNA foci undergoes significant reduction during mitosis, suggesting that mitosis may be accompanied by structural re-arrangement or compaction of specific chromatin domains
Delayed Accumulation of H3K27me3 on Nascent DNA Is Essential for Recruitment of Transcription Factors at Early Stages of Stem Cell Differentiation
Recruitment of transcription factors (TFs) to repressed genes in euchromatin is essential to activate new transcriptional programs during cell differentiation. However, recruitment of all TFs, including pioneer factors, is impeded by condensed H3K27me3-containing chromatin. Single-cell and gene-specific analyses revealed that, during the first hours of induction of differentiation of mammalian embryonic stem cells (ESCs), accumulation of the repressive histone mark H3K27me3 is delayed after DNA replication, indicative of a decondensed chromatin structure in all regions of the replicating genome. This delay provides a critical “window of opportunity” for recruitment of lineage-specific TFs to DNA. Increasing the levels of post-replicative H3K27me3 or preventing S phase entry inhibited recruitment of new TFs to DNA and significantly blocked cell differentiation. These findings suggest that recruitment of lineage-specifying TFs occurs soon after replication and is facilitated by a decondensed chromatin structure. This insight may explain the developmental plasticity of stem cells and facilitate their exploitation for therapeutic purposes
A Model of Polarized X-ray Emission from Twinkling Synchrotron Supernova Shells
Synchrotron X-ray emission components were recently detected in many young
supernova remnants (SNRs). There is even an emerging class - SN1006,
RXJ1713.72-3946, Vela Jr, and others - that is dominated by non-thermal
emission in X-rays, also probably of synchrotron origin. Such emission results
from electrons/positrons accelerated well above TeV energies in the spectral
cut-off regime. In the case of diffusive shock acceleration, which is the most
promising acceleration mechanism in SNRs, very strong magnetic fluctuations
with amplitudes well above the mean magnetic field must be present. Starting
from such a fluctuating field, we have simulated images of polarized X-ray
emission of SNR shells and show that these are highly clumpy with high
polarizations up to 50%. Another distinct characteristic of this emission is
the strong intermittency, resulting from the fluctuating field amplifications.
The details of this "twinkling" polarized X-ray emission of SNRs depend
strongly on the magnetic-field fluctuation spectra, providing a potentially
sensitive diagnostic tool. We demonstrate that the predicted characteristics
can be studied with instruments that are currently being considered. These can
give unique information on magnetic-field characteristics and high-energy
particle acceleration in SNRs.Comment: 7 pages, 8 figures, MNRAS (in press
Cervelleite, Ag4TeS: solution and description of the crystal structure
Copyright: Springer-Verlag Wien 2015. This is the final, post refereeing version. You are advised to consult the publisher's version if you wish to cite from it, http://link.springer.com/article/10.1007%2Fs00710-015-0384-
The Radio Structure of the Supernova Remnant MSH14-63
G315.4-2.3 is an extended shell supernova remnant (SNR) with some
characteristics of evolutionarily young remnants and some of older ones. To
further elucidate some of its characteristics, we present imaging and
polarimetry of this SNR at a frequency of 1.34 GHz with a resolution of 8
arcsec made with the Australia Telescope Compact Array.
The indicators of youth are: Morphologically, the radio emission arises in a
smooth shell without the fine scale filaments seen in the optical. Many of the
optical filaments are Balmer dominated. Where measurable, the orientation of
the magnetic field appears to be radial with respect to the center of the
remnant. There may have been a supernova in that region in AD185.
Indications of older age include: Particularly in RCW86, the bright optical
nebula in the southwestern corner of this extended SNR, but also in other
locations there are several filaments with bright [S II] emission
representative of older shocked filaments in radiative equilibrium. If the
remnant lies at the kinematical distance of 2.8 kpc, it has a diameter of 37 pc
which would be large for a remnant less than two thousand years old.
The remnant seems to be expanding inside a cavity outlined by infrared
emission and so it could well be young and large. Where it is encountering the
walls of the cavity it is slowing rapidly and we observe the radiative
filaments. RCW86 itself is encountering a dense clump of material but may also
be the remains of a more compact lump of ejecta ploughing into the
surroundings.Comment: 15 pages in AAS LaTeX 5.0, 5 figures (2 parts in JPEG, 6 in GIF, 4 in
eps), accepted by Ap
The Relation Between the Surface Brightness and the Diameter for Galactic Supernova Remnants
In this work, we have constructed a relation between the surface brightness
() and diameter (D) of Galactic C- and S-type supernova remnants
(SNRs). In order to calibrate the -D dependence, we have carefully
examined some intrinsic (e.g. explosion energy) and extrinsic (e.g. density of
the ambient medium) properties of the remnants and, taking into account also
the distance values given in the literature, we have adopted distances for some
of the SNRs which have relatively more reliable distance values. These
calibrator SNRs are all C- and S-type SNRs, i.e. F-type SNRs (and S-type SNR
Cas A which has an exceptionally high surface brightness) are excluded. The
Sigma-D relation has 2 slopes with a turning point at D=36.5 pc: (at 1
GHz)=8.4 D
WmHzster (for
WmHzster and D36.5 pc) and (at 1
GHz)=2.7 10 D
WmHzster (for
WmHzster and D36.5 pc). We discussed the theoretical
basis for the -D dependence and particularly the reasons for the change
in slope of the relation were stated. Added to this, we have shown the
dependence between the radio luminosity and the diameter which seems to have a
slope close to zero up to about D=36.5 pc. We have also adopted distance and
diameter values for all of the observed Galactic SNRs by examining all the
available distance values presented in the literature together with the
distances found from our -D relation.Comment: 45 pages, 2 figures, accepted for publication in Astronomical and
Astrophysical Transaction
Magnetic fields in supernova remnants and pulsar-wind nebulae
We review the observations of supernova remnants (SNRs) and pulsar-wind
nebulae (PWNe) that give information on the strength and orientation of
magnetic fields. Radio polarimetry gives the degree of order of magnetic
fields, and the orientation of the ordered component. Many young shell
supernova remnants show evidence for synchrotron X-ray emission. The spatial
analysis of this emission suggests that magnetic fields are amplified by one to
two orders of magnitude in strong shocks. Detection of several remnants in TeV
gamma rays implies a lower limit on the magnetic-field strength (or a
measurement, if the emission process is inverse-Compton upscattering of cosmic
microwave background photons). Upper limits to GeV emission similarly provide
lower limits on magnetic-field strengths. In the historical shell remnants,
lower limits on B range from 25 to 1000 microGauss. Two remnants show
variability of synchrotron X-ray emission with a timescale of years. If this
timescale is the electron-acceleration or radiative loss timescale, magnetic
fields of order 1 mG are also implied. In pulsar-wind nebulae, equipartition
arguments and dynamical modeling can be used to infer magnetic-field strengths
anywhere from about 5 microGauss to 1 mG. Polarized fractions are considerably
higher than in SNRs, ranging to 50 or 60% in some cases; magnetic-field
geometries often suggest a toroidal structure around the pulsar, but this is
not universal. Viewing-angle effects undoubtedly play a role. MHD models of
radio emission in shell SNRs show that different orientations of upstream
magnetic field, and different assumptions about electron acceleration, predict
different radio morphology. In the remnant of SN 1006, such comparisons imply a
magnetic-field orientation connecting the bright limbs, with a non-negligible
gradient of its strength across the remnant.Comment: 20 pages, 24 figures; to be published in SpSciRev. Minor wording
change in Abstrac
Radio emission from Supernova Remnants
The explosion of a supernova releases almost instantaneously about 10^51 ergs
of mechanic energy, changing irreversibly the physical and chemical properties
of large regions in the galaxies. The stellar ejecta, the nebula resulting from
the powerful shock waves, and sometimes a compact stellar remnant, constitute a
supernova remnant (SNR). They can radiate their energy across the whole
electromagnetic spectrum, but the great majority are radio sources. Almost 70
years after the first detection of radio emission coming from a SNR, great
progress has been achieved in the comprehension of their physical
characteristics and evolution. We review the present knowledge of different
aspects of radio remnants, focusing on sources of the Milky Way and the
Magellanic Clouds, where the SNRs can be spatially resolved. We present a brief
overview of theoretical background, analyze morphology and polarization
properties, and review and critical discuss different methods applied to
determine the radio spectrum and distances. The consequences of the interaction
between the SNR shocks and the surrounding medium are examined, including the
question of whether SNRs can trigger the formation of new stars. Cases of
multispectral comparison are presented. A section is devoted to reviewing
recent results of radio SNRs in the Magellanic Clouds, with particular emphasis
on the radio properties of SN 1987A, an ideal laboratory to investigate
dynamical evolution of an SNR in near real time. The review concludes with a
summary of issues on radio SNRs that deserve further study, and analyzing the
prospects for future research with the latest generation radio telescopes.Comment: Revised version. 48 pages, 15 figure
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