588 research outputs found
Radio Emission from the Composite Supernova Remnant G326.3-1.8 (MSH15-56)
High resolution radio observations of the composite supernova remnant (SNR)
G326.3-1.8 or MSH 15-56 with the Australia Telescope Compact Array show details
of both the shell and the bright plerion which is offset about 1/3 of the
distance from the center of the SNR to the shell. The shell appears to be
composed of thin filaments, typical of older shell SNRs. The central part of
the elongated plerion is composed of a bundle of parallel ridges which bulge
out at the ends and form a distinct ring structure on the northwestern end. The
magnetic field with a strength of order 45 microGauss, is directed along the
axis of the ridges but circles around the northwestern ring. This plerion is
large and bright in the radio but is not detected in x-ray or optical
wavelengths. There is, however, a faint hard x-ray feature closer to the shell
outside the plerion. Perhaps if the supernova explosion left a rapidly moving
magnetar with large energy input but initially rapid decay of both relativistic
particles and magnetic field, the observed differences with wavelength could be
explained.Comment: 15 pages, 10 figures, accepted by Ap
Ultraviolet Observations of Super-Chandrasekhar Mass Type Ia Supernova Candidates with Swift UVOT
Among Type Ia supernovae (SNe~Ia) exist a class of overluminous objects whose
ejecta mass is inferred to be larger than the canonical Chandrasekhar mass. We
present and discuss the UV/optical photometric light curves, colors, absolute
magnitudes, and spectra of three candidate Super-Chandrasekhar mass
SNe--2009dc, 2011aa, and 2012dn--observed with the Swift Ultraviolet/Optical
Telescope. The light curves are at the broad end for SNe Ia, with the light
curves of SN~2011aa being amongst the broadest ever observed. We find all three
to have very blue colors which may provide a means of excluding these
overluminous SNe from cosmological analysis, though there is some overlap with
the bluest of "normal" SNe Ia. All three are overluminous in their UV absolute
magnitudes compared to normal and broad SNe Ia, but SNe 2011aa and 2012dn are
not optically overluminous compared to normal SNe Ia. The integrated luminosity
curves of SNe 2011aa and 2012dn in the UVOT range (1600-6000 Angstroms) are
only half as bright as SN~2009dc, implying a smaller 56Ni yield. While not
enough to strongly affect the bolometric flux, the early time mid-UV flux makes
a significant contribution at early times. The strong spectral features in the
mid-UV spectra of SNe 2009dc and 2012dn suggest a higher temperature and lower
opacity to be the cause of the UV excess rather than a hot, smooth blackbody
from shock interaction. Further work is needed to determine the ejecta and 56Ni
masses of SNe 2011aa and 2012dn and fully explain their high UV luminosities.Comment: 12 pages, 8 figures Accepted for publication in the Astrophysical
Journal Data available on the Swift supernova website
http://swift.gsfc.nasa.gov/docs/swift/sne/swift_sn.htm
X-ray generation using carbon nanotubes
Since the discovery of X-rays over a century ago the techniques applied to the engineering of X-ray sources have remained relatively unchanged. From the inception of thermionic electron sources, which, due to simplicity of fabrication, remain central to almost all X-ray applications, there have been few fundamental technological advances. However, with the emergence of ever more demanding medical and inspection techniques, including computed tomography and tomosynthesis, security inspection, high throughput manufacturing and radiotherapy, has resulted in a considerable level of interest in the development of new fabrication methods. The use of conventional thermionic sources is limited by their slow temporal response and large physical size. In response, field electron emission has emerged as a promising alternative means of deriving a highly controllable electron beam of a well-defined distribution. When coupled to the burgeoning field of nanomaterials, and in particular, carbon nanotubes, such systems present a unique technological opportunity. This review provides a summary of the current state-of-the-art in carbon nanotube-based field emission X-ray sources. We detail the various fabrication techniques and functional advantages associated with their use, including the ability to produce ever smaller electron beam assembles, shaped cathodes, enhanced temporal stability and emergent fast-switching pulsed sources. We conclude with an overview of some of the commercial progress made towards the realisation of an innovative and disruptive technology.Clare Collins is studying for the MRes in Ultra Precision,
funded by the EPSRC, at the University of Cambridge.This is the final published version. It first appeared at http://www.nanoconvergencejournal.com/content/2/1/1
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The implications of spatially variable pre-emergence herbicide efficacy for weed management
BACKGROUND
The efficacy of pre-emergence herbicides within fields is spatially variable due to soil heterogeneity. We quantified the effect of soil organic matter on the efficacy of two pre-emergence herbicides; flufenacet and pendimethalin, against A. myosuroides and investigated the implications of variation in organic matter for weed management using a crop-weed competition model.
RESULTS
Soil organic matter played a critical role in determining the level of control achieved. The high organic matter soil had more surviving weeds with higher biomass than the low organic matter soil. In the absence of competition, surviving plants recovered to produce the same amount of seed as if no herbicide were applied. The competition model predicted that weeds surviving pre-emergence herbicides could compensate for sub-lethal effects even when competing with the crop. The ED50 was higher for weed seed production than seedling mortality or biomass. This difference was greatest on high organic matter soil.
CONCLUSION
These results show that the application rate of herbicides should be adjusted to account for within-field variation in soil organic matter. The results from the modelling emphasised the importance of crop competition in limiting the capacity of weeds surviving pre-emergence herbicides to compensate and replenish the seedbank
Operation Moshtarak and the manufacture of credible, “heroic” warfare
Richard Lance Keeble argues that Fleet Street’s coverage of the Afghan conflict has served largely to promote the interests of the military/industrial/media complex – and marginalise the views of the public who have consistently appealed in polls for the troops to be brought back hom
Conservation genomics of an Australian cycad, cycas calcicola and the absence of key genotypes in botanic gardens
Phylogeography and allopatric divergence of cypress species (Cupressus L.) in the Qinghai-Tibetan Plateau and adjacent regions
Additional files can be found at http://www.biomedcentral.com/1471-2148/10/194Background Although allopatric speciation is viewed as the most common way in which species originate, allopatric divergence among a group of closely related species has rarely been examined at the population level through phylogeographic analysis. Here we report such a case study on eight putative cypress (Cupressus) species, which each have a mainly allopatric distribution in the Qinghai-Tibetan Plateau (QTP) and adjacent regions. The analysis involved sequencing three plastid DNA fragments (trnD-trnT, trnS-trnG and trnL-trnF) in 371 individuals sampled from populations at 66 localities. Results Both phylogenetic and network analyses showed that most DNA haplotypes recovered or haplotype-clustered lineages resolved were largely species-specific. Across all species, significant phylogeographic structure (NST > GST, P < 0.05) implied a high correlation between haplotypes/lineages and geographic distribution. Two species, C. duclouxiana and C. chengiana, which are distributed in the eastern QTP region, contained more haplotypes and higher diversity than five species with restricted distributions in the western highlands of the QTP. The remaining species, C. funebris, is widely cultivated and contained very little cpDNA diversity. Conclusions It is concluded that the formation of high mountain barriers separating deep valleys in the QTP and adjacent regions caused by various uplifts of the plateau since the early Miocene most likely promoted allopatric divergence in Cupressus by restricting gene flow and fixing local, species-specific haplotypes in geographically isolated populations. The low levels of intraspecific diversity present in most species might stem from population bottlenecks brought about by recurrent periods of unfavorable climate and more recently by the negative impacts of human activities on species' distributions. Our findings shed new light on the importance of geographical isolation caused by the uplift of the QTP on the development of high plant species diversity in the QTP biodiversity hotspot.Publisher PDFPeer reviewe
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
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