1,299 research outputs found
Integral-field studies of the high-redshift Universe
We present results from a new method of exploring the distant Universe. We
use 3-D spectroscopy to sample a large cosmological volume at a time when the
Universe was less than 3 billion years old to investigate the evolution of
star-formation activity. Within this study we also discovered a high redshift
type-II quasar which would not have been identified with imaging studies alone.
This highlights the crucial role that integral-field spectroscopy may play in
surveying the distant Universe in the future.Comment: From proceedings of Euro3D Spectroscopy annual RTN network meeting
(ed. J. Walsh). 5 pages, accepted for publication in Astron. Nach
Strategies of subcellular fractionation suitable for analysis of peroxisomes and microperoxisomes of animal tissues.
Strategies of subcellular fractionation are reviewed from the perspective of their utility in the
analysis of peroxisomes. The considerable potential inherant in the method of rate dependent banding In
zonal rotors is emphasized. The use of various density gradient solutes IS considered
Loss of scribble causes cell competition in mammalian epithelial cells
Cancer is a disease caused by transformation of cells by the activation or over-expression of oncogenes such as Ras and c-myc, and the loss of tumour suppressor genes such as E-cadherin and scribble. The initial stage of tumourigenesis is the transformation of a single cell in an otherwise normal epithelium. What occurs at this stage is largely unknown - do the transformed cells and normal cells co-exist or is there an antagonism between them? This thesis examines the fate of epithelial cells that lose the tumour suppressor scribble when in an otherwise normal epithelium.
The fate of scribble knockout clones has been studied in Drosophila melanogaster larval imaginal discs. It has been observed that scribble knockout clones are removed from the larval tissues by c-Jun N-terminal kinase (JNK) dependent apoptosis. It is though that this is an innate tumour suppressive mechanism. It is therefore of great interest and importance to understand if a similar phenomenon can be seen in mammalian cells.
Scribble knockdown Madin-Darby canine kidney (MDCK) epithelial cells die only when surrounded by normal MDCK cells. Dead scribble short-hairpin RNA (shRNA) cells are apically extruded from the epithelium after cell death and exhibit classical apoptotic markers such as cytoplasmic condensation, caspase 3 activation and DNA fragmentation. Extrusion of dead scribble knockout cells occurs after initiation of apoptosis as blocking myosin activation results in many dead scribble knockout cells staying in the epithelial monolayer. Prior to cell death they maintain normal cell-cell adhesion with their normal MDCK neighbours and activate the stress induced protein kinase p38, but not c-‐Jun N‐- terminal kinase (JNK)
Transport in the Heavy Fermion Superconductor UPt3
We report new theoretical results and analysis for the transport properties
of superconducting UPt3 based on the leading models for the pairing symmetry.
We use Fermi surface data and the measured inelastic scattering rate to show
that the low-temperature thermal conductivity and transverse sound attenuation
in the A and B phase of UPt3 are in excellent agreement with pairing states
belonging to the two-dimensional orbital E2u representation.Comment: 2 pages, contribution at Int. Conf. LT-22, Helsinki, Finland, 4-11
Aug. 199
Are microseismic ground displacements a significant geomorphic agent?
This paper considers the role that microseismic ground displacements may play in fracturing rock via cyclic loading and subcritical crack growth. Using a coastal rock cliff as a case study, we firstly undertake a literature review to define the spatial locations that may be prone to microseismic damage. It is suggested that microseismic weakening of rock can only occur in ‘damage accumulation zones’ of limited spatial extent. Stress concentrations resulting from cliff height, slope angle and surface morphology may nucleate and propagate a sufficiently dense population of microcracks that can then be exploited by microseismic cyclic loading. We subsequently examine a 32-day microseismic dataset obtained from a coastal cliff-top location at Staithes, UK. The dataset demonstrates that microseismic ground displacements display low peak amplitudes that are punctuated by periods of greater displacement during storm conditions. Microseismic displacements generally display limited preferential directivity, though we observe rarely occurring sustained ground motions with a cliff-normal component during storm events. High magnitude displacements and infrequently experienced ground motion directions may be more damaging than the more frequently occurring, reduced magnitude displacements characteristic of periods of relative quiescence. As high magnitude, low frequency events exceed and then increase the damage threshold, these extremes may also render intervening, reduced magnitude microseismic displacements ineffective in terms of damage accumulation as a result of crack tip blunting and the generation of residual compressive stresses that close microcracks. We contend that damage resulting from microseismic ground motion may be episodic, rather than being continuous and in (quasi-)proportional and cumulative response to environmental forcing. A conceptual model is proposed that describes when and where microseismic ground motions can operate effectively. We hypothesise that there are significant spatial and temporal limitations on effective microseismic damage accumulation, such that the net efficacy of microseismic ground motions in preparing rock for fracture, and hence in enhancing erosion, may be considerably lower than previously suggested in locations where high magnitude displacements punctuate ‘standard’ displacement conditions. Determining and measuring the exact effects of microseismic ground displacement on damage accumulation and as a trigger to macro-scale fracture in the field is not currently possible, though our model remains consistent with field observations and conceptual models of controls on rockfall activity
Transport and the Order Parameter of Superconducting UPt3
We calculate the ultrasonic absorption and the thermal conductivity in the
superconducting state of UPt as functions of temperature and direction of
propagation and polarization. Two leading candidates for the superconducting
order parameter are considered: the and representations. Both
can fit the data except for the ultrasonic absorption in the phase. To do
that, it is necessary to suppose that the system has only a single domain, and
that must be chosen as the most favorable one. However, the theory
requires fine-tuning of parameters to fit the low temperature thermal
conductivity. Thus, transport data favor the theory. Measurements of
the thermal conductivity as a function of pressure at low temperature could
help to further distinguish the two theories.Comment: 7 pages, 4 figure
The State of Self-Organized Criticality of the Sun During the Last Three Solar Cycles. II. Theoretical Model
The observed powerlaw distributions of solar flare parameters can be
interpreted in terms of a nonlinear dissipative system in the state of
self-organized criticality (SOC). We present a universal analytical model of a
SOC process that is governed by three conditions: (i) a multiplicative or
exponential growth phase, (ii) a randomly interrupted termination of the growth
phase, and (iii) a linear decay phase. This basic concept approximately
reproduces the observed frequency distributions. We generalize it to a
randomized exponential-growth model, which includes also a (log-normal)
distribution of threshold energies before the instability starts, as well as
randomized decay times, which can reproduce both the observed occurrence
frequency distributions and the scatter of correlated parametyers more
realistically. With this analytical model we can efficiently perform
Monte-Carlo simulations of frequency distributions and parameter correlations
of SOC processes, which are simpler and faster than the iterative simulations
of cellular automaton models. Solar cycle modulations of the powerlaw slopes of
flare frequency distributions can be used to diagnose the thresholds and growth
rates of magnetic instabilities responsible for solar flares.Comment: Part II of Paper I: The State of Self-Organized Criticality of the
Sun During the Last Three Solar Cycles. I. Observation
Absence of Persistent Magnetic Oscillations in Type-II Superconductors
We report on a numerical study intended to examine the possibility that
magnetic oscillations persist in type II superconductors beyond the point where
the pairing self-energy exceeds the normal state Landau level separation. Our
work is based on the self-consistent numerical solution for model
superconductors of the Bogoliubov-deGennes equations for the vortex lattice
state. In the regime where the pairing self-energy is smaller than the
cyclotron energy, magnetic oscillations resulting from Landau level
quantization are suppressed by the broadening of quasiparticle Landau levels
due to the non-uniform order parameter of the vortex lattice state, and by
splittings of the quasiparticle bands. Plausible arguments that the latter
effect can lead to a sign change of the fundamental harmonic of the magnetic
oscillations when the pairing self-energy is comparable to the cyclotron energy
are shown to be flawed. Our calculations indicate that magnetic oscillations
are strongly suppressed once the pairing self-energy exceeds the Landau level
separation.Comment: 7 pages, revtex, 7 postscript figure
Deterministically Driven Avalanche Models of Solar Flares
We develop and discuss the properties of a new class of lattice-based
avalanche models of solar flares. These models are readily amenable to a
relatively unambiguous physical interpretation in terms of slow twisting of a
coronal loop. They share similarities with other avalanche models, such as the
classical stick--slip self-organized critical model of earthquakes, in that
they are driven globally by a fully deterministic energy loading process. The
model design leads to a systematic deficit of small scale avalanches. In some
portions of model space, mid-size and large avalanching behavior is scale-free,
being characterized by event size distributions that have the form of
power-laws with index values, which, in some parameter regimes, compare
favorably to those inferred from solar EUV and X-ray flare data. For models
using conservative or near-conservative redistribution rules, a population of
large, quasiperiodic avalanches can also appear. Although without direct
counterparts in the observational global statistics of flare energy release,
this latter behavior may be relevant to recurrent flaring in individual coronal
loops. This class of models could provide a basis for the prediction of large
solar flares.Comment: 24 pages, 11 figures, 2 tables, accepted for publication in Solar
Physic
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