6,726 research outputs found
Now the wars are over: The past, present and future of Scottish battlefields
Battlefield archaeology has provided a new way of appreciating historic battlefields. This paper provides a summary of the long history of warfare and conflict in Scotland which has given rise to a large number of battlefield sites. Recent moves to highlight the archaeological importance of these sites, in the form
of Historic Scotlandâs Battlefields Inventory are discussed, along with some of the problems associated with the preservation and management of these important
cultural sites
Self-organization of actin filament orientation in the dendritic-nucleation/array-treadmilling model
Author Posting. © The Author(s), 2006. This is the author's version of the work. It is posted here by permission of National Academy of Sciences of the USA for personal use, not for redistribution. The definitive version was published in Proceedings of the National Academy of Sciences 104 (2007): 7086-7091, doi:10.1073/pnas.0701943104.The dendritic-nucleation/array-treadmilling model provides a conceptual framework for
the generation of the actin network driving motile cells. We have incorporated it into a 2-D,
stochastic computer model to study lamellipodia via the self-organization of filament
orientation patterns. Essential dendritic-nucleation sub-models were incorporated,
including discretized actin monomer diffusion, Monte-Carlo filament kinetics, and flexible
filament and plasma membrane mechanics. Model parameters were estimated from the
literature and simulation, providing values for the extent of the leading edge
branching/capping-protective zone (5.4 nm) and the auto-catalytic branch rate (0.43 /s).
For a given set of parameters the system evolved to a steady state filament count and
velocity, at which total branching and capping rates were equal only for specific
orientations; net capping eliminated others. The standard parameter set evoked a sharp
preference for the ±35 deg. filaments seen in lamellipodial electron micrographs, requiring
~ 12 generations of successive branching to adapt to a 15 deg. change in protrusion
direction. This pattern was robust with respect to membrane surface and bending energies
and to actin concentrations, but required protection from capping at the leading edge and
branching angles greater than 60 deg. A +70/0/-70 deg. pattern was formed with flexible
filaments ~ 100 nm or longer and with velocities less than ~ 20% of free polymerization
rates
End states, ladder compounds, and domain wall fermions
A magnetic field applied to a cross linked ladder compound can generate
isolated electronic states bound to the ends of the chain. After exploring the
interference phenomena responsible, I discuss a connection to the domain wall
approach to chiral fermions in lattice gauge theory. The robust nature of the
states under small variations of the bond strengths is tied to chiral symmetry
and the multiplicative renormalization of fermion masses.Comment: 10 pages, 4 figures; final version for Phys. Rev. Let
Static stress drop associated with brittle slip events on exhumed faults
We estimate the static stress drop on small exhumed strike-slip faults in the Lake Edison granodiorite of the central Sierra Nevada (California). The sub-vertical strike-slip faults were exhumed from 4-15 km depth, and were chosen because they are exposed in outcrop along their entire tip-to-tip lengths of 8-12 m. Slip nucleated on joints and accumulated by ductile shearing (forming quartz mylonites from early quartz vein filling in joints) and successive brittle faulting (forming epidote-bearing cataclasites). The occurrence of thin, < 1 mm wide, pseudotachylytes along some small faults throughout the study area suggests that some portion of the brittle slip was seismic. We suggest that the contribution of seismic slip to the total slip along the studied cataclasite-bearing small faults may be estimated by the length of epidote-filled, rhombohedral dilatational jogs (rhombochasms) distributed semi-periodically along the length of the faults. The interpretation that slip recorded by rhombochasms occurred in single events is based on evidence that: 1) epidote crystals are randomly oriented and undeformed within the rhombochasm; and 2) cataclasite structure in principal slip zones does not include clasts of previous cataclasite. We thereby constrain both the rupture length and slip. Based on these measurements, we calculate stress drops ranging over 90-250 MPa, i.e., one to two orders of magnitude larger than typical seismological estimates for earthquakes, but similar in magnitude to recent observations of small (< M2) earthquakes from the San Andreas Fault Observatory at Depth (SAFOD). These inferred seismic ruptures occurred along small, deep-seated faults, and, given the calculated stress drops and observations that brittle faults exploited joints sealed by quartz-bearing mylonite, we conclude that these were âstrongâ faults
Analytical results for string propagation near a Kaluza-Klein black hole
This brief report presents analytical solutions to the equations of motion of
a null string. The background spacetime is a magnetically charged Kaluza-Klein
black hole. The string coordinates are expanded with the world-sheet velocity
of light as an expansion parameter. It is shown that the zeroth order solutions
can be expressed in terms of elementary functions in an appropriate large
distance approximation. In addition, a class of exact solutions corresponding
to the Pollard-Gross-Perry-Sorkin monopole case is also obtained.Comment: Revtex, 9 pages including two postscript figures, More detailed
discussion and new references adde
Pair Creation of Dilaton Black Holes
We consider dilaton gravity theories in four spacetime dimensions
parametrised by a constant , which controls the dilaton coupling, and
construct new exact solutions. We first generalise the C-metric of
Einstein-Maxwell theory () to solutions corresponding to oppositely
charged dilaton black holes undergoing uniform acceleration for general . We
next develop a solution generating technique which allows us to ``embed" the
dilaton C-metrics in magnetic dilaton Melvin backgrounds, thus generalising the
Ernst metric of Einstein-Maxwell theory. By adjusting the parameters
appropriately, it is possible to eliminate the nodal singularities of the
dilaton C-metrics. For (but not for ), it is possible to further
restrict the parameters so that the dilaton Ernst solutions have a smooth
euclidean section with topology , corresponding to
instantons describing the pair production of dilaton black holes in a magnetic
field. A different restriction on the parameters leads to smooth instantons for
all values of with topology .Comment: 22 pages, EFI-93-51, FERMILAB-Pub-93/272-A, UMHEP-393. (Asymptotics
of Ernst solutions clarified, typos repaired
Matrix-free calcium in isolated chromaffin vesicles
Isolated secretory vesicles from bovine adrenal medulla contain 80 nmol of Ca2+ and 25 nmol
of Mg2+ per milligram of protein. As determined with a Ca2+-selective electrode, a further accumulation
of about 160 nmol of Ca2+/mg of protein can be attained upon addition of the Ca2+ ionophore A23187.
During this process protons are released from the vesicles, in exchange for Ca2+ ions, as indicated by the
decrease of the pH in the incubation medium or the release of 9-aminoacridine previously taken up by the
vesicles. Intravesicular Mg2+ is not released from the vesicles by A23 187, as determined by atomic emission
spectroscopy. In the presence of N H Q , which causes the collapse of the secretory vesicle transmembrane
proton gradient (ApH), Ca2+ uptake decreases. Under these conditions A23 187-mediated influx of Ca2+
and efflux of H+ cease at Ca2+ concentrations of about 4 pM. Below this concentration Ca2+ is even released
from the vesicles. At the Ca2+ concentration at which no net flux of ions occurs the intravesicular matrix
free Ca2+ equals the extravesicular free Ca2+. In the absence of NH4C1 we determined an intravesicular
pH of 6.2. Under these conditions the Ca2+ influx ceases around 0.15 pM. From this value and the known
pH across the vesicular membrane an intravesicular matrix free Ca2+ concentration of about 24 pM was
calculated. This is within the same order of magnitude as the concentration of free Ca2+ in the vesicles
determined in the presence of NH4C1. Calculation of the total Ca2+ present in the secretory vesicles gives
an apparent intravesicular Ca2+ concentration of 40 mM, which is a factor of lo4 higher than the free
intravesicular concentration of Ca2+. It can be concluded, therefore, that the concentration gradient of free
Ca2+ across the secretory vesicle membrane in the intact chromaffin cells is probably small, which implies
that less energy is required to accumulate and maintain Ca2+ within the vesicles than was previously
anticipated
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