27,655 research outputs found
The formation of galaxy disks in a hierarchical universe
The formation of galactic discs and the efficiency of star formation within
them are issues central to our understanding of galaxy formation. We have
developed a detailed and versatile model of disc formation which combines the
strengths of previous studies of isolated discs with those of hierarchical
galaxy formation models. Disc structure is inferred from the distribution of
angular momentum in hot halo gas and the hierarchical build-up of dark matter,
leading to theoretically generated systems where the evolution of surface
density, rotation, velocity dispersion, stability and metallicity is predicted
for annular regions of width 20-100 pc. The model will be used to establish
whether the accepted theory of large-scale structure formation in the universe
is consistent with observed trends in the properties of disc galaxies.
This first paper explicitly examines the importance of embedding such
calculations within a merging hierarchy of dark matter haloes, finding that
this leads to dramatically different formation histories compared to models in
which discs grow in isolation. Different models of star formation are explored,
and are found to have only a secondary influence on the properties of the
resulting galaxy discs, the main governing factor being the infalling gas
supply from the hot halo.Comment: 13 pages, 9 figures, accepted by MNRA
Topologically Alice Strings and Monopoles
Symmetry breaking can produce ``Alice'' strings, which alter scattered
charges and carry monopole number and charge when twisted into loops. Alice
behavior arises algebraically, when strings obstruct unbroken symmetries -- a
fragile criterion. We give a topological criterion, compelling Alice behavior
or deforming it away. Our criterion, that \pi_o(H) acts nontrivially on
\pi_1(H), links topologically Alice strings to topological monopoles. We twist
topologically Alice loops to form monopoles. We show that Alice strings of
condensed matter systems (nematic liquid crystals, helium 3A, and related
non-chiral Bose condensates and amorphous chiral superconductors) are
topologically Alice, and support fundamental monopole charge when twisted into
loops. Thus they might be observed indirectly, not as strings, but as loop-like
point defects. We describe other models, showing Alice strings failing our
topological criterion; and twisted Alice loops supporting deposited, but not
fundamental, monopole number.Comment: 2 figures; this paper consolidates preprints hep-th/0304161 and
hep-th/0304162, to appear in Phys. Rev.
QCD Flux Tubes as Sigma Model Relics
We describe flux tubes and their interactions in a low energy sigma model
induced by SU(\NF) \goto SO(\NF) flavor symmetry breaking in QCD.
Gauge confinement manifests itself in the low energy theory through flux tube
interactions with unscreened sources. The flux tubes which mediate confinement
also illustrate an interesting ambiguity in defining global Alice strings.Comment: 12 pages (REVTEX) plus one figur
Satellite Power System. Concept development and evaluation program, volume 6: Construction and operations
The construction, operation, and maintenance requirements for a solar power satellite, including the space and ground systems, are reviewed. The basic construction guidelines are explained, and construction location options are discussed. The space construction tasks, equipment, and base configurations are discussed together with the operations required to place a solar power satellite in geosynchronous orbit. A rectenna construction technique is explained, and operation with the grid is defined. Maintenance requirements are summarized for the entire system. Key technology issues required for solar power satellite construction operations are defined
Charge Violation and Alice Behavior in Global and Textured Strings
Spontaneous breaking of global symmetries can produce ``Alice'' strings: line
defects which make unbroken symmetries multivalued, induce apparent charge
violation via Aharonov-Bohm interactions, and form point defects when twisted
into loops. We demonstrate this behavior for both divergent and textured global
Alice strings. Both adiabatically scatter charged particles via effective
Wilson lines. For textured Alice strings, such Wilson lines occur at all radii,
and are multivalued only inside the string. This produces measurable effects,
including path-dependent charge violation.Comment: 32 pages, 2 epsfigs, Revte
Simple DFS on the Complement of a Graph and on Partially Complemented Digraphs
A complementation operation on a vertex of a digraph changes all outgoing
arcs into non-arcs, and outgoing non-arcs into arcs. A partially complemented
digraph is a digraph obtained from a sequence of vertex
complement operations on . Dahlhaus et al. showed that, given an
adjacency-list representation of , depth-first search (DFS) on
can be performed in time, where is the number of
vertices and is the number of edges in . To
achieve this bound, their algorithm makes use of a somewhat complicated
stack-like data structure to simulate the recursion stack, instead of
implementing it directly as a recursive algorithm. We give a recursive
algorithm that uses no complicated data-structures
A new mini-navigation tool allows accurate component placement during anterior total hip arthroplasty.
Introduction: Computer-assisted navigation systems have been explored in total hip arthroplasty (THA) to improve component positioning. While these systems traditionally rely on anterior pelvic plane registration, variances in soft tissue thickness overlying anatomical landmarks can lead to registration error, and the supine coronal plane has instead been proposed. The purpose of this study was to evaluate the accuracy of a novel navigation tool, using registration of the anterior pelvic plane or supine coronal plane during simulated anterior THA.
Methods: Measurements regarding the acetabular component position, and changes in leg length and offset were recorded. Benchtop phantoms and target measurement values commonly seen in surgery were used for analysis. Measurements for anteversion and inclination, and changes in leg length and offset were recorded by the navigation tool and compared with the known target value of the simulation. Pearson\u27s
Results: The device accurately measured cup position and leg length measurements to within 1° and 1 mm of the known target values, respectively. Across all simulations, there was a strong, positive relationship between values obtained by the device and the known target values (
Conclusion: The preliminary findings of this study suggest that the novel navigation tool tested is a potentially viable tool to improve the accuracy of component placement during THA using the anterior approach
Hydrogen flash lamps studied
Parameters of gas pressure, type of gas, tube voltage, and electrode gap are tested on the intensity and shape of a radiation pulse from a hydrogen-filled lamp
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