4,782 research outputs found
Torsion Degrees of Freedom in the Regge Calculus as Dislocations on the Simplicial Lattice
Using the notion of a general conical defect, the Regge Calculus is
generalized by allowing for dislocations on the simplicial lattice in addition
to the usual disclinations. Since disclinations and dislocations correspond to
curvature and torsion singularities, respectively, the method we propose
provides a natural way of discretizing gravitational theories with torsion
degrees of freedom like the Einstein-Cartan theory. A discrete version of the
Einstein-Cartan action is given and field equations are derived, demanding
stationarity of the action with respect to the discrete variables of the
theory
A decade of energy and mass balance investigations on the glacier Kongsvegen, Svalbard
Kongsvegen is an Arctic glacier located in northwest Spitzbergen. We use meteorological observations made near the average equilibrium line of Kongsvegen during the decade 2001 to 2010 to drive a glacier energy and mass balance model. Average daily and seasonal cycles are analyzed over the course of a full decade, as well as the interannual variations of the meteorological parameters and of the mass and energy balance components. The calculated average of net radiation is close to zero and the sensible heat flux is the most important and continuous source of energy at the surface. The latent heat flux is a weak source of energy as well. The resultant flux constellation yields a surplus of energy accumulating throughout the decade (9.5 W m-2) and fosters a negative specific surface mass balance throughout the investigated decade (-1.8 m w. eq.). The most significant mass loss occurred during the middle of the decade (2004 until 2006), with positive surface mass balances observed afterward when significant amounts of superimposed ice were formed. This development is well correlated to the total surface mass balance of the glacier. Application of monthly temperature and precipitation perturbations corroborates earlier studies indicating a high sensitivity of the mass balance to energy fluxes depending on temperature conditions during summer. Key PointsEvaluation of a decadal meteorological record of an Arctic glacierUse of SOMARS, validation of results, investigations of climate sensitiviyAnnual, monthly, daily consideration ©2012. American Geophysical Union. All Rights Reserved
The Asiago-ESO/RASS QSO Survey. I.The Catalog and the Local QSO Luminosity Function
This paper presents the first results of a survey for bright quasars (V <
14.5 and R30.
The photometric database is derived from the GSC and USNO catalogs. Quasars are
identified on the basis of their X-ray emission measured in the ROSAT All Sky
Survey. The surface density of quasars brighter than 15.5 mag turns out to be
, about 3 times higher than that estimated by
the PG survey. The quasar optical Luminosity Function (LF) at is computed and shown to be consistent with a Luminosity Dependent
Luminosity Evolution of the type derived by La Franca and Cristiani (1997) in
the range . The predictions of semi-analytical models of
hierarchical structure formation agree remarkably well with the present
observations.Comment: 54 pages Latex, with 7 PostScript figures. Some minor changes.
Astronomical Journal, in pres
Semi-Teleparallel Theories of Gravitation
A class of theories of gravitation that naturally incorporates preferred
frames of reference is presented. The underlying space-time geometry consists
of a partial parallelization of space-time and has properties of Riemann-Cartan
as well as teleparallel geometry. Within this geometry, the kinematic
quantities of preferred frames are associated with torsion fields. Using a
variational method, it is shown in which way action functionals for this
geometry can be constructed. For a special action the field equations are
derived and the coupling to spinor fields is discussed.Comment: 14 pages, LaTe
Diagnosis, treatment and long-term outcome of solitary fibrous tumours of the pleura
Objective: Solitary fibrous tumours of the pleura (SFTP) are rare and can histologically be differentiated into benign and malignant forms. The aim of this study is to present new cases, and discuss up-to-date preoperative examinations, the role of video-assisted thoracic surgery and long-term outcome. Methods: Between 1993 and 2006, 27 SFTPs were diagnosed (14 females, mean age±SD, 62.3±9.6 years) at our institution. Medical records were reviewed, and follow-up was obtained by repeated examinations or contact with general practitioners. Results: SFTPs were associated with symptoms in 63% of all cases. In the six patients in which positron emission tomography (PET) with 18F-fluorodeoxyglucose (FDG) was performed preoperatively, malignant lesions were all found to be positive. Complete resection was achieved by video-assisted thoracic surgery in 15 and anterolateral thoracotomy in 12 patients. Mean hospital stay was shorter for patients operated by video-assisted thoracic surgery compared to thoracotomy, 4.5 (range 3-6) versus 7.5 (range 4-25) days, respectively (p≪0.01). Histology revealed 17 benign and 10 malignant SFTP. Mean±SD tumour diameter of malignant SFTPs was larger than in benign forms, 11.9±7.1 versus 6.1±3.5cm, respectively (p≪0.01). Tumour recurrence was recognised in four patients with malignant SFTPs at a median time interval after surgery of 38 (range 6-122) months, two late deaths occurred resulting from tumour recurrences. Conclusions: SFTPs can be treated minimally invasively by video-assisted thoracic surgery with short hospital stay. Large SFTPs with increased FDG-uptake have a high likelihood for malignancy. Long-term follow-up is mandatory in malignant SFTPs because of late recurrences associated with deat
The Josephson plasmon as a Bogoliubov quasiparticle
We study the Josephson effect in alkali atomic gases within the two-mode
approximation and show that there is a correspondence between the Bogoliubov
description and the harmonic limit of the phase representation. We demonstrate
that the quanta of the Josephson plasmon can be identified with the Bogoliubov
excitations of the two-site Bose fluid. We thus establish a mapping between the
Bogoliubov approximation for the many-body theory and the linearized pendulum
Hamiltonian.Comment: 9 pages, LaTeX, submitted to J. Phys.
Regge Calculus in Teleparallel Gravity
In the context of the teleparallel equivalent of general relativity, the
Weitzenbock manifold is considered as the limit of a suitable sequence of
discrete lattices composed of an increasing number of smaller an smaller
simplices, where the interior of each simplex (Delaunay lattice) is assumed to
be flat. The link lengths between any pair of vertices serve as independent
variables, so that torsion turns out to be localized in the two dimensional
hypersurfaces (dislocation triangle, or hinge) of the lattice. Assuming that a
vector undergoes a dislocation in relation to its initial position as it is
parallel transported along the perimeter of the dual lattice (Voronoi polygon),
we obtain the discrete analogue of the teleparallel action, as well as the
corresponding simplicial vacuum field equations.Comment: Latex, 10 pages, 2 eps figures, to appear in Class. Quant. Gra
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