544 research outputs found
Characterisation and representation of non-dissipative electromagnetic medium with a double light cone
We study Maxwell's equations on a 4-manifold N with a medium that is
non-dissipative and has a linear and pointwise response. In this setting, the
medium can be represented by a suitable (2,2)-tensor on the 4-manifold N.
Moreover, in each cotangent space on N, the medium defines a Fresnel surface.
Essentially, the Fresnel surface is a tensorial analogue of the dispersion
equation that describes the response of the medium for signals in the geometric
optics limit. For example, in isotropic medium the Fresnel surface is at each
point a Lorentz light cone. In a recent paper, I. Lindell, A. Favaro and L.
Bergamin introduced a condition that constrains the polarisation for plane
waves. In this paper we show (under suitable assumptions) that a slight
strengthening of this condition gives a pointwise characterisation of all
medium tensors for which the Fresnel surface is the union of two distinct
Lorentz null cones. This is for example the behaviour of uniaxial medium like
calcite. Moreover, using the representation formulas from Lindell et al. we
obtain a closed form representation formula that pointwise parameterises all
medium tensors for which the Fresnel surface is the union of two distinct
Lorentz null cones. Both the characterisation and the representation formula
are tensorial and do not depend on local coordinates
Form Geometry and the 'tHooft-Plebanski Action
Riemannian geometry in four dimensions, including Einstein's equations, can
be described by means of a connection that annihilates a triad of two-forms
(rather than a tetrad of vector fields). Our treatment of the conformal factor
of the metric differs from the original presentation of this result, due to
'tHooft. In the action the conformal factor now appears as a field to be
varied.Comment: 12pp, LaTe
Neighbours of Einstein's Equations: Connections and Curvatures
Once the action for Einstein's equations is rewritten as a functional of an
SO(3,C) connection and a conformal factor of the metric, it admits a family of
``neighbours'' having the same number of degrees of freedom and a precisely
defined metric tensor. This paper analyzes the relation between the Riemann
tensor of that metric and the curvature tensor of the SO(3) connection. The
relation is in general very complicated. The Einstein case is distinguished by
the fact that two natural SO(3) metrics on the GL(3) fibers coincide. In the
general case the theory is bimetric on the fibers.Comment: 16 pages, LaTe
Geochemistry of lavas from Mohns Ridge, Norwegian-Greenland Sea: implications for melting conditions and magma sources near Jan Mayen
Mohns Ridge lavas between 71 and 72°30âČN (âŒ360â
km) have heterogeneous compositions varying between alkali basalts and incompatible-element-depleted tholeiites. On a large scale there is a continuity of incompatible element and isotopic compositions between the alkali basalts from the island Jan Mayen and Mohns Ridge tholeiites. The variation in isotopes suggests a heterogeneous mantle which appears to be tapped preferentially by low degree melts (âŒ5%) close to Jan Mayen but also shows its signature much further north on Mohns Ridge. Three lava types with different incompatible element compositions [e.g. chondrite-normalized (La/Sm)N2] occur in the area at 72°N and were generated from this heterogeneous mantle. The relatively depleted tholeiitic melts were mixed with a small degree melt from an enriched source. The elements Ba, Rb and K of the enriched melt were probably buffered in the mantle by residual amphibole or phlogopite. That such a residual phase is stable in this region of oceanic mantle suggests both high water contents and low mantle temperatures, at odds with a hotspot origin for Jan Mayen. Instead we suggest that the melting may be induced by the lowered solidus temperature of a âwetâ mantle. Mohns MORB (mid ocean ridge basalt) and Jan Mayen area alkali basalts have high contents of Ba and Rb compared to other incompatible elements (e.g. Ba/Laâ
>10). These ratios reflect the signature of the mantle source. Ratios of Ce/Pb and Rb/Cs are normal MORB mantle ratios of 25 and 80, respectively, thus the enrichments of Ba and Rb are not indicative of a sedimentary component added to the mantle source but were probably generated by the influence of a metasomatizing fluid, as supported by the presence of hydrous phases during the petrogenesis of the alkali basalts. Geophysical and petrological models suggest that Jan Mayen is not the product of hotspot activity above a mantle plume, and suggest instead that it owes its existence to the unique juxtaposition of a continental fragment, a fracture zone and a spreading axis in this part of the North Atlantic
Geochemical and Isotopic Evolution of Late Oligocene Magmatism in Quchan, NE Iran
Magmatic activity that accompanied the collision between Arabia and Eurasia at âŒ27 Ma, provides unique opportunities for understanding the triggers and magma reservoirs for collisional magmatism and its different styles in magmatic fronts and back-arcs. We present new ages and geochemical-isotopic results for magmatic rocks that formed during the collision between Arabia and Eurasia in NE Iran, which was a back-arc region to the main magmatic arcs of Iran. Our new zircon U-Pb ages indicate that collisional magmatism began at âŒ24 Ma in the NE Iran back-arc, although magmatism in this area started in the Late Cretaceous time and continued until the Pleistocene. The collisional igneous rocks are characteristically bimodal, and basaltic-andesitic and dacitic-rhyolitic components show significant isotopic differences; ΔNd(t) = +4.4 to +7.4 and ΔHf(t) = +5.4 to +9.5 for mafic rocks and ΔNd(t) = +0.2 to +8.4 and ΔHf(t) = +3.4 to +12.3 for silicic rocks. The isotopic values and modeling suggest that fractional crystallization and assimilation-fractional crystallization played important roles in the genesis of felsic rocks in the NE Iran collisional zone. Trace element and isotopic modeling further emphasize that the main triggers of the magmatism in NE Iran comprise a depleted to the enriched mantle and the Cadomian continental crust of Iran. Our results also emphasize the temporal magmatic variations in the NE Iran back-arc from Late Cretaceous to Pleistocene. © 2021. The Authors
Radiation dose of chaperones during common pediatric computed tomography examinations
Background
One main challenge in pediatric imaging is to reduce motion artifacts by calming young patients. To that end, the Radiological Society of North America (RSNA) as early as 1997 stated the necessity of adults accompanying their child during the childâs examination. Nonetheless, current research lacks data regarding radiation dose to these chaperones.
Objective
The aim of this study was to measure the radiation dose of accompanying adults during state-of-the-art pediatric CT protocols.
Materials and methods
In addition to a 100-kV non-contrast-enhanced chest CT (Protocol 1), we performed a 70-kV contrast-enhanced chest protocol (Protocol 2) using a third-generation dual-source CT. We acquired data on the radiation dose around the scanner using digital dosimetry placed right at the gantry, 1 m away, as well as beside the gantry. We acquired the CT-surrounding radiation dose during scanning of a pediatric phantom as well as 12 pediatric patients.
Results
After conducting 10 consecutive phantom scans using Protocol 1, we found the location with the highest cumulative dose acquired was right next to the gantry opening, at 3 ΌSv. Protocol 2 showed highest cumulative dose of 2 ΌSv at the same location. For Protocol 1, the location with the highest radiation doses during pediatric scans was right next to the gantry opening, with doses of 0.75±0.70 ΌSv. For Protocol 2, the highest radiation was measured 1 m away at 0.50±0.60 ΌSv. No radiation dose was measured at any time beside the gantry.
Conclusion
Our results provide proof that chaperones receive low radiation doses during state-of-the-art CT examinations. Given knowledge of these values as well as the optimal spots with the lowest radiation doses, parents as well as patients might be more relaxed during the examination
Determination of electromagnetic medium from the Fresnel surface
We study Maxwell's equations on a 4-manifold where the electromagnetic medium
is described by an antisymmetric -tensor . In this setting,
the Tamm-Rubilar tensor density determines a polynomial surface of fourth order
in each cotangent space. This surface is called the Fresnel surface and acts as
a generalisation of the light-cone determined by a Lorentz metric; the Fresnel
surface parameterises electromagnetic wave-speed as a function of direction.
Favaro and Bergamin have recently proven that if has only a principal
part and if the Fresnel surface of coincides with the light cone for a
Lorentz metric , then is proportional to the Hodge star operator of
. That is, under additional assumptions, the Fresnel surface of
determines the conformal class of . The purpose of this paper is
twofold. First, we provide a new proof of this result using Gr\"obner bases.
Second, we describe a number of cases where the Fresnel surface does not
determine the conformal class of the original -tensor . For
example, if is invertible we show that and have
the same Fresnel surfaces.Comment: 23 pages, 1 figur
The variable mass loss of the AGB star WX Psc as traced by the CO J=1-0 through 7-6 lines and the dust emission
Low and intermediate mass stars lose a significant fraction of their mass
through a dust-driven wind during the Asymptotic Giant Branch (AGB) phase.
Recent studies show that winds from late-type stars are far from being smooth.
Mass-loss variations occur on different time scales, from years to tens of
thousands of years. The variations appear to be particularly prominent towards
the end of the AGB evolution. The occurrence, amplitude and time scale of these
variations are still not well understood.
The goal of our study is to gain insight into the structure of the
circumstellar envelope (CSE) of WX Psc and map the possible variability of the
late-AGB mass-loss phenomenon.
We have performed an in-depth analysis of the extreme infrared AGB star WX
Psc by modeling (1) the CO J=1-0 through 7-6 rotational line profiles and the
full spectral energy distribution (SED) ranging from 0.7 to 1300 micron. We
hence are able to trace a geometrically extended region of the CSE.
Both mass-loss diagnostics bear evidence of the occurrence of mass-loss
modulations during the last ~2000 yr. In particular, WX Psc went through a high
mass-loss phase (Mdot~5e-5 Msun/yr) some 800 yr ago. This phase lasted about
600 yr and was followed by a long period of low mass loss (Mdot~5e-8 Msun/yr).
The present day mass-loss rate is estimated to be ~6e-6 Msun/yr.
The AGB star WX Psc has undergone strong mass-loss rate variability on a time
scale of several hundred years during the last few thousand years. These
variations are traced in the strength and profile of the CO rotational lines
and in the SED. We have consistently simulated the behaviour of both tracers
using radiative transfer codes that allow for non-constant mass-loss rates.Comment: 12 pages, accepted for publication in A&
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