20,694 research outputs found
Alignment of the Angular Momentum Vectors of Planetary Nebulae in the Galactic Bulge
We use high-resolution H {\alpha} images of 130 planetary nebulae (PNe) to
investigate whether there is a preferred orientation for PNe within the
Galactic Bulge. The orientations of the full sample have an uniform
distribution. However, at a significance level of 0.01, there is evidence for a
non-uniform distribution for those planetary nebulae with evident bipolar
morphology. If we assume that the bipolar PNe have an unimodal distribution of
the polar axis in Galactic coordinates, the mean Galactic position angle is
consistent with 90{\deg}, i.e. along the Galactic plane, and the significance
level is better than 0.001 (the equivalent of a 3.7{\sigma} significance level
for a Gaussian distribution).
The shapes of PNe are related to angular momentum of the original star or
stellar system, where the long axis of the nebula measures the angular momentum
vector. In old, low-mass stars, the angular momentum is largely in binary
orbital motion. Consequently, the alignment of bipolar nebulae that we have
found indicates that the orbital planes of the binary systems are oriented
perpendicular to the Galactic plane. We propose that strong magnetic fields
aligned along the Galactic plane acted during the original star formation
process to slow the contraction of the star forming cloud in the direction
perpendicular to the plane. This would have produced a propensity for wider
binaries with higher angular momentum with orbital axes parallel to the
Galactic plane. Our findings provide the first indication of a strong,
organized magnetic field along the Galactic plane that impacted on the angular
momentum vectors of the resulting stellar population.Comment: There are two effective parts. The main paper consists of the first
17 pages and includes 8 figures and 7 tables. The remaining 10 pages will be
published as an online supplement that is made up of 4 multi-part figures.
Accepted for publication in MNRAS Main Journa
Descriptions of reversed yielding in bending
Existence of Bauschinger effect in bending-unbending of copper beams has been shown from experiment. In modelling of the Bauschinger effect, it is shown that a significant second plastic penetration can occur with the release of the moment required for an elasticplastic bending of a beam. The theory is given for both linear and parabolic hardening material models. The elastic and plastic strains are developed from each hardening model to express the beam curvature of the unstressed neutral axis. Conditions are expressed, using the normalized stress—strain response of a rectangular beam section, for which the release is purely elastic and elastic—plastic. Under the latter the depth to which a second zone of plasticity penetrates is given. Two stress distributions: one for applying the moment and the other for its release, are sufficient to derive the residual stress. Residuals found for parabolic hardening are believed to be more realistic than those from simpler linear or perfectly plastic models, particularly, where a second penetration is evident
How the Ji’kmaqn Came to Spiro: Possible Additions to the Inventory of Sound-Making Instruments Depicted in the Spiro Engravings
While doing research on turtle shell rattles the author stumbled onto a photograph of a rare and unusual idiophone whose exact likeness appears twice in one of the engraved shell images from Spiro. This paper describes the instrument and the Spiro image and discusses how an instrument currently found only in the Maritime Provinces of Canada may have come to be portrayed on a marine shell cup found at Spiro
Preconditioning iterative methods for the optimal control of the Stokes equation
Solving problems regarding the optimal control of partial differential equations (PDEs) – also known as PDE-constrained optimization – is a frontier area of numerical analysis. Of particular interest is the problem of flow control, where one would like to effect some desired flow by exerting, for example, an external force. The bottleneck in many current algorithms is the solution of the optimality system – a system of equations in saddle point form that is usually very large and ill-conditioned. In this paper we describe two preconditioners – a block-diagonal preconditioner for the minimal residual method and a block-lower triangular preconditioner for a non-standard conjugate gradient method – which can be effective when applied to such problems where the PDEs are the Stokes equations. We consider only distributed control here, although other problems – for example boundary control – could be treated in the same way. We give numerical results, and compare these with those obtained by solving the equivalent forward problem using similar technique
The Optimal Rate of Decline of an Inefficient Industry
This paper considers the problem of the optimal time path of contraction of an industry which has been hit by foreign competition, and shows that in general, along the optimal path, a production subsidy is warranted. The optimal subsidy trades off the benefit of unemployment in speeding up the approach to the new long-run equilibrium against the cost of lost output in the ‘inefficient’ industry. The dynamic shadow price of labour in this industry is also derived and shown to be always positive, though below the industry wage rat
Reprocessing of radiation by multi-phase gas in Low Luminosity Accretion Flows
We discuss the role that magnetic fields in low luminosity accretion flows
can play in creating and maintaining a multi-phase medium, and show that small
magnetically-confined clouds or filaments of dense cold gas can dramatically
reprocess the `primary' radiation from tori. In particular, radio emission
would be suppressed by free-free absorption, and an extra (weak) component
would appear at optical wavelengths. This is expected to be a common process in
various environments in the central regions of Active Galaxies, such as broad
line regions, accretion disk coronae and jets.Comment: submitted to MNRAS; 4 pages, 1 figure (MNRAS LaTex style
Chebyshev semi-iteration in Preconditioning
It is widely believed that Krylov subspace iterative methods are better than Chebyshev semi-iterative methods. When the solution of a linear system with a symmetric and positive definite coefficient matrix is required then the Conjugate Gradient method will compute the optimal approximate solution from the appropriate Krylov subspace, that is, it will implicitly compute the optimal polynomial. Hence a semi-iterative method, which requires eigenvalue bounds and computes an explicit polynomial, must, for just a little less computational work, give an inferior result. In this manuscript we identify a specific situation in the context of preconditioning when the Chebyshev semi-iterative method is the method of choice since it has properties which make it superior to the Conjugate Gradient method
Reverse Transplant Tourism
In this article, we propose a novel form of kidney swap, which we label “Reverse Transplant Tourism.” This proposal has the potential to increase the number of successful transplants in the US at a time of great need, while reducing costs. It also will provide benefits to impoverished international patients with willing, compatible donors who otherwise would have no access to transplantation. Instead of non-US kidney donors being offered money through a black market middleman in exchange for one of their kidneys, Reverse Transplant Tourism would provide a legal and ethical exchange of living donor kidneys through kidney-paired donation. In this way, the donors will not receive money for their kidneys, but rather will receive a transplant for someone they love, while also helping a US pair who would otherwise be unable to transplant due to biological incompatibility
Religious actors, civil society, and the development agenda: The dynamics of inclusion and exclusion
This article uses the World Bank\u27s engagement with religious actors to analyse their differentiated role in setting the development agenda raising three key issues. First, engagements between international financial institutions (IFIs) and religious actors are formalised thus excluding many of the actors embedded within communities in the South. Secondly, the varied politics of religious actors in development are rarely articulated and a single position is often presented. Thirdly, the potential for development alternatives from religious actors excluded from these engagements is overlooked, due in part to misrecognition of the mutually constitutive relationship between secular and sacral elements in local contexts
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