3,562 research outputs found
Optical control of solar sails using distributed reflectivity
The dynamics of solar sails with a variable surface reflectivity distribution are investigated. When changing the reflectivity across the sail film, which can be achieved using electro-chromic coatings, the solar radiation pressure forces and torques across the sail film can be controlled without changing the attitude of the spacecraft relative to the Sun and without using mechanical systems. The paper presents two approaches. First, a continuous reflectivity distribution is presented to control the sail attitude under the influence of, for example, gravity gradient torques in Earth orbit. The second approach assumes discrete on/o reflectivity regions across the surface. Both concepts of `optical reconfiguration' of solar sails enable a more flexible steering of the spacecraft and minimize actuation effort
Strange Quark Contribution to the Nucleon Spin from Electroweak Elastic Scattering Data
The total contribution of strange quarks to the intrinsic spin of the nucleon
can be determined from a measurement of the strange-quark contribution to the
nucleon's elastic axial form factor. We have studied the strangeness
contribution to the elastic vector and axial form factors of the nucleon, using
elastic electroweak scattering data. Specifically, we combine elastic
and scattering cross section data from the Brookhaven E734
experiment with elastic and quasi-elastic and -He scattering
parity-violating asymmetry data from the SAMPLE, HAPPEx, G0 and PVA4
experiments. We have not only determined these form factors at individual
values of momentum-transfer (), but also have fit the -dependence of
these form factors using simple functional forms. We present the results of
these fits, along with some expectations of how our knowledge of these form
factors can be improved with data from Fermilab experiments.Comment: 3 pages, 1 figure, CIPANP 201
Hat hier jemand gesagt, der Kaiser sei nackt? Eine Verteidigung der Geussschen Kritik an Rawls? idealtheoretischem Ansatz [?Did Somebody Just Say the Emperor is Naked? In Defence of Geuss's Objections to Rawls's Ideal-Theoretical Approach?]
In this paper, we take up two objections Raymond Geuss levels against John Rawls? ideal theory in Philosophy and Real Politics. We show that, despite their fundamental disagreements, the two theorists share a common starting point: they both (a) reject doing political philosophy by way of applying an independently derived moral theory; and (b) grapple with the danger of unduly privileging the status quo. However, neither Rawls? characterization of politics nor his ideal theoretical approach as response to the aforementioned danger is adequate or so we argue. Moreover, contrary to received opinion, Geuss? political philosophy is the more reflective and the more philosophical of the two. In a final section, we highlight another agreement: both think that political philosophers should develop conceptual innovations as a way of clarifying and overcoming practical problems. We demonstrate that Geuss could offer a number of reasons for finding Rawls? conceptual innovations wanting
Binary Central Stars of Planetary Nebulae Discovered Through Photometric Variability. III. The Central Star of Abell 65
A growing number of close binary stars are being discovered among central stars of planetary nebulae. Recent and ongoing surveys are finding new systems and contributing to our knowledge of the evolution of close binary systems. The push to find more systems was largely based on early discoveries which suggested that 10%–15% of all central stars are close binaries. One goal of this series of papers is confirmation and classification of these systems as close binaries and determination of binary system parameters. Here we provide time-resolved multi-wavelength photometry of the central star of Abell 65 as well as further analysis of the nebula and discussion of possible binary–nebula connections. Our results for Abell 65 confirm recent work showing that it has a close, cool binary companion, though several of our model parameters disagree with the recently published values. With our longer time baseline of photometric observations from 1989 to 2009 we also provide a more precise orbital period of 1.0037577 days
Approximate roots of a valuation and the Pierce-Birkhoff Conjecture
This paper is a step in our program for proving the Piece-Birkhoff Conjecture
for regular rings of any dimension (this would contain, in particular, the
classical Pierce-Birkhoff conjecture which deals with polynomial rings over a
real closed field). We first recall the Connectedness and the Definable
Connectedness conjectures, both of which imply the Pierce - Birkhoff
conjecture. Then we introduce the notion of a system of approximate roots of a
valuation v on a ring A (that is, a collection Q of elements of A such that
every v-ideal is generated by products of elements of Q). We use approximate
roots to give explicit formulae for sets in the real spectrum of A which we
strongly believe to satisfy the conclusion of the Definable Connectedness
conjecture. We prove this claim in the special case of dimension 2. This proves
the Pierce-Birkhoff conjecture for arbitrary regular 2-dimensional rings
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