3,950 research outputs found
Conformal Orthosymplectic Quantum Mechanics
We present the most general curvature obstruction to the deformed parabolic
orthosymplectic symmetry subalgebra of the supersymmetric quantum mechanical
models recently developed to describe Lichnerowicz wave operators acting on
arbitrary tensors and spinors. For geometries possessing a
hypersurface-orthogonal homothetic conformal Killing vector we show that the
parabolic subalgebra is enhanced to a (curvature-obstructed) orthosymplectic
algebra. The new symmetries correspond to time-dependent conformal symmetries
of the underlying particle model. We also comment on generalizations germane to
three dimensions and new Chern--Simons-like particle models.Comment: 27 pages LaTe
Multi-limbed locomotion systems for space construction and maintenance
A well developed technology of coordination of multi-limbed locomotory systems is now available. Results from a NASA sponsored study of several years ago are presented. This was a simulation study of a three-limbed locomotion/manipulation system. Each limb had six degrees of freedom and could be used either as a locomotory grasping hand-holds, or as a manipulator. The focus of the study was kinematic coordination algorithms. The presentation will also include very recent results from the Adaptive Suspension Vehicle Project. The Adaptive Suspension Vehicle (ASV) is a legged locomotion system designed for terrestrial use which is capable of operating in completely unstructured terrain in either a teleoperated or operator-on-board mode. Future development may include autonomous operation. The ASV features a very advanced coordination and control system which could readily be adapted to operation in space. An inertial package with a vertical gyro, and rate gyros and accelerometers on three orthogonal axes provides body position information at high bandwidth. This is compared to the operator's commands, injected via a joystick to provide a commanded force system on the vehicle's body. This system is, in turn, decomposed by a coordination algorithm into force commands to those legs which are in contact with the ground
The preliminary design of bearings for the control system of a high-temperature lithium-cooled nuclear reactor
The design of bearings for the control system of a fast reactor concept is presented. The bearings are required to operate at temperatures up to 2200 F in one of two fluids, lithium or argon. Basic bearing types are the same regardless of the fluid. Crowned cylindrical journals were selected for radially loaded bearings and modified spherical bearings were selected for bearings under combined thrust and radial loads. Graphite and aluminum oxide are the materials selected for the argon atmosphere bearings while cermet compositions (carbides or nitrides bonded with refractory metals) were selected for the lithium lubricated bearings. Mounting of components is by shrink fit or by axial clamping utilizing differential thermal expansion
New Challenges For Wind Shock Models: The Chandra Spectrum Of The Hot Star Delta Orionis
The Chandra spectrum of delta Ori A shows emission lines from hydrogen- and helium-like states of Si, Mg, Ne, and O, along with N VII Lyalpha and lines from ions in the range Fe XVII-Fe XXI In contrast to the broad lines seen in zeta Pup and zeta Ori (850 +/- 40 and 1000 +/- 240 km s(-1) half-width at half-maximum [HWHM], respectively), these lines are broadened to only 430 +/- 60 km s(-1) HWHM. This is much lower than the measured wind terminal velocity of 2000 km s(-1). The forbidden, intercombination, and resonance (fir) lines from He-like ions indicate that the majority of the X-ray line emission does not originate at the base of the wind, in agreement with the standard wind shock models for these objects. However, in that model the X-ray emission is distributed throughout an expanding, X-ray-absorbing wind, and it is therefore surprising that the emission lines appear relatively narrow, unshifted, and symmetric. We compare the observed line profiles to recent detailed models for X-ray line pro le generation in hot stars, but none of them offers a fully satisfactory explanation for the observed line profiles
Development of biaxial test fixture includes cryogenic application
Test fixture has the capability of producing biaxial stress fields in test specimens to the point of failure. It determines biaxial stress by dividing the applied load by the net cross section. With modification it can evaluate materials, design concepts, and production hardware at cryogenic temperatures
X-ray Emission from Magnetically Torqued Disks of Oe/Be Stars
We focus attention on the Oe/Be stars to test the concept that the disks of
these stars form by magnetic channeling of wind material toward the equator.
Calculations are made of the X-rays expected from the Magnetically Torqued Disk
(MTD) model for Be stars discussed by Cassinelli et al. (2002), by Maheswaran
(2003), and by Brown et al. (2004). The dominant parameters in the model are
the value of the velocity law, the rotation rate of the star, ,
and the ratio of the magnetic field energy density to the disk gravitational
energy density, .
The model predictions are compared with the observations obtained for
an O9.5 star Oph from \Berghofer et al. (1996) and for 7 Be stars from
Cohen et al. (1997). Extra considerations are also given here to the well
studied Oe star Oph for which we have observations of the
X-ray line profiles of the triad of He-like lines from the ion Mg XI.Comment: 28 pages with 6 figures. Accepted for publication in Ap
Cosmology as Relativistic Particle Mechanics: From Big Crunch to Big Bang
Cosmology can be viewed as geodesic motion in an appropriate metric on an
`augmented' target space; here we obtain these geodesics from an effective
relativistic particle action. As an application, we find some exact (flat and
curved) cosmologies for models with N scalar fields taking values in a
hyperbolic target space for which the augmented target space is a Milne
universe. The singularities of these cosmologies correspond to points at which
the particle trajectory crosses the Milne horizon, suggesting a novel
resolution of them, which we explore via the Wheeler-deWitt equation.Comment: 17 pages, 3 figures, references and comments adde
Cosmological D-instantons and Cyclic Universes
For models of gravity coupled to hyperbolic sigma models, such as the
metric-scalar sector of IIB supergravity, we show how smooth trajectories in
the `augmented target space' connect FLRW cosmologies to non-extremal
D-instantons through a cosmological singularity. In particular, we find closed
cyclic universes that undergo an endless sequence of big-bang to big-crunch
cycles separated by instanton `phases'. We also find `big-bounce' universes in
which a collapsing closed universe bounces off its cosmological singularity to
become an open expanding universe.Comment: 21 pages, 4 figures. v2: minor change
An Extensive Collection of Stellar Wind X-ray Source Region Emission Line Parameters,Temperatures, Velocities, and Their Radial Distributions as Obtained from Chandra Observations of 17 OB Stars
Chandra high energy resolution observations have now been obtained from
numerous non-peculiar O and early B stars. The observed X-ray emission line
properties differ from pre-launch predictions, and the interpretations are
still problematic. We present a straightforward analysis of a broad collection
of OB stellar line profile data to search for morphological trends. X-ray line
emission parameters and the spatial distributions of derived quantities are
examined with respect to luminosity class. The X-ray source locations and their
corresponding temperatures are extracted by using the He-like f/i line ratios
and the H-like to He-like line ratios respectively. Our luminosity class study
reveals line widths increasing with luminosity. Although the majority of the OB
emission lines are found to be symmetric, with little central line
displacement, there is evidence for small, but finite, blue-ward line-shifts
that also increase with luminosity. The spatial X-ray temperature distributions
indicate that the highest temperatures occur near the star and steadily
decrease outward. This trend is most pronounced in the OB supergiants. For the
lower density wind stars, both high and low X-ray source temperatures exist
near the star. However, we find no evidence of any high temperature X-ray
emission in the outer wind regions for any OB star. Since the temperature
distributions are counter to basic shock model predictions, we call this the
"near-star high-ion problem" for OB stars. By invoking the traditional OB
stellar mass loss rates, we find a good correlation between the fir-inferred
radii and their associated X-ray continuum optical depth unity radii. We
conclude by presenting some possible explanations to the X-ray source problems
that have been revealed by this study.Comment: Published in 2007, ApJ, 668, 456. An Erratum scheduled for
publication in 2008, ApJ, 680, is included as an Appendix. The Erratum
corrects some tabulated data in 5 tables and 2 figure
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