36,237 research outputs found
Conversion Efficiencies of Heteronuclear Feshbach Molecules
We study the conversion efficiency of heteronuclear Feshbach molecules in
population imbalanced atomic gases formed by ramping the magnetic field
adiabatically. We extend the recent work [J. E. Williams et al., New J. Phys.,
8, 150 (2006)] on the theory of Feshbach molecule formations to various
combinations of quantum statistics of each atomic component. A simple
calculation for a harmonically trapped ideal gas is in good agreement with the
recent experiment [S. B. Papp and C. E. Wieman, Phys. Rev. Lett., 97, 180404
(2006)] without any fitting parameters. We also give the conversion efficiency
as an explicit function of initial peak phase space density of the majority
species for population imbalanced gases. In the low-density region where
Bose-Einstein condensation does not appear, the conversion efficiency is a
monotonic function of the initial peak phase space density, but independent of
statistics of a minority component. The quantum statistics of majority atoms
has a significant effect on the conversion efficiency. In addition,
Bose-Einstein condensation of an atomic component is the key element
determining the maximum conversion efficiency.Comment: 46 pages, 32 figure
Photon Bubbles and the Vertical Structure of Accretion Disks
We consider the effects of "photon bubble" shock trains on the vertical
structure of radiation pressure-dominated accretion disks. These density
inhomogeneities are expected to develop spontaneously in radiation-dominated
accretion disks where magnetic pressure exceeds gas pressure, even in the
presence of magnetorotational instability. They increase the rate at which
radiation escapes from the disk, and may allow disks to exceed the Eddington
limit by a substantial factor. We first generalize the theory of photon bubbles
to include the effects of finite optical depths and radiation damping.
Modifications to the diffusion law at low optical depth tend to fill in the
low-density regions of photon bubbles, while radiation damping inhibits the
formation of photon bubbles at large radii, small accretion rates, and small
heights above the equatorial plane. Accretion disks dominated by photon bubble
transport may reach luminosities of 10 to >100 times the Eddington limit (L_E),
depending on the mass of the central object, while remaining geometrically
thin. However, photon bubble-dominated disks with alpha-viscosity are subject
to the same thermal and viscous instabilities that plague standard radiation
pressure-dominated disks, suggesting that they may be intrinsically unsteady.
Photon bubbles can lead to a "core-halo" vertical disk structure. In
super-Eddington disks the halo forms the base of a wind, which carries away
substantial energy and mass, but not enough to prevent the luminosity from
exceeding L_E. Photon bubble-dominated disks may have smaller color corrections
than standard accretion disks of the same luminosity. They remain viable
contenders for some ultraluminous X-ray sources and may play a role in the
rapid growth of supermassive black holes at high redshift.Comment: 38 pages, 2 figures, accepted for publication in The Astrophysical
Journa
Advanced expander test bed program
The Advanced Expander Test Bed (AETB) is a key element in NASA's Chemical Transfer Propulsion Program for development and demonstration of expander cycle oxygen/hydrogen engine technology component technology for the next space engine. The AETB will be used to validate the high-pressure expander cycle concept, investigate system interactions, and conduct investigations of advanced missions focused components and new health monitoring techniques. The split-expander cycle AETB will operate at combustion chamber pressures up to 1200 psia with propellant flow rates equivalent to 20,000 lbf vacuum thrust
A Pseudopolynomial Algorithm for Alexandrov's Theorem
Alexandrov's Theorem states that every metric with the global topology and
local geometry required of a convex polyhedron is in fact the intrinsic metric
of a unique convex polyhedron. Recent work by Bobenko and Izmestiev describes a
differential equation whose solution leads to the polyhedron corresponding to a
given metric. We describe an algorithm based on this differential equation to
compute the polyhedron to arbitrary precision given the metric, and prove a
pseudopolynomial bound on its running time. Along the way, we develop
pseudopolynomial algorithms for computing shortest paths and weighted Delaunay
triangulations on a polyhedral surface, even when the surface edges are not
shortest paths.Comment: 25 pages; new Delaunay triangulation algorithm, minor other changes;
an abbreviated v2 was at WADS 200
Real-time content-aware texturing for deformable surfaces
Animation of models often introduces distortions to their parameterisation, as these are typically optimised for a single frame. The net effect is that under deformation, the mapped features, i.e. UV texture maps, bump maps or displacement maps, may appear to stretch or scale in an undesirable way. Ideally, what we would like is for the appearance of such features to remain feasible given any underlying deformation. In this paper we introduce a real-time technique that reduces such distortions based on a distortion control (rigidity) map. In two versions of our proposed technique, the parameter space is warped in either an axis or a non-axis aligned manner based on the minimisation of a non-linear distortion metric. This in turn is solved using a highly optimised hybrid CPU-GPU strategy. The result is real-time dynamic content-aware texturing that reduces distortions in a controlled way. The technique can be applied to reduce distortions in a variety of scenarios, including reusing a low geometric complexity animated sequence with a multitude of detail maps, dynamic procedurally defined features mapped on deformable geometry and animation authoring previews on texture-mapped models. © 2013 ACM
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