4,612 research outputs found
Silicate luminescence and remote compositional mapping
Silicate luminescence and remote compositional mappin
Pseudo-prototyping of aerospace mechanical dynamic systems with a generalized computer program
The ADAMS computer program for automated analysis of mechanisms and machines is described. The program automatically formulates mathematical models for prototype or existing mechanisms with the minimum necessary physical and geometric data. The model can then be analyzed in various modes of analysis. The outputs (displacements, velocities, acceleration and forces) can be produced in tabular and graphical (plots, wire frame graphics) form. The application of this computer program to simulating satellite docking maneuvers is illustrated
Binary neutron star mergers: a jet engine for short gamma-ray bursts
We perform magnetohydrodynamic simulations in full general relativity (GRMHD)
of quasi-circular, equal-mass, binary neutron stars that undergo merger. The
initial stars are irrotational, polytropes and are magnetized. We explore
two types of magnetic-field geometries: one where each star is endowed with a
dipole magnetic field extending from the interior into the exterior, as in a
pulsar, and the other where the dipole field is initially confined to the
interior. In both cases the adopted magnetic fields are initially dynamically
unimportant. The merger outcome is a hypermassive neutron star that undergoes
delayed collapse to a black hole (spin parameter )
immersed in a magnetized accretion disk. About ms following merger, the region above the black hole poles
becomes strongly magnetized, and a collimated, mildly relativistic outflow ---
an incipient jet --- is launched. The lifetime of the accretion disk, which
likely equals the lifetime of the jet, is s. In contrast to black hole--neutron star mergers, we find
that incipient jets are launched even when the initial magnetic field is
confined to the interior of the stars.Comment: 6 pages, 3 figures, 1 table, matches published versio
Stress-Energy Tensor Induced by Bulk Dirac Spinor in Randall-Sundrum Model
Motivated by the possible extension into a supersymmetric Randall-Sundrum
(RS) model, we investigate the properties of the vacuum expectation value (VEV)
of the stress-energy tensor for a quantized bulk Dirac spinor field in the RS
geometry and compare it with that for a real scalar field. This is carried out
via the Green function method based on first principles without invoking the
degeneracy factor, whose validity in a warp geometry is a priori unassured. In
addition, we investigate the local behavior of the Casimir energy near the two
branes. One salient feature we found is that the surface divergences near the
two branes have opposite signs. We argue that this is a generic feature of the
fermionic Casimir energy density due to its parity transformation in the fifth
dimension. Furthermore, we investigate the self-consistency of the RS metric
under the quantum correction due to the stress-energy tensor. It is shown that
the VEV of the stress-energy tensor and the classical one become comparable
near the visible brane if k ~ M ~ M_Pl (the requirement of no hierarchy
problem), where k is the curvature of the RS warped geometry and M the
5-dimensional Planck mass. In that case the self-consistency of RS model that
includes bulk fields is in doubt. If, however, k <~ M, then an approximate
self-consistency of the RS-type metric may still be satisfied.Comment: 7 pages with 2 figure
Electromagnetic semitransparent -function plate: Casimir interaction energy between parallel infinitesimally thin plates
We derive boundary conditions for electromagnetic fields on a
-function plate. The optical properties of such a plate are shown to
necessarily be anisotropic in that they only depend on the transverse
properties of the plate. We unambiguously obtain the boundary conditions for a
perfectly conducting -function plate in the limit of infinite
dielectric response. We show that a material does not "optically vanish" in the
thin-plate limit. The thin-plate limit of a plasma slab of thickness with
plasma frequency reduces to a -function plate
for frequencies () satisfying . We show that the Casimir interaction energy between two parallel perfectly
conducting -function plates is the same as that for parallel perfectly
conducting slabs. Similarly, we show that the interaction energy between an
atom and a perfect electrically conducting -function plate is the usual
Casimir-Polder energy, which is verified by considering the thin-plate limit of
dielectric slabs. The "thick" and "thin" boundary conditions considered by
Bordag are found to be identical in the sense that they lead to the same
electromagnetic fields.Comment: 21 pages, 7 figures, references adde
Casimir energy and realistic model of dilute dielectric ball
The Casimir energy of a dilute homogeneous nonmagnetic dielectric ball at
zero temperature is derived analytically for the first time for an arbitrary
physically possible frequency dispersion of dielectric permittivity
. A microscopic model of dielectrics is considered,
divergences are absent in calculations because an average interatomic distance
is a {\it physical} cut-off in the theory. This fact has been
overlooked before, which led to divergences in various macroscopic approaches
to the Casimir energy of connected dielectrics.Comment: 12 pages, 1 figure, latex 2e, journal versio
Benthic invertebrates that form habitat on deep banks off southern California, with special reference to deep sea coral
There is increasing interest in the potential impacts that fishing activities have on megafaunal benthic invertebrates occurring in continental shelf and slope ecosystems. We examined how the structure, size, and high-density aggregations of invertebrates provided structural relief for fishes in continental shelf and slope ecosystems off southern California. We made 112 dives in a submersible at 32−320 m water depth, surveying a variety of habitats from high-relief rock to flat sand and mud. Using quantitative video transect methods, we made 12,360 observations of 15 structure-form-ing invertebrate taxa and 521,898 individuals. We estimated size and incidence of epizoic animals on 9105 sponges, black corals, and gorgonians. Size variation among structure-form-ing invertebrates was significant and 90% of the individuals were <0.5 m high. Less than 1% of the observations of organisms actually sheltering in or located on invertebrates involved fishes. From the analysis of spatial associations between fishes and large invertebrates, six of 108 fish species were found more often adjacent to invertebrate colonies than the number of fish predicted by the fish-density data from transects. This finding indicates that there may be spatial associations that do not necessarily include physical contact with the sponges and corals. However, the median distances between these six fish species and the invertebrates were not particularly small (1.0−5.5 m). Thus, it is likely that these fishes and invertebrates are present together in the same habitats but that there is not necessarily a functional relationship between these groups of organisms. Regardless of their associations with fishes, these invertebrates provide structure and diversity for continental shelf ecosystems off southern California and certainly deserve the attention of scientists undertaking future conservation efforts
- …