27,761 research outputs found
Optical memory disks in optical information processing
We describe the use of optical memory disks as elements in optical information processing architectures. The optical disk is an optical memory devicew ith a storage capacity approaching 1010b its which is naturally suited to parallel access. We discuss optical disk characteristics which are important in optical computing systems such as contrast, diffraction efficiency, and phase uniformity. We describe techniques for holographic storage on optical disks and present reconstructions of several types of computer-generated holograms. Various optical information processing architectures are described for applications such as database retrieval, neural network implementation, and image correlation. Selected systems are experimentally demonstrated
Radiative transfer in very optically thick circumstellar disks
In this paper we present two efficient implementations of the diffusion
approximation to be employed in Monte Carlo computations of radiative transfer
in dusty media of massive circumstellar disks. The aim is to improve the
accuracy of the computed temperature structure and to decrease the computation
time. The accuracy, efficiency and applicability of the methods in various
corners of parameter space are investigated. The effects of using these methods
on the vertical structure of the circumstellar disk as obtained from
hydrostatic equilibrium computations are also addressed. Two methods are
presented. First, an energy diffusion approximation is used to improve the
accuracy of the temperature structure in highly obscured regions of the disk,
where photon counts are low. Second, a modified random walk approximation is
employed to decrease the computation time. This modified random walk ensures
that the photons that end up in the high-density regions can quickly escape to
the lower density regions, while the energy deposited by these photons in the
disk is still computed accurately. A new radiative transfer code, MCMax, is
presented in which both these diffusion approximations are implemented. These
can be used simultaneously to increase both computational speed and decrease
statistical noise. We conclude that the diffusion approximations allow for fast
and accurate computations of the temperature structure, vertical disk structure
and observables of very optically thick circumstellar disks.Comment: Accepted for publication in A&
Relativistic slim disks with vertical structure
We report on a scheme for incorporating vertical radiative energy transport
into a fully relativistic, Kerr-metric model of optically thick, advective,
transonic alpha disks. Our code couples the radial and vertical equations of
the accretion disk. The flux was computed in the diffusion approximation, and
convection is included in the mixing-length approximation. We present the
detailed structure of this "two-dimensional" slim-disk model for alpha=0.01. We
then calculated the emergent spectra integrated over the disk surface. The
values of surface density, radial velocity, and the photospheric height for
these models differ by 20%-30% from those obtained in the polytropic,
height-averaged slim disk model considered previously. However, the emission
profiles and the resulting spectra are quite similar for both types of models.
The effective optical depth of the slim disk becomes lower than unity for high
values of the alpha parameter and for high accretion rates.Comment: 15 pages, 18 figures (2 new), A&A in pres
PAHs in protoplanetary disks: emission and X-ray destruction
We study the PAH emission from protoplanetary disks. First, we discuss the
dependence of the PAH band ratios on the hardness of the absorbed photons and
the temperature of the stars. We show that the photon energy together with a
varying degree of the PAH hydrogenation accounts for most of the observed PAH
band ratios without the need to change the ionization degree of the molecules.
We present an accurate treatment of stochastic heated grains in a vectorized
three dimensional Monte Carlo dust radiative transfer code. The program is
verified against results using ray tracing techniques. Disk models are
presented for T Tauri and Herbig Ae stars. Particular attention is given to the
photo-dissociation of the molecules. We consider beside PAH destruction also
the survival of the molecules by vertical mixing within the disk. By applying
typical X-ray luminosities the model accounts for the low PAH detection
probability observed in T Tauri and the high PAH detection statistics found in
Herbig Ae disks. Spherical halos above the disks are considered. We show that
halos reduce the observed PAH band-to-continuum ratios when observed at high
inclination. Finally, mid-IR images of disks around Herbig Ae disks are
presented. We show that they are easier to resolve when PAH emission dominate.Comment: Accepted for publication in A&A. 10 pages, 7 figures, 2 tabl
NSSDC Conference on Mass Storage Systems and Technologies for Space and Earth Science Applications, volume 1
Papers and viewgraphs from the conference are presented. This conference served as a broad forum for the discussion of a number of important issues in the field of mass storage systems. Topics include magnetic disk and tape technologies, optical disks and tape, software storage and file management systems, and experiences with the use of a large, distributed storage system. The technical presentations describe, among other things, integrated mass storage systems that are expected to be available commercially. Also included is a series of presentations from Federal Government organizations and research institutions covering their mass storage requirements for the 1990's
MOA-II Galactic Microlensing Constraints: The Inner Milky Way has a Low Dark Matter Fraction and a Near Maximal Disk
Microlensing provides a unique tool to break the stellar to dark matter
degeneracy in the inner Milky Way. We combine N-body dynamical models fitted to
the Milky Way's Boxy/Peanut bulge with exponential disk models outside this,
and compute the microlensing properties. Considering the range of models
consistent with the revised MOA-II data, we find low dark matter fractions in
the inner Galaxy: at the peak of their stellar rotation curve a fraction
of the circular velocity is baryonic (at , at ). These results are in agreement with constraints from the
EROS-II microlensing survey of brighter resolved stars, where we find
at . Our fiducial model of a disk with scale length
2.6kpc, and a bulge with a low dark matter fraction of 12%, agrees with both
the revised MOA-II and EROS-II microlensing data. The required baryonic
fractions, and the resultant low contribution from dark matter, are consistent
with the NFW profiles produced by dissipationless cosmological simulations in
Milky Way mass galaxies. They are also consistent with recent prescriptions for
the mild adiabatic contraction of Milky Way mass haloes without the need for
strong feedback, but there is some tension with recent measurements of the
local dark matter density. Microlensing optical depths from the larger OGLE-III
sample could improve these constraints further when available.Comment: 14 pages, 13 figures, submitted to MNRA
Robo-line storage: Low latency, high capacity storage systems over geographically distributed networks
Rapid advances in high performance computing are making possible more complete and accurate computer-based modeling of complex physical phenomena, such as weather front interactions, dynamics of chemical reactions, numerical aerodynamic analysis of airframes, and ocean-land-atmosphere interactions. Many of these 'grand challenge' applications are as demanding of the underlying storage system, in terms of their capacity and bandwidth requirements, as they are on the computational power of the processor. A global view of the Earth's ocean chlorophyll and land vegetation requires over 2 terabytes of raw satellite image data. In this paper, we describe our planned research program in high capacity, high bandwidth storage systems. The project has four overall goals. First, we will examine new methods for high capacity storage systems, made possible by low cost, small form factor magnetic and optical tape systems. Second, access to the storage system will be low latency and high bandwidth. To achieve this, we must interleave data transfer at all levels of the storage system, including devices, controllers, servers, and communications links. Latency will be reduced by extensive caching throughout the storage hierarchy. Third, we will provide effective management of a storage hierarchy, extending the techniques already developed for the Log Structured File System. Finally, we will construct a protototype high capacity file server, suitable for use on the National Research and Education Network (NREN). Such research must be a Cornerstone of any coherent program in high performance computing and communications
Scattering line polarization in rotating, optically thick disks
To interpret observations of astrophysical disks it is essential to
understand the formation process of the emitted light. If the disk is optically
thick, scattering dominated and permeated by a Keplerian velocity field,
Non-Local Thermodynamic Equilibrium radiative transfer modeling must be done to
compute the emergent spectrum from a given disk model. We investigate Non-local
thermodynamic equilibrium polarized line formation in different simple disk
models and aim to demonstrate the importance of both radiative transfer effects
and scattering as well as the effects of velocity fields. We self-consistently
solve the coupled equations of radiative transfer and statistical equilibrium
for a two level atom model by means of Jacobi iteration. We compute scattering
polarization, that is Q/I and U/I line profiles. The degree of scattering
polarization is significantly influenced by the inclination of the disk with
respect to observer, but also by the optical thickness of the disk and the
presence of rotation. Stokes U shows double-lobed profiles with amplitude which
increases with the disk rotation. Our results suggest that the line profiles,
especially the polarized ones, emerging from gaseous disks differ significantly
from the profiles predicted by simple approximations. The profiles are diverse
in shape, but typically symmetric in Stokes Q and antisymmetric in Stokes U. A
clear indicator of disk rotation is the presence of Stokes U, which might prove
to be a useful diagnostic tool. We also demonstrate that, for moderate
rotational velocities, an approximate treatment can be used, where non-local
thermodynamic equilibrium radiative transfer is done in the velocity field-free
approximation and Doppler shift is applied in the process of spatial
integration over the whole emitting surface.Comment: 16 pages; 12 figures; Accepted with revision for A&A. This is the
version after first round of referee's suggestion
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