18,645 research outputs found
Recommended from our members
Prediction of the Thermal Conductivity of Beds Which Contain Polymer Coated Metal Particles
Structural parts of ceramics or metals can, in principle, be made by laser sintering
polymer coated ceramic or metal powders, followed by conventional methods for removing
the binder and sintering in ovens. Understanding the laser sintering of coated materials
requires knowledge of the behavior of beds containing composite particles. Many
correlations for predicting the effective thermal conductivity of a bed of solid particles exist
in literature, but little work has been done on beds of coated particles. We coated lead
shots(high conductivity) with a styrene acrylic acid copolymer (low conductivity) to study
the effect of coating thickness on the thermal conductivity. The thermal conductivity ofthe
coated particle·hed was found to •drop rapidly in the beginning·· and then level off with
increasing coating thickness. We also developed an equation that yields the equivalent
conductivity of a coated spherical particle subjected to axial heat flow. The predicted results
with the experimental measurements of bed conductivity obtained by an unsteady
state method.Mechanical Engineerin
Evidence for a neural model to evaluate symmetry in V1
50 years ago Hubel and Wiesel discovered simple and complex cells in V1, but there is still no consensus on their functional roles. It is agreed that complex cells are more often selective for direction of motion than simple cells, that there are differences in the way they combine information within their receptive fields, and that complex cells probably receive most of their input from simple cells, but what this serial hierarchy achieves is not understood. There is another puzzling dichotomy that we think is related, namely that of cross-correlation, which is widely accepted as the operation performed on the input image by simple cells, and auto-correlation, which some think underlies the perception of Glass patterns, and possibly motion. We propose the hypothesis that complex cells signal auto-correlations in the visual image, but to evaluate them they require the preliminary analysis done by simple cells, and also pinwheels - structures intervening between simple cells and complex cells that were quite unknown to Hubel and Wiesel. We shall first present psychophysical evidence, using a new kind of random dot display, which suggests that both cross-correlation and auto-correlation are performed in early vision. We then point to recent evidence on the micro-circuitry of pinwheels, and mappings of their intrinsic activity, which shows how pinwheels might enable complex cells to respond selectively to autocorrelations in the input image that activates the simple cells. Auto-correlation is a powerful tool for detecting symmetry, and many may be surprised by evidence that such an abstract property is detected so early in visual perception
Evidence for Auto-Correlation and Symmetry Detection in Primary Visual Cortex
The detectability of patterns in random dot arrays was measured as a function of dot density and compared with the statistical limit set by different methods of detecting the pattern. For filtering, cross-correlation, convolution, or template matching, the limit is expected to be inversely proportional to the square root of dot density. But for auto-correlation, which can detect symmetries of various types, the limit is unaffected by dot density under many conditions. Confirming previous results, we found that the coherence-threshold is often constant for Glass patterns, but the range of constancy depends on details of the display procedure. Coherence-thresholds were found to increase when the average number of dots expected at each location rose towards or exceeded a value of one; we therefore think it results from the non-linear effects of occlusion that occur when a later-programmed dot falls in the same location as an earlier one. To test this, these non-linear effects were prevented by arranging the luminance of each location to be directly proportional to the number of times that location was covered by a dot. Millions of dots can be used for these images, and they retain the streakiness of Glass patterns, while discrete dots disappear. The constant coherence threshold for detecting this streakiness is maintained over a huge range of dot densities, extending right down to the range where discrete dots become visible and up to patterns that are essentially full-tone images with no discrete dots. At threshold, all these patterns have similar auto-correlation functions, as we can see from the way both low dot-number Glass-patterns and these mega-dot, multi-tone, Glass-like images are formed. This startling fact raises the question whether primary visual cortex computes auto-correlations as well as, or even instead of, the local, Fourier-type, wavelet analysis of the currently popular paradigm
An eclipsing post common-envelope system consisting of a pulsating hot subdwarf B star and a brown dwarf companion
Hot subdwarf B stars (sdBs) are evolved, core helium-burning objects located
on the extreme horizontal branch. Their formation history is still puzzling as
the sdB progenitors must lose nearly all of their hydrogen envelope during the
red-giant phase. About half of the known sdBs are in close binaries with
periods from 1.2 h to a few days, a fact that implies they experienced a
common-envelope phase. Eclipsing hot subdwarf binaries (also called HW Virginis
systems) are rare but important objects for determining fundamental stellar
parameters. Even more significant and uncommon are those binaries containing a
pulsating sdB, as the mass can be determined independently by asteroseismology.
Here we present a first analysis of the eclipsing hot subdwarf binary
V2008-1753. The light curve shows a total eclipse, a prominent reflection
effect, and low--amplitude pulsations with periods from 150 to 180 s. An
analysis of the light-- and radial velocity (RV) curves indicates a mass ratio
close to , an RV semi-amplitude of , and an
inclination of . Combining these results with our spectroscopic
determination of the surface gravity, , the best--fitting
model yields an sdB mass of 0.47 and a companion mass of . As the latter mass is below the hydrogen-burning limit,
V2008-1753 represents the first HW Vir system known consisting of a pulsating
sdB and a brown dwarf companion. Consequently, it holds great potential for
better constraining models of sdB binary evolution and asteroseismology.Comment: 9 pages, 8 figures, accepted for A&
Three-Dimensional Ionisation, Dust RT and Chemical Modelling of Planetary Nebulae
The assumption of spherical symmetry is not justified for the vast majority
of PNe. The interpretation of spatially-resolved observations cannot rely
solely on the application of 1D codes, which may yield incorrect abundances
determinations resulting in misleading conclusions. The 3D photoionisation code
MOCASSIN (Monte CAarlo SimulationS of ionised Nebulae) is designed to remedy
these shortcomings. The 3D transfer of both primary and secondary radiation is
treated self-consistently without the need of approximations. The code was
benchmarked and has been applied to the study of several PNe. The current
version includes a fully self-consistent radiative transfer treatment for dust
grains mixed within the gas, taking into account the microphysics of dust-gas
interactions within the geometry-independent Monte Carlo transfer. The new code
provides an excellent tool for the self-consistent analysis of dusty ionised
regions showing asymmetries and/or density and chemical inhomogeneities. Work
is currently in progress to incorporate the processes that dominate the thermal
balance of photo-dissociation regions (PDRs), as well as the formation and
destruction processes for all the main molecular species.Comment: 3 pages, to appear in Proc. IAU Symp. 234, Planetary Nebulae in Our
Galaxy and Beyond (3-7 Apr 2006), eds. M.J. Barlow & R.H. Mendez (Cambridge
Univ. Press
Dust yields in clumpy SN shells: SN 1987A revisited
We present a study of the effects of clumping on the emergent spectral energy
distribution (SED) from dusty supernova (SN) shells illuminated by a diffuse
radiation source distributed throughout the medium. (...) The fully 3D
radiation transport problem is solved using a Monte Carlo code, MOCASSIN, and
we present a set of models aimed at investigating the sensitivity of the SEDs
to various clumping parameters. We find that, contrary to the predictions of
analytical prescriptions, the combination of an optical and IR observational
data set is sufficient to constrain dust masses even in the case where
optically thick clumps are present. Using both smoothly varying and clumped
grain density distributions, we obtain new estimates for the mass of dust
condensed by the Type II SN 1987A by fitting the optical and infrared
spectrophotometric data of Wooden et al. (1993) at two epochs (day 615 and day
775). (...) From our numerical models we derive dust masses for SN 1987A that
are comparable to previous analytic clumped graphite grain mass estimates, and
at least two orders of magnitude below the 0.1-0.3 Msol that have been
predicted to condense as dust grains in primordial core collapse supernova
ejecta. This low condensation efficiency for SN 1987A is in contrast to the
case of SN 2003gd, for which a dust condensation efficiency as large as 0.12
has recently been estimated. (Abridged)Comment: accepted for publication in MNRAS. The paper contains 15 figures and
1 tabl
The timing and location of dust formation in the remnant of SN 1987A
The discovery with the {\it Herschel Space Observatory} of bright far
infrared and submm emission from the ejecta of the core collapse supernova
SN\,1987A has been interpreted as indicating the presence of some
0.4--0.7\,M of dust. We have constructed radiative transfer models of
the ejecta to fit optical to far-infrared observations from the literature at
epochs between 615 days and 24 years after the explosion, to determine when and
where this unexpectedly large amount of dust formed.
We find that the observations by day 1153 are consistent with the presence of
310M of dust. Although this is a larger amount than has
previously been considered possible at this epoch, it is still very small
compared to the amount present in the remnant after 24 years, and significantly
higher dust masses at the earlier epochs are firmly ruled out by the
observations, indicating that the majority of the dust must have formed at very
late times. By 8515-9200 days after the explosion, 0.6--0.8\,M of dust
is present, and dust grains with radii greater than 2\,m are required to
obtain a fit to the observed SED. This suggests that the dust mass increase at
late times was caused by accretion onto and coagulation of the dust grains
formed at earlier epochs.
These findings provide further confirmation that core collapse supernovae can
create large quantities of dust, and indicate that the reason for small dust
masses being estimated in many cases is that the vast majority of the dust
forms long after most supernovae have been detectable at mid-infrared
wavelengths.Comment: 13 pages, 16 figures. Accepted for publication in MNRA
A study of the drooped leading edge airfoil
Wind tunnel tests were conducted to examine various aspects of the drooped-leading edge airfoil which reduces the tendency for an airplane to enter a spin after stall occurs. Three baseline models were used for tests of two dimensional models: NACA 0015, 0014.6, and 0014.2. The 14.6% and 14.2% models were derived from NACA 0015 sections by increasing the chord and matching the profiles aft section. Force, balance data (lift, drag, pitching moment) were obtained for each model at a free-steam Reynold's number of 2.66 x 10 to the 6th power/m. In addition, oil flow visualization tests were performed at various angles of attack. An existing NACA 64 sub 1 A211 airfoil was used in a second series of tests. The leading edge flap was segmented in three parts which allowed various baseline/drooped leading edge configurations to be tested. Force balance and flow visualization tests were completer at chord Renolds numbers of 0.44 x 10 to the 6th power, 1.4 x 10 to the 6th power, and 2.11 x 10 to the 6th power. Test results are included
3D Photoionisation Modelling of NGC 6302
We present a three-dimensional photoionisation and dust radiative transfer
model of NGC 6302, an extreme, high-excitation planetary nebula. We use the 3D
photoionisation code Mocassin} to model the emission from the gas and dust. We
have produced a good fit to the optical emission-line spectrum, from which we
derived a density distribution for the nebula. A fit to the infrared coronal
lines places strong constraints on the properties of the unseen ionising
source. We find the best fit comes from using a 220,000 K hydrogen-deficient
central star model atmosphere, indicating that the central star of this PN may
have undergone a late thermal pulse.
We have also fitted the overall shape of the ISO spectrum of NGC 6302 using a
dust model with a shallow power-law size distribution and grains up to 1.0
micron in size. To obtain a good fit to the infrared SED the dust must be
sufficiently recessed within the circumstellar disk to prevent large amounts of
hot dust at short wavelengths, a region where the ISO spectrum is particularly
lacking. These and other discoveries are helping to unveil many properties of
this extreme object and trace it's evolutionary history.Comment: 8 pages, 4 figures; for the proceedings of "Asymmetric Planetary
Nebuale IV," R. L. M. Corradi, A. Manchado, N. Soker ed
Mocassin: A fully three-dimensional Monte Carlo photoionization code
The study of photoionized environments is fundamental to many astrophysical
problems. Up to the present most photoionization codes have numerically solved
the equations of radiative transfer by making the extreme simplifying
assumption of spherical symmetry. Unfortunately very few real astronomical
nebulae satisfy this requirement. To remedy these shortcomings, a
self-consistent, three-dimensional radiative transfer code has been developed
using Monte Carlo techniques. The code, Mocassin, is designed to build
realistic models of photoionized nebulae having arbitraries geometry and
density distributions with both the stellar and diffuse radiation fields
treated self-consistently. In addition, the code is capable of tretating on or
more exciting stars located at non-central locations. The gaseous region is
approximated by a cuboidal Cartesian grid composed of numerous cells. The
physical conditions within each grid cell are determined by solving the thermal
equilibrium and ionization balance equations This requires a knowledge of the
local primary and secondary radiation fields, which are calculated
self-consistently by locally simulating the individual processes of ionization
and recombination. The main structure and computational methods used in the
Mocassin code are described in this paper. Mocassin has been benchmarked
against established one-dimensional spherically symmetric codes for a number of
standard cases, as defined by the Lexington/Meudon photoionization workshops
(Pequignot et al., 1986; Ferland et al., 1995; Pequignot et al.,
2001)\citep{pequignot86,ferland95, pequignot01}. The results obtained for the
benchmark cases are satisfactory and are presented in this paper. A performance
analysis has also been carried out and is discussed here.Comment: 17 pages, 4 figures, 1 appendix Changes: appendix adde
- …